PRINCIPLES FOR FORCING TULIPS, HYACINTHS, DAFFODILS, EASTER LILIES AND DUTCH IRISES

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1 Scientia Horticulturae, 2(1974) Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands PRINCIPLES FOR FORCING TULIPS, HYACINTHS, DAFFODILS, EASTER LILIES AND DUTCH IRISES AUGUST DE HERTOGH Department of Horticulture, Michigan State University, East Lansing, Mich. (U.S.A.) Michigan Agricultural Experiment Station Journal Article No (Received March 14th, 1974) ABSTRACT De Hertogh, A., Principles for forcing tulips, hyacinths, daffodils, Easter lilies and Dutch irises. Scientia Hort., 2: Systematic investigations on the forcing of tulips, hyacinths, daffodils, Easter lilies and Dutch iris have produced a fairly complete set of scientific principles. In this review, these principles have been classified into a 3-phase concept of forcing. These are: production, programming, and greenhouse. The production phase is defined as all processes which occur during bulb production and it terminates with the harvesting of the bulbs. The programming phase comprises all handling of the bulbs from harvesting until they are placed under greenhouse conditions. The greenhouse phase is the accelerated development of the bulbs until anthesis or marketing of the plants. These phases have been discussed relative to floral and root development and the basic environmental requirements of the bulb species. INTRODUCTION The floricultural greenhouse industry produces potted plants and cut flowers from a variety of bulbous species (Table I). The system by which the potted plant or cut flower is produced from a storage organ, e.g., bulb, corm, tuber, is called forcing. To accomplish this requires a knowledge of many factors related to the developmental cycle of the species and cultivars. The principal requirement in forcing is environmental control and, more specifically, temperature control. In some instances, forcing is accomplished by utilizing naturally occurring climatic conditions in combination with artificially controlled ones. In other cases, the plants are controlled entirely by artificial means, both environmental and chemical. The exact technique to be used is dependent on the species and cultivar being forced, how and when the pot plant or cut flower is to be marketed and the climatic conditions existing in the forcing locality. The physiology of flower-bulb forcing has been reviewed periodically (Purvis, 1937, 1938; Went, 1948; Hartsema, 1961; Rees, 1966). Recently,

2 314 TABLE I Spectrum of bulbous plants used for greenhouse forcing, their principal commercial uses and normal periods of availability Bulb Principal commercial usage Normal period of availability Cut flower Pot plant Alstroemeria spp. X April to December Convallaria majalis X Year-round Crocus spp. X December to March Eucharis grandiflora X October to April Freesia spp. X September to May Gloriosa spp. X May to September Hippeastrum hybrida X December to May Hyacinthus orientalis X X December to April Iris hollandica X October to May Iris reticulata X December to March Lilium spp. X X Year-round Muscari armeniacum X January to March Narcissus spp. X X November to April Nerine sarniensis X July to November Tulipa spp. X X December to May Zantedeschia spp. X November to May two extensive reference sources have become available. In 1970, the First International Symposium on Flower-Bulbs was held in The Netherlands and the papers have been released in two volumes (Bergman et al., 1971). In 1972, Rees published the first book devoted to flower-bulb physiology. These have been important contributions o this field of research and are highly recommended. For those interested in the techniques used in commercial bulb forcing, there are several manuals and bulletins available (Stuart et al., 1955; Gould, 1957; De Munk, 1965; Brown, 1967; Durieux and D e Pagter, 1967; Hoogeterp, 1967, 1968; Kiplinger and Langhans, 1967; Anonymous, 1969; Bing, 1971; De Hertogh and Wilkins, 1971; Segers, 1971; Ball, 1972; De Pagter, 1972; Schenk, 1972; De Hertogh, 1973). I should point out that there is an extensive reference library on bulbous plants at the Royal Dutch Bulb Growers Association in Hillegom, The Netherlands. Bulbous plants are an extremely heterogeneous group (Table I). Based on morphological and anatomical characteristics, they have been divided into five broad classes: bulbs, corms, tubers, tuberous roots and rhizomes (Hartmann and Kester, 1968). The cultural requirements of this group vary too greatly to be covered in a single review. I have, therefore, limited this review to the forcing of tulips, hyacinths, daffodils, Easter lilies, and Dutch iris. In

3 315 addition, only those principles related to forcing in the Northern Hemisphere will be considered. This group of bulbous plants was selected because their developmental cycle either requires or is markedly accelerated by low temperature treatments and because they represent the majority of bulbs forced. Other bulbous plants and their requirements will be mentioned only when the information available complements them. The principal aim of this review was to summarize the theoretical aspects of bulb forcing and to demonstrate how this research information provides the basic principles for commercial utilization. I will discuss the information using a phasic concept of forcing (see Section "Forcing procedures", page 322). While doing this, it must be realized that it is impossible to cite all the papers on forcing of the selected bulbs. I have utilized only those papers which are readily available in the literature or which clearly review or illustrate the principle discussed. It has been my intention to present the material in such a manner that it will be beneficial to researchers~ extension agents, students, and commercial bulb forcers. This has been done because it is essential that scientists and commercial groups work together and understand each other's problems and goals. The need for integrated efforts toward the goal of increasing our knowledge of bulbous plants is paramount today and only a cooperative effort can achieve progress. GENERAL ASPECTS Historical aspects of flower-bulb forcing and research. -- Although bulbs are produced in many countries (see Section "World bulb production, export and forcing", p.316), The Netherlands is the center of the world flower-bulb industry. When one studies the history of these plants, emphasis is always laid on how the bulbs came to be cultured in The Netherlands, the resultant 'Tulpomania' and the magnitude of bulb export (Krelage, 1946; Blunt, 1950; Janse, 1966). Very little, however, has been written concerning the history of flowerbulb forcing. According to Janse (1966), the forcing of hyacinth bulbs was already commonplace in the early 18th century. J.F.Ch. Dix (personal communication, 1973) indicated that tulip forcing began in the late 18th century. Early forcers resorted to planting the bulbs in boxes and then placing them in protected areas so that they would flower earlier than normal. This technique forms the basis for natural forcing and is still utilized to some degree. Systematic research on the influence of temperature on bulb development did not begin until the early 20th century (Janse, 1966; Schenk, 1971a). Nicolaas Dames is credited with the initial investigations on "preparing" hyacinths and can be called the "father of flower-bulb physiology". What is truly outstanding about his efforts is that he was a bulb grower and not a scientist per se. However, by keen observation and a logical approach he was

4 316 able to demonstrate that Dutch-grown hyacinths could be forced into t]ower before Christmas by a combination of early harvesting and a sequence of postharvest temperature treatments. Following this pioneering work, the two well-known Dutch research laboratories came into existence (Janse, 1966). In 1917, Professor Blaauw and his co-workers began their research at Wageningen on the influence of temperature on the development of a wide range of bulbous plants. Their elegant series of investigations formed the foundation for many of the current scientific and practical endeavors. While many of these reports are only available in Dutch, it is fortunate that Annie Hartsema, who was one of Blaauw's colleagues, has reviewed much of this work and published it in English (Hartsema, 1961). In 1920, the Laboratory for Flower-Bulb Research, under the direction of Professor E. van Slogteren, was established at Lisse, The Netherlands. Initially, most of the efforts of this laboratory were directed toward the disease and insect problems associated with bulbous plants. Later, however, the scope of the research was expanded. Today, under the direction of Professor P.K. Schenk, this laboratory maintains a focal point of bulb research in the world. In addition to these two major groups, laboratories whose primary goal has been to further the understanding of bulb physiology have been established in various countries. Among them are: the United States Department of Agriculture at Beltsville, Md. and various agricultural experiment stations and universities in the United States, Canada, Great Britain, Israel, Germany, Japan, France, and Norway. For a rather complete listing of currently active groups, consult the First International Symposium on Flower-Bulbs (Bergman et al., 1971). From this brief historical review, one can see that the art of bulb forcing is centuries old, while scientific endeavors can only be measured in decades. Bulbous plants utilized for forcing. -- For those familiar with bulbous plants, it is apparent that most bulbous species can be forced to some degree. Some of the more important ones were listed in Table I, The table is very general and merely serves as a guide. It is possible to construct such a table for each forcing area. For instance, almost all lilies forced in the U.S. are L. longiflorum and are used principally as pot plants for Easter. In contrast, in Western Europe, almost all lilies are forced as cut flowers and many more species and cultivars are used. Thus, the bulb world is very diverse and when flower-bulb physiology is discussed, it is possible to make generalizations. These must, however, always be tempered by considerations of the bulb species, commercial usage, cultivar, bulb production areas, etc. World bulb production, export and forcing. -- World bulb production is concentrated heavily in The Netherlands, Great Britain, the United States, and Japan (Rees, 1972). It is interesting to note that only a few of the bulbs produced are native to these countries, a major exception being Lilium, many

5 317 species of which are indigenous to the Japanese islands. Bulb export is carried out principally by The Netherlands, the United States, and Japan. At present, Great Britain consumes most of the bulbs it produces, but this appears to be changing. Although most bulbs are exported in some quantity, tulips, hyacinths, daffodils, crocus, lilies, Dutch iris and gladioli comprise the major bulb exports. The mode, duration and conditions during transport greatly influence the physiological ana morphological development of the bulbs (Beijer and Van Slogteren, 1931; Van Slogteren, 1934, 1935). These conditions must be considered both theoretically and practically. The Netherlands leads the world in the number of bulb flowers produced in greenhouses. There is, however, a sizable production in other countries such as German Federal Republic, Great Britain, Sweden, Denmark, Japan, Israel, South Africa and the United States. GROWTH HABIT AND CULTURAL REQUIREMENTS General. -- To develop a systematic forcing program for any bulbous Species, a knowledge of many specific factors is required. Among these are: the native habitat of the species, current centers of bulb production, plant and bulb morphology, the periodicity of growth, environmental requirements, type of forced product desired, time of flowering desired, cultivar responses, the nature of diseases and insects affecting the species, and the sensitivity to ethylene. It has been the role of the applied plant physiologist and horticulturist to investigate these factors and to integrate them into a practical forcing system. In this section, I will cover some of the basic factors which significantly influence forcing programs. In subsequent sections, specific details concerning the five major bulb species selected for this review will be discussed, Plant material. -- A comprehensive understanding of the plant material is essential. Each bulb species, and even the cultivars, has specific requirements. It is advisable to know the origin of the species. Bailey (1949) provides an excellent source of such information. The significance of this type of information has been reviewed by Rees (1966, 1972). He points out that many of the bulbous plants originated between north and south latitude. As a result, the bulbous habit appears to have developed as a result of alternating conditions of warm- ~dry summers and cool--moist winters. This type of information can be useful in determining the major environmental requirements of the specific species. More than likely, the exact requirements will have been modified by the fact that changes have occurred through adaptation of the species to production regimes, e.g., the tulip in The Netherlands, and by hybridization and horticultural selection. Through man's exploitation, it has been possible for species to lose certain characteristics, or to acquire new ones. It is also possible to develop different strains through adaptation. Thus, 'Wedgwood' iris grown in the State of Washington does not necessarily correspond

6 318 identically to 'Wedgwood' produced in The Netherlands or elsewhere. This aspect is frequently overlooked. Morphological considerations are also important. The fundamental types of bulbous plants have been described by Hartmann and Kester (1968) and Rees (1972). In addition to knowing whether the storage organ is an enlarged stem, root, leaf or hypocotyl, an awareness of the fact that bulbous species are alive and undergo ~narked morphological changes over short periods of time is critical. Thus, in the case of the tulip, the meristem is normally vegetative at the time of harvest and, if all inductive factors are normal, the apical meristem undergoes a rapid transformation to the reproductive state with the complete immature flower being developed within a few weeks. Flower development can be regulated by post-harvest temperatures (Luyten et al., 1925; see also Fig.1 on p.328). It has also been demonstrated that the normal complement of floral parts, which is 15, can be increased or decreased by suboptimal temperatures (see Hartsema, 1961). This indicates that precise temperature control is needed throughout the development cycle of the plant. Another characteristic of importance is the horticultural production cycle of the species. Some multiply by annual replacement (tulip and crocus), others by offset production (daffodil) and still others by modified organ culture (hyacinths and lilies). The specific cycle will determine the age of the bulb used for forcing. The specific characteristics for flowering must be determined for each bulb species (Hartsema, 1961). One parameter is minimum bulb size. Normally only large bulbs are used for forcing and this factor is routinely eliminated. The requirement for leaf number is also important. With some bulb types, such as the tulip, all leaves must be initiated before floral initiation can occur. On the other hand, with a bulb like the hyacinth, the leaf initiating process can be interrupted by early harvesting and floral initiation takes place. It is this characteristic which Nicolaas Dames observed and which led to the technique of "preparing" Dutch-grown hyacinths for Christmas flowering. A major aspect, which has been given little consideration in bulb forcing, is the fact that bulbous plants are similar in structure to seeds (Table II). A similar table could be constructed comparing them to buds on a tree. This similarity is very important because damage by physical abuse, improper environmental conditions, diseases or insects to a vital organ such as the apical meristem or root primordia can lead to reduced percentages of flowering. Thus, with the millions of bulbs handled annually in the bulb trade, it becomes economically important to establish for each bulb species and cultivar a reasonable percentage of loss. This would be comparable to the percentage germination rating given to seed. From this discussion, it should not be implied that bulbs are purposely abused. However, even with the emphasis laid on bulb handling, it is reasonable to assume that losses will occur. In essence, bulbous plants have become domesticated and with this process comes an assortment of problems.

7 319 i:!:... TABLE II Morphological similarities of bulbous plants and seeds Bulb organ Tunic (when present) Enlarged stemsl leaves, roots, or hypocotyls for storage Apical meristem(s) Basal plate (when present) Root primordia Seed organ Seed coat(s) Cotyledon(s) or endosperm Plumule of embryo Hypocotyl and epicotyl Radicle Dormancy. -- The question of "dormancy" in bulbous plants is highly academic and somewhat controversial. The fact is that bulb species have evolved mechanisms to survive adverse climatic conditions and man has attempted to describe these processes. Kamerbeek et al. (1972) have recently put forth their views on this subject. They have divided several of the major bulb species into three classes of "dormancy" based on the ability of the shoot to develop new organs and to elongate. Group I, which consists of Lilium, Allium, and Gladioli, exhibits relatively long periods of dormancy. In general, the dormancy period occurs shortly after the formation of the storage organs and prior to the onset of active formation of vegetative tissue, e,g., stem and leaves. Group II demonstrates only a slight period of dormancy and consists of Tulipa, Hyacinthus, and Narcissus. The apparent dormancy period of this group occurs after floral organ formation and is associated with elongation of the scape. Group III shows no rest period, unless imposed by man, and is best represented by Iris hollandica. I feel from both a practical and theoretical point of view that an awareness of the fact that bulbs are never physiologically at rest is very critical. Close periodic examinations of bulbs will reveal marked developmental changes (Hartsema, 1961). Luyten et al. (1932) recognized this fact ee/rly in their work and described "after" or "indirect" effects of suboptimal temperatures given during the summer and fall. Thus, both researchers and commercial bulb forcers should always monitor and control the temperatures from bulb harvest to flowering. Periods of several days or weeks without control can result in abnormal plant development. In the course of the investigations in my laboratory, I have come to the conclusion that bulbs, like most plants, contain temperature monitoring system(s). They function like "biocomputers" and forcing can be viewed as a system of inputs and outputs. Thus, if one examines the literature with a computer system in mind, it is possible to divide the forcing system into three distinct phases: (a) production; (b) programming; (c) greenhouse. Each of

8 320 these phases can be applied to a bulb species and the specific type of forcing utilized. In subsequent sections, I shall demonstrate how this classification of phases can be utilized in forcing. To return briefly to the monitoring ability of the bulbs, it is obvious that each bulb varies in its response characteristics and the site(s) of this monitoring system are not known. Wellensiek (1965) has suggested that premitotic or mitotic cells are involved in those plants in which temperature induces flowering. Perhaps a similar situation occurs in the bulbous plants. Systematic investigations in this area of bulb physiology would be extremely valuable, especially since the low temperature responses can be divided into at least two distinct classes, e.g., bulbing and scape elongation in the tulip (Le Nard and Cohat, 1968). Environmental factors controlling bulb forcing. ~ To place the entire forcing sequence in perspective, one must take into consideration the fact that the entire flowering process can be divided into five stages: (a) induction; (b) initiation; (c) organogenesis; (d) maturation; and (e) anthesis (Lang, 1965; Nitsch, 1965). If one discusses the influence of specific environmental fac/~ors on the development of bulbs for forcing, an assumption must be made that the bulbs utilized are free of harmful diseases and insects and that all requirements for flowering induction, e.g., minimum bulb size, have been satisfied. Also, the individual bulb species and/or cultivar must have the genetic potential to be flowered out of season. It is well known that some cultivars of tulips are easily manipulated while Others do not readily force. The questions of cultivar responses will be dealt with in more detail in the specific forcing sections. Temperature, light, moisture and nutrients all influence the forcing of bulbs. The most important factor is temperature (Hartsema, 1961). However, most of these factors have specific interactions. Many types of bulbs respond to cyclic periods of warm and low temperatures and this phenomenon has been called annual thermoperiodism by Went (1948). Each bulb type has a specific requirement for a certain number of weeks for both the warm and low temperature requirement. This aspect will be covered in more detail in the forcing sections. It is the ability to control the specific stages of growth and development by temperature in combination with other factors which permits the forcing of the bulbs to take place. In forcing, two basic goals are set forth. The bulbs are either accelerated to flower at the earliest date or they are delayed or "retarded" to flower at the latest date. The limits of these manipulations are under genetic control. For some bulbs, e.g., Dutch iris and lilies, it has been possible to develop a year-round flowering schedule. Blaauw and co-workers (see Purvis, 1937, 1938; Hartsema, 1961) studied the influence of temperature on many bulbous species. They observed that there were "special temperature optima" for different stages of development,

9 321 e.g., flower initiation, root growth, etc. By linking these optima a "general temperature optima" could be obtained for each cultivar and desired flowering period. This was accomplished by effecting a compromise of the important factors. Thus, when early flowering was desired, flower size and total scape length were compromised. In general, the earlier the flowering the greater the number of compromises; as forcing becomes less accelerated, the plants become more "normal" in appearance. Too often, this necessary compromise is not recognized by forcers. The so-called standards for judging forcing performance have to be modified to fit the flowering period. The specific temperature treatments for forcing tulips, hyacinths, daffodils, Easter lilies and Dutch iris will be discussed in the respective sections. Closely linked to temperature in the control of bulb development is moisture. Precooling Easter lilies for early flowering is a cold--moist treatment (Stuart, 1954). Low moisture content of the peat causes significant delays in flowering. With bulbs, as with all plants, moisture plays an important role in the development of an adequate root system. This factor is always important after the bulbs have been planted and should be considered in the total forcing program regardless of the forcing techniques utilized. The effect of light on the development of the five bulbous species selected is summarized in Table III, For some species, such as Tulipa, Hyacinthus and TABLE III Effects of light on growth and development of forced tulips, hyacinths, daffodils, Easter lilies and Dutch iris Type of Basic responses to: References bulb Photoperiodic Light intensity Tulip None Leaf shape, stem Wassink, 1965 length and diameter Bulb growth Hyacinth None Plant quality Unpublished Bulb growth observations Daffodil None Plant quality Unpublished Bulb growth observations Easter lily Floral initiation Waters and Wilkins, promoted under LD Plant height reduced Kohl and Nelson, under SD. Floral bud 1963 Einert and Box, 1967 abortion under low light Dutch iris Flower abortion Floral abortion Fortanier and greater on less than under low light Zevenbergen, h day

10 322 Narcissus, neither light intensity nor photoperiod markedly influence anthesis. In fact, these plants can be brought to anthesis in'absolute darkness. However, overall quality is adversely affected due to the etiolated growth of the plants. In the forcing of species such as the Easter lily and Dutch iris, Iight intensity is important. It has been demonstrated that under low light intensities in winter, flower blasting can occur in Dutch iris (Fortanier and Zevenbergen, 1973) and Easter lilies (Einert and Box, 1967), This effect can be modified by low greenhouse temperatures. In situations such as this there appears to be a competitive relationship between leaf growth and flower stalk development. With exception of the Easter lily, photoperiod techniques are not utilized in the forcing of bulbs. Water and Wilkins (1967) have reported that a night break applied to emerging shoots of bulbs which have had a minimum of 3 weeks at 4.5 C can substitute for remaining low temperature requirement and thus complete the programming of the lily. This system has proved to be effective as an insurance policy for timing the Easter lily in the U.S, and Canada. In addition, Kohl and Nelson (1963) have demonstrated that an 8 h photoperiod can reduce the total plant height of potted Easter lilies. Photoperiod is also involved in the bulbing and tuberization responses of several bulbous plants (Heath and Holdsworth, 1943; Gregory, 1965; Moser and Hess, 1968). This, of course, is not directly related to forcing, but primarily effects production. As a general rule, nutrition does not play a highly significant role in the forcing of low-temperature requiring bulbs. Most bulbs contain sufficient nutrients and energy reserves which are recycled during forcing. The Easter lily is an exception (Boodley, 1967). Because of the long period in the greenhouse, the Easter lily requires periodic fertilization. Failure to fertilize properly can result in alteration of plant height, loss of leaves and flowers and a reduced vigor. In certain instances, the tulip can benefit from an application of Ca 2. Algera (1968} has pointed out that the physiological disorder "stem topple" can be prevented by Ca(NO3)2 applications. He concluded that Ca 2 translocation is not sufficient during the period of rapid scape elongation and that cell wall and membrane integrity are lost. Forcing procedures. -- A close analysis of the influence of temperature, light, moisture, and nutrients on the development of tulips, hyacinths, daffodils, Easter lilies and Dutch iris on the basic process of flower development shows that they can be di~cided into two broad groups. The first group consists of tulips, hyacinths and daffodils. In this group, floral initiation and organogenesis occur either before or shortly after harvesting and are completed prior to the application of a low temperature treatment (Hartsema, 1961). During the low temperature treatment, floral maturation takes place and the plant subsequently reaches anthesis when placed in a warm greenhouse or, if outdoors, when Spring arrives. As pointed

11 323 out in Table III, light does not play an important role in the flowering process, but does affect overall quality. Moisture is important in the development of the root system. The second group consists of the Dutch iris and Easter lilies. In this group, the bulbs are vegetative at the time of harvesting and floral initiation and organogenesis occur either during (Dutch iris) or subsequent (Easter lilies) to a low temperature treatment (Hartsema, 1961). In contrast to the previous group, light (see Table III) plays a significant role in floral development of these plants. Moisture is important not only for root development but also for programming of the Easter lily. When this grouping is combined with the five stages of bulb production, which are: (a) harvesting and preplanting storage; (b) planting, rooting and low temperature mobilization for flowering and/or bulbing; (c) bolting; (d) flowering; and (e) increase in bulb weight and size, it is possible to develop a 3-phase forcing system: production, programming and greenhouse. The production phase includes all of the above five stages. The programming phase for forcing includes all procedures from the time of harvesting to placing the bulbs in the greenhouse. Depending on the exact technique utilized this can include both pre- and post-planting treatments. The greenhouse phase lasts from the time the bulbs are placed in the greenhouse until the product is marketed either as a pot plant or cut flower. In subsequent sections, the development of specific forcing techniques for each bulb species will be described using this phase concept as a model. It is important to realize that the number and types of forcing procedures utilized will vary with the bulb species, product desired, and forcing locality. While the exact techniques will vary the basic principles are, however, the same, i.e., the environmental requirements of the bulb for flowering must be satisfied. Other factors affecting bulb forcing. -- In addition to the environmental factors discussed in Section "Environmental factors controlling bulb forcing", p.320, there are several other factors which affect the forcing performance of bulb species. It would be possible to write a review on several of them, e.g., disease problems associated with forcing, and such articles would be most welcomed. In this section, I will only briefly mention some of the more important factors. When pertinent, they will be dealt with in more detail under the appropriate bulb forcing sections. It is well known that there are many diseases and insects which can affect the development of bulbous crops. There are a few manuals available which should be consulted for information on the subject (Moore, 1949; Kiplinger and Langhans, 1967; Schenk, 1971b). The planting medium is also important (Rees, 1972). For forcing, the medium should be well drained and yet hold sufficient water, so that the plants are not under moisture stress. It should anchor the plants. The medium should have a ph of , and be low in soluble salts. Roots are sensitive

12 324 to these factors. As pointed out previously, only the Easter lily is normally fertilized during forcing. In many instances, bulbs used for either experimental or commercial forcing must be transported over long distances. Problems can occur in transit. Beyer and Van Slogteren (1931) and Van Slogteren (1934, 1935) have pointed out the problem of "heating in transit". This is an aspect which should not be ignored. There are methods available either to overcome or at least to monitor this factor. Bulbs can be transported by air. This can reduce the length of time in transit from weeks to a few days. It is also possible to utilize controlled temperature containers, both on and off ships. Regardless of the mode of transport used, it is wise to monitor the temperatures en route utilizing automatic temperature recorders. It is well known that ethylene is an important regulator of many plant processes (Abeles, 1973). Recently, the effects of this chemical on bulb development have received renewed attention, particularly with respect to the tulip (De Munk and De Rooy, 1971; De Munk, 1972, 1973a, b). It should be pointed out, however, that many other bulbous plants respond to ethylene (Hitchcock et al., 1932). With the exception of the accelerated flowering shown with 'Wedgwood' iris (Stuart et al., 1966), most of the effects of ethylene are undesirable. Thus, a general rule is never to expose bulbous plants to ethylene. Another factor which requires special attention and which will be discussed in greater detail in subsequent sections is the effect of applications of various growth regulators on bulb forcing. At present, growth regulators are utilized principally to control the height of lilies (Kiplinger and Langhans, 1967; Hasek et al., 1971; Dicks and Rees, 1973). Recent studies (Farnham and Hasek, 1972; Shoub and De Hertogh, 1974) have demonstrated that the tulip is responsive to ancymidol treatments. In the future, this chemical may be used to control plant height of potted tulips. Chemical height control has not been demonstrated with Dutch iris. Hyacinths and daffodils will respond to ethephon (unpublished data), but the procedures are still experimental. TULIP FORCING General aspects. -- The tulip is the premier of forced flower bulbs. Millions are forced annually in various parts of the world. They are utilized both as cut flowers and potted plants; however, the emphasis varies greatly from country to country. For example, in Western Europe, most are forced as cut flowers, while in the United States the majority are utilized as potted plants. In the Northern Hemisphere, the flowering season normally begins in early December and terminates in early May. It is possible, however, to have tulips on a year-round basis using a freezing technique (Slootweg, 1968). I refer to these tulips as 'Eskimo' tulips to denote the type of forcing procedure utilized. At present, because of cost and demand, this technique is utilized only on a

13 325 limited scale. It will, therefore, not be discussed in detail in this section. The major emphasis will be placed on the traditional forcing techniques and season. In addition, while it is possible to retard the floral development of the tulip for shipment to the Southern Hemisphere (Table IV), this handling will also be excluded. TABLE IV Treatments for year-round and/or storage treatments for retarding bulbs for shipments to the Southern Hemisphere Type of Description of treatment References bulb Immediately after Subsequent storage harvest treatments Tulip -1 C, apical meristem after 5--8 months, Blaauw et al., 1930; must be in Stage I transfer to C Hartsema, 1961 for flower development Hyacinth 30 C to mid-october -1/2 C to January, then Beijer and Van by 25.5 C Slogteren, 1933; Hartsema, 1961 Daffodil 30 C to mid-october -1]2 C to January, then by 25.5 C Easter lily -0.5 C and in a medium plant directly in so bulbs do not greenhouse dessicate 0~C and 100% N2 program as described in the section "Programming phase", p.341 Dutch iris 30 C in relatively precool at C high humidity for 6 weeks prior to planting Belier, 1938; Hartsema, 1961 Stuart, 1952 Stuart et al., 1970 Durieux and De Pagter, 1967 Developmental cycle of the tulip. -- The tulip requires a warm--cool--warm annual temperature sequence for growth and development (Hartsema, 196l; Rees, 1966, 1972). Briefly, the developmental cycle is as follows. At harvest time, the apical meristem is vegetative. The bulbs initially require temperatures of C to complete leaf initiation and C for floral initiation and organogenesis to proceed. Subsequently, the bulbs require low temperatures (1--9 C) for mobilization of the bulb for the floral stalk elongation and floral maturation. After the low temperature requirement has been satisfied, the bulbs are placed at C, scape growth ensues and anthesis is reached. To force the tulip, two basic techniques have been developed based on this temperature controlled cycle. One is called 'Standard Forcing' (Table V). The other is called 'Special Precooling' (Table VI). Both techniques have a common

14 326 TABLE V Concept of standard forcing of tulips and hyacinths. Important events Programming phase Greenhouse phase Summer Fall Winter Spring Harvesting of bulbs Planting and Mobilization Flower stalk rooting under for flowering, elongation Preplanting storage cool-tnoist under low and anthesis under warm temperatures conditions temperatures under warm for floral initiation temperatures and organogenesis TABLE VI Concept of special precooling technique for tulips. Important events Programming phase Greenhouse phase Summer Fall--winter Spring Harvesting of bulbs Preplanting storage under warm temperatures for floral initiation and organogenesis Mobilization for flowering under low temperatures Rooting, flower Stalk elongation and anthesis under warm temperatures starting point in that floral initiation and organogenesis must be completed prior to movement of bulbs from warm temperatures to low temperatures. The major difference between the two techniques is the application of the total low temperature treatment and the development of the roots in relation to the programming and greenhouse phases. In the standard forcing technique, all or part (bulbs are sometimes given a precooling treatment up to 6 weeks) of the low temperature treatment is applied after planting. This can be accomplished either in controlled temperature rooting rooms or outside under naturally occurring conditions which approximate the temperature and moisture requirements of the bulbs. This means that rooting takes place during the programming phase and that the bulbs are well rooted and shoot growth has started when placed in the greenhouse. In the special precooling technique, all of the low temperature is applied as a dry storage treatment in controlled temperature rooms and root and shoot growth take place after planting the bulbs in the greenhouse. Both techniques are used commercially. The commercial forcing of tulips presents certain difficulties, the principal one being the magnitude of cultivars available for forcing. Over 100 cultivars are utilized. In addition, we have the two different techniques for forcing.

15 327 Both have distinct advantages and disadvantages and they complicate cultivar usage. For precise capabilities, each forcing procedure should be investigated for each cultivar. Also, from year to year, the production phase of development will be slightly different and this must be taken into account. In general, it takes a minimum of 2 years of trials and preferably 3--5 years to be assured of exact cultivar responses. Too often, this aspect is not fully appreciated in practice. Standard forcing. -- The concept for standard forcing was outlined in Table V on p.326. The major controlling factor is temperature. To understand the evolution of technique, the major developmental stages of the tulip will be discussed with reference to the effects of temperature. Programming phase. -- The initial step in standard forcing is the control of floral initiation and organogenesis. When the tulip is harvested immediately after senescence of the foliage, the apical meristem is vegetative. If the bulb is too small to initiate floral primordia, it will only form one leaf (Rees, 1972). If it is large enough to flower, it will form 3--5 leaves. An exception to the rule is the disorder "antholyse" (Schenk, 1971b). When this disorder occurs, a normal leaf complement is formed but no floral primordia are initiated. This condition appears to be that of a truly "blind" plant. This is in direct contrast to other tulips traditionally called "blind" which had, in fact, initiated flowers but were subsequently aborted. A knowledge of various stages of floral organogenesis in the tulip is important. These stadia are described in the review by Hartsema (1961). Briefly, the meristem proceeds through seven stages, the last of which is commonly known as stage "G". At this stage, the primordia of six tepals, six androecium and a trilobed gynoecium are visible. Blaauw and his co-workers carried out detailed studies on floral initiation and organogenesis. Using 'Pride of Haarlem', Luyten et al. (1925) showed that floral development to stage G occurred most rapidly at C (Fig.l). Storage of the bulbs at temperatures above 20 C, but below 25.5 C, permitted floral development to occur but at a slower rate. A similar response occurred at temperatures of C. Since this work was carried out nearly 50 years ago and many new cultivars have been developed, the question may be asked: do the present day cultivars respond similarly? Hoogeterp (1971) has repeated this type of study and found that all seven cultivars tested had an optimal temperature range of C. In recent years, an additional refinement has been made. It has been found that for earliest flowering some cultivars will respond to one week of 34 C when applied immediately after harvesting and prior to the transfer to the C (Rees and Turquand, 1967). These investigations have provided the basis for development programs for early and late forcings. Bulbs for early forcings are harvested early, given a week of 34 C andplaced at C. Bulbs for late forcings are stored at

16 'Pride of Haarlem' 8 ~ 6 E~ 2 I I I i I i 5 i Temperature ( C) Fig.1. Influence of temperature on flower formation of the tuiip 'Pride of Haarlem'. Adapted from Luyten et al., C to initially delay floral development. The bulbs are subsequently placed at 17 C to continue floral development. This stepwise reduction in temperature preserves the high quality forcing characteristics of the cultivars. Regardless of the temperatures used, the major objective is to develop and maintain a viable flower. The elongation of the tulip scape is directly correlated with both the degree and length of the low temperature treatment. The total period of the cold treatment must include the precooling treatment whenever utilized. The overall effects of the duration of the low temperature treatments have been concisely described by Rees (1969). His results (Fig.2) with 'Paul Richter' illustrate several principles of the effects of low temperature on tulip forcing. The flowering data show that the first plants placed in the greenhouse are not the first to flower. For most rapid flowering, the bulbs needed a minimum of 12 weeks of low temperatures. The data also demonstrate that the number of days in the greenhouse can be markedly reduced by increasing the weeks of cold. The influence of increased duration of low temperatures on total scape length is also significant. The longer the cold treatment, the longer the scape

17 329 ~ i00 P au ~l Richt e_~r o ~ 75 ~ a) ~ 5c "~.~ E~ 25 ~0 I i o i0 Lowest Leaf J i i i i 9 ii Length Of cold (weeks) Fig.2. Effects of duration of low temperature treatments subsequent to flower organogenesis on development of the forced 'Paul Richter' tulip. Adapted from Rees, until the genetic limits are reached. Commercially, consideration must be given to whether the product desired is a pot plant or a cut flower. This determines the number of weeks of cold to be utilized. Another interesting point is the fact that the internode between the highest leaf and the base of the flower appears relatively insensitive to the duration of the cold treatment In contrast, the lower internodes of the scape increased with increased weeks of cold. This characteristic is a useful indicator of the length of the cold treat ment utilized. In the standard forcing technique, the low temperature can be applied in combination with a precooling treatment or entirely after planting. Some precooling is generally used for standard forcing when the earliest possible flowering is desired and when natural temperatures are too high. It must be determined if the particular cultivar will respond to such a treatment. The bulbs must be transferred from the warm to cool temperature for precooling when the proper stage of floral development is reached. Initially, it was felt

18 330 that stage P1 was suitable (Mulder and Blaauw, 1925). It is now known that it is cultivar dependent but most cultivars can be transferred upon reaching stage G. Some, such as 'Christmas Marvel,, 'Merry Widow', and 'Most Miles' should be kept at 17 C for 7 to 14 days after they have reached stage G. This has been called the "inbetween" temperature {Hoogeterp and Slootweg, 1968). Hoogeterp (1971) has shown that the exact date of reaching stage G varies from year to year and from cultivar to cultivar. In addition, the region of bulb production will influence this date. Therefore, yearly periodic sampling of each cultivar is mandatory. De Hertogh and Aung (1968) have described a quick, simple, inexpensive technique for identifying stage G in the tulip. The question of what temperature to use for precooling is also one which is dependent on the cultivar used, the product to be produced {cut or pot) and the flowering period desired. Investigations by Hartsema et al. {1930) using 'William Copland' indicated that tulips could be precooled at 8 or 9 C prior to planting. They also noted that 5 C for 6 weeks gave greater acceleration but resulted in a loss of quality. Thus, indications that temperatures near 5 C could be used for precooling appeared quite early in the literature. Recently, Rees and Turquand {1969) have re-examined the effects of various precooling temperatures. They precooled 'Paul Richter', 'Rose Copland', 'Merry Widow', and 'Apeldoorn' at 4.4, 6.7, and 8.9 C for 4,6 or 8 weeks. The bulbs were subsequently planted and rooted at 8.9 C until the plants were carried into the greenhouse. They found that temperatures of 4.4 and 6.7 C increased the total length of 'Apeldoorn', 'Merry Widow' and 'Rose Copland' over the 8.9 C-treated plants. The lower temperatures also hastened flowering with 'Rose Copland' and 'Apeldoorn'. Flower size was reduced in 'Paul Richter', 'Rose Copland' and 'Merry Widow'. In some cases, the percentage of marketable flowers was also reduced by the lower precooling temperature. They suggested that when temperatures below 8.9 C are used for precooling, the precooling period be 4 weeks or less. In our studies, we have compared similar temperatures for precooling of Dutchigrown tulips and found that a temperature of 5--7 C was optimal for 'Cantor', 'Demeter', and 'Dix's Favourite', while 7--9 C was optimal for 'Most Miles' and 'Gander' (unpublished data). Thus, in practice, the factors of costs, compromised quality, time of flowering, percentage of loss and cultivar usage must be considered in deciding what precooling temperature should be utilized in developing a forcing program. In general, it is good practice to use temperatures below 8--9 C for as short a time as needed in order to maintain plant quality. After planting, precooled tulips are placed at 9 C. This continues the low temperature treatment and also allows rooting to proceed. Fundamentally, this is the forcing scheme initially proposed by Hartsema et al. {1930). The optimal length of the total cold treatment will vary with each cultivar. Rees et al. (1972a) demonstrated that when the bulbs were precooled at 9 C,

19 :: ( 331 the minimum cold requirement for earliest flowering of 'Apeldoorn', 'Paul Richter', and 'Rose Copland' was 159,128 and 140 days, respectively. In conjunction with a 35 C treatment for 1 week immediately after harvesting and then either 4.4 or 6.7 C precooling, the minimum could be reduced to 142, 123, and 134 days, respectively. Information on precise temperatures for handling of nonprecooled bulbs after planting is somewhat lacking. In practice, the bulbs are initially placed at 9 C. In examining the effects of temperature on rooting of nonprecooled tulips (unpublished data), we found that the optimal temperature range was C and that 9 C was slightly suboptimal. Thus, the use of a 9 C rooting temperature provides a compromise between development of the roots and cooling of the bulbs. It also is beneficial in deterring the development of certain diseases. Exactly what temperature is optimal for maximum cold-unit efficiency after planting has not been conclusively demonstrated. There are indications that it is near 2 C. Kosugi et al. (1968) found that for early forcing of 21 cultivars of Japanese-grown tulips that 2 C for 55 days was superior to either 5 or 9 C for 55 days. In practice, after nonprecooled tulips become well rooted at 9 C, the temperature is progressively lowered to 5 and then to 2 C to retard shoot growth. This is an essential sequence for late forcings in order to preserve plant quality. Greenhouse phase. -- The final stage in the temperature controlled process of forcing is the greenhouse phase. Hartsema (1961) stated that 20 C is the maximum temperature to be utilized. Temperatures above 20 C can induce the disorder known as "stem topple" (Algera, 1968). A study (Fig.3) carried out at Michigan State University shows that the length of the greenhouse phase is markedly increased as temperatures are decreased from 18 to 13 C. There was no difference in the number of days in the greenhouse between the 18 and 24 C treatments, but stem topple was observed. Another effect of the greenhouse temperature on plant growth was that plants forced at 13 C had significantly longer tepals. This is a quality factor which can be of commercial benefit. Thus, a compromise between rate of forcing and other quality factors must be considered. In addition to temperature control, moisture is also very important. The plants must be kept moist to prevent bud blasting. Special precooling. -- General. -- The technique of special precooling is also known as "5 C Forcing" and "Direct Forcing" (Rees, 1972). I propose, and prefer, the term special precooling since this more closely implies the technique which must be employed to program the bulbs (see Table VI). The use of temperature terminology such as 5 C is restrictive because only one temperature is implied. It is well known that other temperatures can and should be used (Hoogeterp, 1973). The technique was originally proposed by Stuart and Gould (1953). Using the cultivar 'William Pitt', they found that the bulbs could be given a dry

20 332 5O 4o o ~ 3o ~c.~ 20 '~ i0 i I i i i ~ Temperature ~ C ) Fig.3. Influence of temperature during the greenhouse phase on number of days to flower the tulip. Curve is a composite of data from 'Elmus', 'Apeld0orn', 'Robinea', 'Sulphur Glory', and 'White House' given a 13.5 week low temperature treatment and placed in greenhouse at the various temperatures on February 8th. storage treatment of 4.5 C for 6 weeks followed by 4 weeks at 10 C with subsequent direct planting and placing of the bulbs in the greenhouse. Thus, the bulbs received no cooling after planting. They suggested that such a technique of "conditioning" the bulbs could be developed into a commercial forcing program. Dickey (1953, 1957) and Gill et al. (1957) have demonstrated that this procedure could also be applied to tulips for outdoor use in areas which do not naturally receive sufficient winter cold. The principal use for special precooling is for early~cut flower production. It has not been utilized to any extent for pot plants but the potential exists, as will be discussed in "Special considerations", p.333 and Table VII. Programming phase. -- This phase comprises all handling of the bulbs from harvest to planting and placing of the bulbs in the greenhouse. The initial steps of controlling floral initiation and organogenesis are identical to those of standard forcing, discussed in "Programming phase" on p.327. Slootweg and Hoogeterp (1971) have described the influence of warm temperatures immediately after harvesting and prior to precooling on the forcing of 'Apeldoorn'. They found that storage of bulbs for one week at 34 C immediately after harvesting would accelerate the leaf initiating process and consequently speed up floral initiation and organogenesis. Compared to bulbs kept at 20 C, bulbs receiving this treatment reached stage G 8 days earlier. They found that not all cultivars tested responded to this treatment. Hoogeterp (1967b, 1968) has also reported that an "inbetween" tempera-

21 333 ture of 17 C for 1--3 weeks prior to precooling is beneficial for some cultivars. In practice, this is of extreme value since it can reduce the percentage of blasted flowers. The term "5 C forcing" was derived from the fact that temperatures very near to 5 C were suitable for special precooling. This temperature is one which has been most widely utilized. Recently, Hoogeterp (1973) has suggested the use of 2 C for late forcing. It is because of this that I have put forth the term "Special Precooling" to denote the overall technique. Then, depending on the cultivar, forcing time and production area, the specific temperatures needed can be utilized. Although it has been known for many years (Hoogeterp, 1967b, 1968) that the length of the low temperature programming treatment is critical for special precooling of tulips, it is only recently that this principle has been reported in detail (Moe and Wickstr m, 1973). They carried out a study in which they compared the length of special precooling at 5 C vs nonprecooling (21 C) for 'Paul Richter' and 'Apeldoorn', and found that nonprecooled bulbs were very short, took very long to force, and had low percentage of flowering plants compared to those which received a precooling treatment. 'Paul Richter' had optimal precooling requirement of weeks, while 'Apeldoorn' produced an optimal of weeks. Thus, the forcing of special precooled tulips is also a function of the cultivar as well as the degree and length of the low temperature utilized. Greenhouse phase. -- The greenhouse phase of the special precooling technique is more critical than that for standard forcing. The basic reason is that during this phase the tulips must develop and maintain a viable root system and at the same time, the shoot must elongate. Thus, the factors of soil temperature and disease control must be considered. Schenk and Bergman (1969) have reported that Fusarium can be observed in greenhouses when special precooled bulbs are used. De Rooy and Vink (1969) have found that benomyl is an effective fungicide in helping to prevent this disease. Pythium can also be a problem (De Rooy and Vink, 1970). To minimize problems of this nature, three steps are normally taken. First, all bulbs must be individually examined to insure that only healthy bulbs are planted. Second, proper soil preparation to include soil sterilization should be combined with fungicidal treatments. Third, soil temperatures in the greenhouse must be maintained below 16 C. This results in a long greenhouse phase but reduces losses due to diseases. Special considerations. -- In addition to the persistent problems of disease and insect control (Schenk, 1971b), there are a few areas which deserve special consideration in tulip forcing. Since the desired product of tulip forcing is a flowering plant, it is essential that floral abortion, commonly known as bud blasting, be prevented. This is a very complicated problem and many factors have been implicated (Rees, 1972). Two principal causes have been investigated and deserve special

22 334 mention. These are: (a) ethylene; and (b) improper temperatures. The wide range of ethylene effects on developing bulbous plants was first reported by Hitchcock et al. (1932), who found that immature flower buds of tulips could be aborted by ethylene and that other tepal malformations could occur. More recently, De Munk (1973a,b) has investigated this aspect in detail. He has demonstrated Chat ethylene can cause varying extremes of floral abortion, e.g., abnormal tepal coloration, anther abortion, and complete floral abortion. The significance of these effects are far reaching, since it is known that tulips infected with Fusarium oxysporum produce ethylene (Kamerbeek et al., 1971). Thus, during transport and long term storage, it is essential that the bulbs be well ventilated and not be placed in the presence of ethylene-producing materials such as fruit. Also, routine inspections for the presence of Fusarium are mandatory. A second cause of blasting is improper temperatures. This can occur either accidentally or as part of a special treatment. Hartsema and Luyten (1950) and Rees (1967) have investigated the use of high temperatures ( C), after flower organogenesis has been completed, to induce floral abortion and to promote bulb production. This is a technique referred to as "Blindstoken". In principle this same effect can occur accidentally during transit (Van Slogteren, 1934, 1935) and has been called,'heating in transit". It can also occur by malfunctioning of a controlled temperature room. In any event, the effects are similar and the end result is the loss of the flower, which is undesirable for the bulb forcer. Another area worthy of special mention is the control of plant height. In practice, four methods are used. These are: (1) cultivar selection; (2) the length of the low temperature treatment; (3) the use of dark-stretching in the early part of the greenhouse phase; and (4) the shifting of a cultivar from one flowering period to another. At present, another method shows promise for pot plant forcing. Farnham and Hasek (1972) have reported that final plant height of standard forced tulips can be reduced by applications of ancymidol. They found that a soil drench was superior to a foliar spray and that as a soil drench the chemical must be applied within a few days after moving the plants to the greenhouse. They observed activity at rates as low as 0.1 mg active ingredient per 10 cm pot, but rates between mg were the most effective. Shoub and De Hertogh (1974) have found that the effect of ancymidol can be completely reversed by a subsequent soil application of GA4 + 7 and that GA3 was ineffective, Furthermore, they observed that the mechanism of growth retardation occurred through an inhibition of cell division in the intercalary meristems and an inhibition of cell elongation in the basal internodes of the tulip scape. Recent experiments (Table VII) at Michigan State University have indicated that ancymidol can be combined with the special precooling technique. The data reveal that an application of 0.5 mg applied within 2 weeks after planting gave optimal height control for pot plant use. Similar responses were also obtained with 'Golden Melody' and 'Hibernia'. This method is still experimen-

23 335 TABLE VII Effect of ancymidol on growth of special precooled 'Paul Richter' tulips. Bulbs were precooled at 5 C from October 10th to January 4th, then potted and placed in C greenhouse Ancymidol Time of application Avergge % of plants Length (cm) (rag/15 cm pot) (weeks in greenhouse) days to flowering flower Flower Total First plant internode : HSD (5% level) N.S. N.S. N.S tal and needs considerably more study since not all cultivars are adaptable. It is also possible to force some tulip cultivars under artificial light and this can replace the greenhouse phase (Schenk, 1972). Thus, with the use of artificial lights it is possible to carry out controlled forcing of the tulip from harvest to flowering, without the use of naturally occurring climatic conditions. To some individuals, this could be considered the ultimate in tulip forcing. There are, however, as with all forcing techniques, certain advantages and disadvantages. One last point worthy of consideration is the tunic of the bulb. While serving beneficial purposes such as prevention of damage to the fleshy scales, and regulating gas exchange (Rees, 1972) the tunic does play a role in inhibiting rapid root development. For the special precooling technique, it is advantageous to remove the tunic prior to planting to aid in promoting rapid root development. HYACINTH FORCING General aspects. -- In general, the temperature requirements for hyacinths are 5--7 C higher than those of the tulip. Normally, hyacinths are not as susceptible to abnormal temperature effects, diseases, o 7 other forcing problems. Perhaps this explains the lack of recent scientific papers on hyacinth

24 336 forcing. Most of the principles were established by Blaauw's group in the nineteen-twenties and -thirties (Hartsema, 1961) and they appear to be as valid now as they were then. The majority of the bulbs are forced starting in late November and extending to mid-april. By temperature manipulation, it is possible to force these bulbs on a year-r0und schedule. This is an outgrowth of the system used for retarding hyacinth for export to the Southern Hemisphere (Table IV, see page 325). Most of the world's hyacinth production is located in The Netherlands (Rees, 1972). In contrast to the tulip, there are fewer cultivars of hyacinths available for forcing. This combined with the observation that most hyacinths basically fall into only two classes for forcing, early and late, has led to easier forcing schedules for the hyacinth. Also, as will be discussed below, only the standard forcing technique is normally used. It is possible, however, to utilize the special precooling technique for December flowering (Hoogeterp, 1967a). This technique is not routinely used and only the standard forcing technique will be discussed in this section. Developmental cycle of the hyacinth. -- The hyacinth requires a warm--cool-- warm temperature sequence for proper growth and development (Hartsema, 1961; Rees, 1966, 1972). The developmental sequence is quite similar to the tulip. At harvest, the apical meristem is vegetative. The bulbs are subsequently placed at a temperature of 25.5 C to terminate leaf initiation and begin floral initiation and organogenesis. This is followed by a low temperature (9--13 C) requirement for mobilization for floral stalk and leaf elongation. The elongation sequence occurs during the second warm temperature ( C) treatment. Most hyacinths are forced using standard forcing procedures, as mentioned in Table V, p.326. In contrast to the tulip, the bulbs are normally not precooled. All cooling of the bulbs takes place after planting. Thus, the overall forcing process as well as temperature requirements are more straightforward than that of the tulip. A 3-step technique is utilized. The development of the forcing program will be discussed with reference to the effects of temperature. Like the tulip, there is a minimal bulb size for flowering. In practice, only large bulbs (15 cm and over) are utilized and this problem is readily circumvented. zrogrammingphase. -- The first step in forcing is to initiate the floral spike of the hyacinth. The stadia of floral initiation are described in the review by Hartsema (1961). A major point to remember is that the hyacinth forms a floral spike consisting of many florets. Blaauw and co-workers have carried out detailed studies on flower initiation and organogenesis in the hyacinth. Luyten et al. (1932) found that for earliest flowering the bulbs should initially be placed at 34 C for 5 days. Floral development then proceeds most rapidly at 25.5 C (Blaauw, 1924). As soon

25 337 as the uppermost floret reaches stage P2, the bulbs should be placed at 17 C for 3 weeks. For later forcings, the 34 C treatment is omitted. Beijer (1936) has found that it is possible to develop fasciated floral stalks. To do this, the bulbs are placed immediately after harvest at 20 C for 10 days and then transferred to 25.50C. After floral organogenesis has been completed and the 17 C post-organogenesis treatment has been given for 3 weeks, bulbs for early flowering are planted at C. During this low temperature treatment, the root system is developed and the bulbs are mobilized for elongation of the flowering stalk and leaves. Luyten et al. (1932) have reported that the low temperature requirement can be Satisfied at 13 C arid that this is the optimal temperature for earliest flowering. Again, this is several degrees higher than the tulip. For later forcings, it is necessary to utilize a stepwise reduction in temperature down to 1 C in order to retard shoot growth in the dark. This reduction should occur only after the bulbs are well rooted. Versluys (1927) has reported that the optimal temperature for root growth in the fall is 17 C. Thus, the 13 C used for early flowering is a compromise temperature between the rooting optima and the cooling optima. She also found that root growth in the fall was influenced by the pre-planting temperatures. A period of 4 weeks at 17 C after storage at 25.5 C promoted more rapid root growth. This is essentially the treatment used in practice. The length of the cold treatment needed for most rapid growth has not been reported in detail. In general, the minimum requirements for hyacinths prepared for early forcing is weeks and that of the regular hyacinths weeks (De Hertogh, 1973). Greenhouse phase. -- In the greenhouse, the hyacinth can withstand temperatures up to 23 C without difficulty (Luyten et al., 1932). Thus, there are few problems associated with temperature control in the greenhouse. While the most rapid flowering occurs at 23 C, most forcers utilize this temperature only for the early forcings. Subsequently, they use temperatures of C, which is far more economical. Special considerations. -- The major problem associated with the forcing of hyacinths is known as "spouwen", spitting, or loose-bud. There are two forms of the disorder, physiological and mechanical. Beijer (1947) initially described the physiological type which is characterized by the presence of an exudate in the middle of the flower stalk. He postulated that this type of disorder is caused by an imbalance of water uptake and utilization. Later, Beijer (1963) described the mechanical type which does not involve tissue infiltration. Instead, there is a mechanical abscission of the floral stalk at the junction of the basal plate. Apparently, abrupt temperature changes can mediate this response. By observation, I have concluded that two factors play an important role. First, rooted hyacinths should never be frozen or, if this accidentally

26 338 occurs, they should be thawed very slowly. Also, there is a definite cultivar effect. Some, such as 'Eros' and 'Queen of the Pinks', are far more susceptible than others. As with all disorders, the precise causes are not completely known. Hyacinths can be forced in rooms With artificial light and this can replace the greenhouse phase (Krabbendam, 1968). The hyacinth lends itself to this procedure more readily than the tulip. DAFFODIL FORCING General aspects. --The Narcissus, also called daffodils, are forced extensively in Western Europe and North America. They are utilized both as cut flowers and pot plants, but cut flowers predominate. In the Northern Hemisphere, the forcing season normally begins in early December and ends in April. Besides the greenhouse forcings, there are many millions of daffodils flowered outdoors for cut flower usage. This section will concentrate, however, on greenhouse forcing. Bulbs for forcing are extensively produced in The Netherlands, Great Britain and the United States (Rees, 1972). Besides the normal greenhouse season, it is possible to retard the development of the bulbs for the Southern Hemisphere and thus they could be flowered on a year-round schedule (Table IV on p.325). In many respects the requirements for forcing daffodils resemble those for both the tulip and the hyacinth. The optimal temperature conditions are similar to those of the tulip, but the leaf and floral stalk habit resemble that of the hyacinth, In general, daffodils give fewer problems in forcing than the tulip and in this way also resemble the hyacinth. Developmental cycle of daffodils. -- As with the hyacinth and tulip, daffodils require a warm- cool--warm temperature sequence for proper growth and development (Hartsema, 1961; Rees, 1966, 1972). The developmental sequence differs from that of the tulip and hyacinth in that floral initiation and organogenesis precede the harvesting of the bulbs (Huisman and Hartsema, 1933). Thus, the initial warm temperature requirement after harvesting is needed only to complete floral organogenesis. This is followed by a low temperature requirement to promote floral stalk and leaf elongation and a subsequent warm temperature treatment to permit flowering. To force daffodils, the standard forcing technique is employed (Table VIII). The development of this technique will be described below. Programming phase. -- It has been reported by Huisman and Hartsema (1933), Gerritsen and Van der Kloot (1936) and Preece and Morrison (1963) that ~ flower initiation in daffodil occurs while the bulbs are still in the field but after anthesis. Thus, by harvest time floral organogenesis will be either complete or in the last stages. For a complete illustration of floral develop-

27 339 TABLE VIII Concept of standard forcing of daffodils. Important events Programming phase Greenhouse phase Sumraer Fall Winter Spring Harvesting of bulbs Preplanting storage under warm temperatures to complete floral organogenesis Planting and Mobilization Flower stalk rooting under cool--moist for flowering under low elongation and anthesis conditions temperatures under warm temperatures ment in the daffodil see Huisman and Hartsema (1933). It should also be pointed out that most daffodils form more than one flower, especially when they are double-nosed bulbs. Studies have been carried out on the effects of warm temperatures applied immediately after harvesting. Beijer (1955) has reported that a period of 1 week at C was beneficial in promoting early flowering. It was not necessary, however, for late forcing. This work has been subsequently confirmed by Rees et al. (1972b). This treatment is most effective when bulbs are harvested earlier than normal to encourage early forcing. Daffodils can be precooled for early flowering or kept at 17 C until planted and directly placed at low temperatures. The optimal temperature for precooling daffodils for the earliest forcings appears to be 6.7 C for 8 weeks (Rees et al., 1972b). These investigators found that 6.7 C precooling reduced the days to flower for 'Carlton', 'Fortune', and 'Golden Harvest' by days when compared to 9 C. This was dependent on the year in which the study was conducted as well as the cultivar. While this treatment encourages early flowering, it did tend to produce shorter plants and areduction in flower quality. As the season progresses, 9 C precooling is more suitable. This is the temperature which Hartsema and Luyten (1938) and Beijer and Van Slogteren (1931) suggested. It should be pointed out that Hartsema and Luyten (1938) obtained accelerated flowering when 'King Alfred' was precooled at 7 C. They, too, felt that this temperature resulted in a loss of flower quality. After planting, the optimal temperature for the combined functions of cooling and rooting appears to be 9 C (Van Slogteren, 1938; Hartsema and Luyten, 1938). This work has been reviewed by Rees (1972). While a temperature of 13 C appears more suitable for root and shoot growth, it does not adequately satisfy the need for cold for rapid flowering and total plant height (Hartsema and Luyten, 1938). As with the forcings of tulips and hyacinths, when the forcing season is delayed, temperatures of 5 C and later 1--2 C are needed in the rooting room to retard shoot growth in the dark.

28 340 Greenhouse phase. -- Definitive studies on the effects of greenhouse tempera-.tures on forcing of daffodils are not available. Rees (1972) has summarized what little information has been published and the range of C is usually suggested. Plants forced at 13 C take longer to force than those at 18 C. In our studies, I have observed that plants forced at 18 C tend to be shorter than those forced at 13 C. This can be a beneficial treatment for pot plant but detrimental to the cut flower forcer. Special considerations. -- In general, there are few disorders in daffodils. What few there are, appear to be due to improper temperature treatments (Rees, 1972). Thus, hot water treatments given for nematode control can affect floral development. Improper watering and very high greenhouse temperatures can produce bull-noses (failure of flower to open). So far, efforts to control plant height by chemical treatments are still experimental. EASTER LILY FORCING General aspects. -- Lily bulbs used for forcing are produced primarily in the United States, Japan, and The Netherlands. They are utilized both as potted plants and cut flowers. In the U.S. and Canada, almost all of the lilies forced are L. longiflorum Thunb. cvs 'Ace', 'Nellie White', 'Georgia', 'Arai', and 'Harson', and they are utilized as potted plants for Easter. Very few other lily species are used for any other purpose in these countries. However, with the advent of ancymidol, which can reduce the height of tall growing lilies (Hasek et al., 1971; Dicks and Rees, 1973), there should be a change in this tradition. In contrast, in Western Europe lilies are used as cut flowers. They are available on a limited scale on a year-round basis and are readily available from March to November (Schenk and Boontjes, 1970). Species other than L. longiflorum are used for forcing and the mid-century hybrid 'Enchantment' is the leading cultivar. In the U.S., much of the research on lily forcing has been carried out on L. longiflorum for Easter pot plant production. This review will concentrate specifically on Easter lily forcing and will inject other studies only when relevant. The basic forcing principles are applicable to most lilies. It must be pointed out, however, that with regard to cultural requirements L. longiflorum is a diverse species and it is essential to know the cultivar, area of production, harvest dates, and many other factors to know specifically how these plants will perform (A.N. Roberts, personal communication, 1973). Developmental cycle of the Easter lily. -- Commercial Easter lily cultivars have a need for a warm--low- lwarm temperature cycle when forced (Stuart, 1967). When the bulbs are harvested in late summer, the apical meristem is vegetative. After harvest, the bulbs are placed under cool--moist conditions (2--10 C) for a 6-week period, either after planting or during storage in moist peat. Subsequently, the bulbs are placed in a warm (16- t8 C) greenhouse

29 341 during which floral initiation, organogenesis, maturation and anthesis occur. Thus, in contrast to the tulip, hyacinth and daffodil, floral development in the Easter lily occurs during the greenhouse phase of forcing rather than in the early part of the programming phase. Based on this developmental cycle, a 3-phase forcing concept for Easter lilies has been developed (De Hertogh and Wilkins, 1971). The production phase determines the maturity of the bulb at harvest time, which in turn affects the basic ability of the bulb to be programmed and the flower bud potential. The programming phase determines the timing ability and uniformity of the crop by terminating the leaf initiation capacity of the bulb. The greenhouse phase is the follow-through phase during which the flowers are initiated and developed. It is a very important phase in lily forcing, more so than for tulips, hyacinths and daffodils. The basic goals of Easter lily pot plant forcing are to: (1) time the crop for Easter; (2) develop as large a number of flowers as is economically feasible; and (3) control plant height. Initially, I will discuss the basic parameters related to the programming and greenhouse phases. In a subsequent section, the influence of other factors related to timing, flower number and plant height control will be covered. Programming phase. --The programming phase has been defined as that period from harvest to placing the plants in the greenhouse. The overall concept and techniques of programming California and Oregon-grown Easter lilies has been reviewed by De Hertogh and Wilkins (1971). Briefly, they are as follows. (1) Precooling (PC). Bulbs are shipped in cases in moist peat either directly to commercial cold storage facilities or to the forcer in early October. Ideally, 'Ace' and 'Nellie White' lilies are placed for a period of 6 weeks at 2--5 C and 5--7 C, respectively. They are then planted and placed directly in the greenhouse. Planting normally takes place around December 1st. (2) Natural cooling (NC). Bulbs are shipped directly to the forcer in early October. On arrival, the bulbs are potted immediately, watered and then placed in unheated greenhouses, packing sheds or outdoor areas. Under these conditions the bulbs begin to root; shoot growth can occur, and the 6 weeks of chilling is accumulated through the naturally occurring temperatures. The plants are placed in the greenhouse around December 25th. (3) Controlled temperature forcing (CTF). Bulbs are shipped directly to the forcer in early October. On arrival, the bulbs are potted immediately, watered and then placed in a controlled temperature area at 17 C for 2--4 weeks. Under these conditions, the bulbs begin to root, and shoot growth can occur. The temperature is subsequently lowered to C for 6 weeks. The plants are placed in the greenhouse on December 20th--22nd. The fundamental basis of all these methods is the control of temperature and moisture (Stuart, 1954, 1967). The temperature used must be of sufficient duration to optimally control the development of the bulbs. The exact tem-

30 342 perature treatment will vary from year to year depending on bulb maturity at harvest. For the low temperature to be fully functional, an optimal moisture content is also required (Stuart, 1954). If delayed development is desired, the moisture content can be reduced. It is important, however, to ensure that the bulbs do not desiccate. To store bulbs for long periods of time, 0--1 C is used (Table IV on p.325). Stuart et al. (1970) have found that storage in a 100% N2 atmosphere and at 0 C can be used for long-term storage. At present, this method has not been put into practice. There are six overall effects of low temperature programming on Oregon and California produced 'Ace' and 'Nellie White' Easter lilies. These are: (1) promotion of bolting of the shoot; (2) reduction in the total number of leaves; (3) reduction in the number of days to flower; (4) reduction in the span of days from first to last plant flowering; (5) reduction in total plant height at time of flowering; and (6) reduction in the total number of flowers produced. A close examination of these six effects reveals that the first five are positive factors for pot plant forcing. Only the reduction in the number of flowers is detrimental. Stuart (1952) has demonstrated the effect of the length of low temperature cooling on bolting of the 'Creole' lily. He found that a 4-week treatment would satisfy this requirement. Observations have indicated that the number of weeks of cold required will vary from year to year. When the bulbs are fully mature only 1 or 2 weeks is necessary. Immature bulbs will require up to 5 weeks. The reduction in leaf number, days to flower, and total number of flowers is well documented (Langhans and Weiler, 1967). These effects, as well as that of uniformity of flowering, are illustrated in Fig.4. It should be recognized that each lot of bulbs consists of a population of bulbs. Thus, in order to ensure uniform development of the population, those bulbs which are highly responsive must be over-cooled in order to satisfy those bulbs which are less responsive. It is this type of treatment that makes the Easter lily a commercial product. The effect of low temperature duration on height control is illustrated in Fig.5. With less than 6 weeks of cold, leaf number (see Fig.4) and consequently the internode number is high and the plants are taller. Bulbs receiving more than 6 weeks of cold have a lower leaf number, but internodal length is enhanced. Therefore, the plants are again taller. In this discussion, it is assumed that the bulbs have been stored at near optimum conditions prior to cooling. It has been demonstrated, however, that temperatures above 20 C prior to cooling can influence the response to subsequent low temperature treatments (Miller and Kiplinger, 1966a). This technique has not been applied commercially. In addition, Miller and Kiplinger (1966b) have demonstrated that a 20 C treatment after 6 weeks of 4.5 C can also partially nullify the low temperature treatment. Again, this has not been applied commercially because of its undesirable effects on timing and uniformity of flowering. A technique which has received acceptance for timing the lily is that of a long-day treatment originally described by Waters and Wilkins (1967). As

31 Days to Flower ist Flower ]~ Last Flower No. of No. of leaves Floral Buds i I i ~ i ~ 13 i~lo ~o Fig.4. Influence of duration of low temperature (4.5 C) treatment on flowering and development of 'Ace' Easter lily. Bulbs were 17/20 cm and were programmed using CTF technique described in the section "Programming phase" on p.341. Greenhouse temperature was 16 C night temperature and 18 C day temperature. 9] 8o ~ 6o 5O 0 40 ' ~ ~ TO / Pedicel ~ ' c ~ ~ u v I~ i i I I Weeks at 4.5 C Fig.5. Influence of duration of low temperature treatment on total plant height and height to pedicel of 'Ace' Easter lily. Bulbs were 17/20 cm and were programmed using CTF technique described in the section "Programming phase" on p.341. Greenhouse temperatures were 16 C night temperature and 18 C day temperature.

32 344 discussed above, the low temperature treatment promotes both bolting and timing of the plants. Wilkins and co-workers (1967, 1968) found that by partially cooling the bulbs to promote emergence, the remaining requirement for flowering could be satisfied by interrupting the night from 10 p.m. to 3 a.m. with low intensity (5--10 ft c) lighting as the plants emerge from the pots. This type of programming is carried out during the early part of Stage I of the greenhouse phase. It should also be pointed out that there is evidence which indicates that the low temperature treatments used for flowering are quantitative in nature (Roberts and Moeller, 1971). They found that by leaving the bulbs in the ground until fall, the optimal low temperature requirement was lower than for those harvested in the summer. This type of investigation provides some insight into the reasons for wide variation in temperature reported by investigators when bulbs are grown in one area and forced in another (Stuart, 1967). Regardless of which programming technique is used, the bulbs must be planted prior to placing them in the greenhouse. De Hertogh and Blakely (1972) have investigated the influence of temperature on rapid root development of both nonprecooled and precooled 'Ace' lilies. When the effects of high temperatures either prior to or immediately after cooling (Miller and Kiplinger, 1966a, b) were integrated with root growth, they concluded that 17 C was the most suitable temperature for rooting lilies for forcing. It should be pointed out, however, that lilies root most rapidly at approx. 20 C (White, 1940), but this temperature results in a delay of flower development as discussed above. Greenhouse phase. -- The greenhouse phase for Easter lily forcing consists of three stages. These are: (I) time from start of greenhouse forcing to floral initiation; (II) time from floral initiation to buds visible in foliage; and (III) time from buds visible in foliage to flowering. Emsweller and Pryor (1943) have shown that Stage I, which is terminated by the transition of the apical meristem(s) from a vegetative to a reproductive state, is principally controlled by the programming phase. Presumably, greenhouse temperatures immediately after placing the bulbs in the greenhouse will also affect the time of flower initiation (Kohl, 1958; Smith and Langhans, 1962). Unfortunately, these investigators did not determine the exact dates of floral initiation under the various greenhouse temperature sequences. In fact, most investigators do not determine the exact dates of floral initiation for specific treatments. This is one item of data which should always be obtained since it is a vital reference point in floral development. The stages of floral development of the lily have been presented by Krijthe (1938) for Liliurn regale, by Pfeiffer (1935) and De Hertogh and Wilkins (1971) for L. longiflorum. The stage symbols have been summarized by Belier (1942). Stage IIis highly dependent on the temperature in the greenhouse. This is a very important stage in establishing the number of flowers which will be developed. Kohl (1958) using 'Ace' lilies has shown that temperatures above

33 345 or below C will decrease the number of flowers. Similar results have been reported by Smith and Langhans (1962) for 'Croft'. Because timing of the Easter lily is so important, the "leaf counting" technique (Blaney et al., 1967; Wilkins, 1969) has been developed for measuring the development rate of the lily in Stage II. The technique is simple. Once a lily undergoes the transition from a vegetative state to a reproductive state, the number of leaves is fixed. By utilizing a representative number of plants and making a physical count of the number of leaves left to be unfolded from the spindle, the leaf unfolding rate for the bulbs can be determined. By continuously monitoring the plants, alterations in greenhouse temperatures can be made to influence the date of seeing the visible buds in the foliage and consequently affecting the date of anthesis. Stage III is the time period from visible floral buds in the foliage to opening of the first flower. Roh and Wilkins (1973) have studied this stage in detail using L. longiflorum 'Ace'. They found, under a 12 h photoperiod and all possible day and night temperature combinations of 15.6, 21.1, 26.7 and 32.2 C, that day--night temperature combination of 21.1 C was the most satisfactory when the factors of days to flower, number of buds flowering and aborted, total plant height and number of senescing leaves were considered. Special considerations. -- Besides the influence of temperature during the programming and greenhouse phases, the utilization of a long-day treatment, and moisture levels, there are a few other factors which deserve special consideration. Many of these are discussed in detail in the Easter Lily Manual (Kiplinger and Langhans, 1967). It is also imperative that proper disease, insect, fertilizer and watering practices be followed. Light intensity. -- Light intensity can affect both flower bud abscission and plant height. It has been reported by Mastalerz (1965)and Einert and Box (1967)that low light intensities promote floral bud abortion in L. longiflorum. Similar results have been obtained by Kamerbeek and Durieux (1971a) for the mid-century lily 'Enchantment'. This is a problem in forcing lilies in mid-winter in extreme northern latitudes, but it can be overcome with supplemental lighting and temperature control. Plants grown under high light intensities are shorter than those grown under 50 or 75% of full sunlight (Einert and Box, 1967). Photoperiod. -- In addition to the long-day treatment previously described, it has been found that photoperiod can influence the final plant height (Kohl and Nelson, 1963). These effects have been reviewed by Kohl (1967). Lilies grown in the greenhouse on an 8-h photoperiod will be shorter than those grown on a 16-h photoperiod. Forcers desiring short pot plants can use shade cloth from 4 p.m. to 8 a.m. Cultivar and bulb size. P Each cultivar and/or bulb size will produce different numbers of flowers. De Hertogh et al. (1969) have compared

34 346 various bulb sizes of L. longiflorum 'Ace' and 'Nellie White' using the CTF and PC programming techniques. They found that when the same bulb size was compared, 'Ace' produced ca two more flowers than 'Nellie White'. Also, within the cultivar, the larger bulbs produced more flowers than the smaller bulbs. Chemical control of plant height. -- Investigations by Stuart et al. (1961) demonstrated that the final plant height of the flowering lilies could be reduced by soil drench applications of 'Phosphon'. They noted, however, that the plants were not always uniform and that stem strength was sometimes adversely affected. Recently, Hasek et al. (1971) have demonstrated that the 'Georgia' Easter lily is very responsive to ancymidol, which was effective when incorporated into the planting media prior to planting, and as a soil drench or foliar spray after the plants had emerged. Foliar sprays of 50 or 100 ppm produced the most desirable control of plant height and was the least laborious method of application. Dicks and Rees (1973) compared ancymidol with chlormequat, chlorphonium chloride, and ethephon on 'Enchantment' and 'Joan Evans', two of the mid-century hybrids. Ancymidol at 0.25 mg/pot was the most effective chemical in reducing plant height. Thus, it is possible that in the future lilies such as 'Enchantment' will become utilized as pot plants when programmed in conjunction with a chemical such as ancymidol. DUTCH IRIS FORCING General aspects. -- The bulbous irises (Iris hollandica Hoog), commonly known as Dutch iris, used for forcing are produced primarily in the state of Washington in the U.S. and The Netherlands (Rees, 1972). They are used exclusively as cut flowers. Through the manipulation of temperatures and cultivar selection it is possible to flower these bulbs on a year-round schedule (Table IV on p.325). This is done in some forcing areas of Western Europe. In other areas, the normal season is from December to May, the season being limited primarily because of high temperatures during the late spring, summer, and early fall. In some areas, low light intensity during the winter months restricts forcing. Because of the exacting requirements of temperature and light (Fortanier and Zevenbergen, 1973), Dutch iris can be somewhat difficult to force from year to year and location to location. Also, as Stuart et al. (1955) have indicated, the maturity at the time of harvesting is a variable factor, especially for very early flowering. Developmental cycle of Dutch iris. -- Compared to the tulip, hyacinth, daffodil and Easter lily, the Dutch iris have a much different cycle for flower development. Like the other bulbs, the Dutch iris have a requirement for a

35 347 warm--cool--warm temperature sequence (Hartsema, 1961; Rees, 1972). At harvest, the apical meristem is vegetative. The bulbs then require a storage period of several weeks at temperatures near 30 C. This is followed by transfer of the bulbs to C for 6--8 weeks which is the optimum range for early flower initiation (Blaauw, 1941). Floral initiation occurs after 4--5 weeks (Rodrigues-Pereira, 1962). After these temperature treatments, the bulbs are planted and placed in a C greenhouse for continued flower development. Programming phase. --The programming phase of Dutch iris is fundamentally divided into three types. First, there is the control of development for earliest flowering, then a "normal" developmental period and finally the retardation of the bulbs for year-round forcing. The programming and greenhouse phases for forcing Dutch iris are given in Table IX. Investigations on the effects of warm temperatures after harvest to stimulate early flowering have been carried out by Blaauw and co-workers (see review by Beijer, 1952; Stuart et al., 1955; Hartsema, 1961; Kamerbeek and Beijer, 1964; Kimura and Stuart, 1972). The warm temperatures are utilized to accomplish one of two functions. They can be utilized to accelerate the development of the bulb for early flowering. This aspect has been reviewed recently by Kimura and Stuart (1972), who analyzed the data from several studies, including their own, and calculated that freshly harvested 'Ideal' iris need 10 days at 32 C or 10 Equivalent Units (EU) of heat treatment. They also found that the bulbs did not respond to the first 2 days of the heat treatment, however, this time was needed for the initial adjustment of the bulb to the high temperature. During TABLE IX Concept of forcing Dutch iris. Important events Programming phase Greenhouse phase Summer Fall and winter Spring Harvesting of bulbs Warm temperatures for either promoting early flowering or retarding development Low temperature requirement for terminating leaf formation and initiating floral primordia Planting of bulbs Floral stalk elongation, floral maturation and anthesis days 6th--10th, the maximum response to the heat treatment was observed. Thus, the leafiness of the plants was decreased and early flowering encouraged. The per day effect of the 32 C treatment declined markedly during the llth-- 15th days. A careful examination of the various individual studies will reveal

36 348 minor differences, many of which can be attributed to seasonal and production area effects. For instance, Kamerbeek and Beijer (1964) used 35 C for 2 weeks followed by 40 C for 3 days for earliest flowering of Dutch-grown 'Wedgwood'. The second usage of the high temperatures is to retard the development of the terminal bud. In early studies, Beijer (1952) showed that a temperature of 25 C could be used for long-term storage. This was later revised to 30 C (Durieux and De Pagter, 1967). Kamerbeek (1962) has demonstrated the effects of various temperatures on respiration. He found that respiration of the bulbs was minimal at 30 C and temperatures above or below 30 C enhanced respiration. Thus, the 30 C temperature results in a conservation of reserve materials for long term storage. After the warm temperature treatment, an intermediate temperature of 17 C has been suggested. Research by Kamerbeek and Beijer (1964) has indicated, that a 2-week treatment at 17 C immediately after the low temperature treatment increased plant quality and decreased losses due to flower blasting. This was referred to as an "end treatment". When the bulbs are placed at low temperatures, the apical meristem is vegetative. The optimal temperature for floral initiation is 13 C for 'Wedgwood' (Hartsema and Luyten, 1955)and 9 C for 'Imperator' (Blaauw, 1941). The stages of organogenesis in Dutch iris have been described by Blaauw (1935), Beijer (1942), and Uhring (1973). The length of storage at low temperatures should not be less than 6 weeks and not more than 9 weeks. This low temperature treatment is best given as a precooling treatment since low temperatures applied after planting delayed floral development (Hartsema and Luyten, 1962). Greenhouse phase. -- After the completion of the low temperature with or without the end-treatment, the bulbs are planted either in ground beds or in flats in the greenhouse. Three important factors are involved in the greenhouse phase. They are: temperature, moisture, and light intensity (see review by Fortanier and Zevenbergen, 1973). Kamerbeek and Beijer (1964) have indicated that the day temperature is more critical than the night temperature. They found that optimal day temperature was C, while the night temperature could rise as high as 23 C. Fortanier and Zevenbergen (1973) suggested that moisture stress is an important factor in these types of temperature studies. The influence of light during the greenhouse phase is significant. Low light intensity can lead to flower blasting (Hartsema and Luyten, 1962). When low light intensities prevail, lower greenhouse temperatures or high intensity supplemental lighting will tend to counteract this effect. The effects of these interactions on the forcing of iris has been reviewed recently by Fortanier and Zevenbergen (1973). From these and other studies, it appears that, in contrast to the tulip, hyacinth and daffodil, photosynthetic energy is required for forcing Dutch iris. In this respect, the Dutch iris are similar to the Easter lily.

37 349 Special considerations. -- In addition to temperature and light, there are several diseases and insects which can affect Dutch iris (Gould, 1957). In general, most of these can be' controlled with present-day fungicides and insecticides. One aspect of iris forcing which has not yet been put into practice, but which deserves consideration, is the finding of Stuart et al. (1966) that ethylene can accelerate flowering of Dutch iris. These investigators exposed 'Wedgwood' to either 1, 5, or 10 ppm ethylene for 5 days prior to precooling at 10 C. They found that flowering was accelerated and more uniform. Also, leaf length was reduced. Uhring (1973) subsequently found that ethylene could accelerate floral initiation in 'Wedgwood'. It is known that temperature can influence ethylene production in other plants (Pratt and Goeschl, 1969). Also, ethylene can promote sprouting in some storage organs such as gladiolus (Denny, 1930) and potato (Burton, 1952). Perhaps the heat treatments used promote earlier flowering via an ethylene-induced mechanism. These positive effects of ethylene are in direct contrast to the negative effects observed for tulips and other bulbs (Hitchcock et al., 1933; De Munk, 1973a, b). A serious problem with forcing of Dutch iris is blindness. Stuart (1952) using 'Wedgwood' has shown this disorder to be associated not only with temperature treatments but also bulb size. He found that it is best to use 10 cm or larger bulbs for the earliest forcings. As the season progresses, smaller sizes can be utilized. He also noted that a precooling temperature of 4.4 C increased the percentage of blindness and that heat curing immediately after harvest tended to reduce blindness. Thus, it is apparent that blindness as well as flower blasting in Dutch iris is a very complicated phenomenon. ACKNOWLEDGEMENTS The author wishes to acknowledge the financial support provided by the American--Canadian group of the Dutch Bulb Exporters Association, Hillegom, The Netherlands and the Netherlands Flower-bulb Institute, New York, N.Y. Without their assistance, the Michigan State University bulb research program would never have become a reality. Also, the author wishes to thank his colleagues, Dr K.C. Sink, Dr A.N. Roberts, and Dr H.M. Cathey for their constructive remarks on the manuscript, and Mrs LuAnn Kent for her secretarial assistance. REFERENCES Abeles, F.B., Ethylene in Plant Biology. Academic Press, New York, N.Y., 302 pp. Algera, L., Topple disease of tulips. Phytopathol. Z., 62: Anon., Lilies as Cut-Flowers. Netherlands Flower-bulb Institute, Market Information Service, Hillegom, 22 pp. Bailey, L.H., Manual of Cultivated Plants, 2nd edition. MacMillan Co., New York, N.Y., 1116 pp.

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