Freeze and Frst Prtectin with Aqueus Fam Field Experiments Christpher Y. Chi 1 and Gene Giacmelli 2 ADDITIONAL INDEX WORDS. freeze and frst damage, plant cld prtectin, radiatin shield, bidegradable aqueus fam, and mulching. SUMMARY. Newly frmulated aqueus fam was tested in the field. The fam demnstrated the lngevity necessary fr practical field use. Sil temperatures beneath an insulatin layer f aqueus fam were measured t determine the effectiveness f fam as sil mulch. Leaf temperature within a canpy was mnitred t bserve the mdificatin f plant leaf temperature, and t evaluate the phyttxic effects f fam applied directly t the leaf canpy. Leaves were nt damaged after being cvered with the fam fr tw weeks. The fam-prtected sil was effectively insulated, and the aqueus fam prved t be an effective radiatin shield against the cld night sky. Temperature differences as high as 5 C (9 F) were measured between the fam-cvered and uncvered cpper metal plates, which were used t simulate plant leaves. The fam cvered plates were 80% as effective as the aluminum fil cvered plates in reducing radiatin heat transfer. The develpment and evaluatin f the gelatin-sugarbased aqueus fam was presented by Chi et al. (1999). Many pssible applicatins f this lng-lasting fam may be fund if its effectiveness fr cld prtectin is prven in the field. The fam can be used as sil mulch in a NJAES Paper N. D-03232-18-99. Supprted by state funds. The cst f publishing this paper was defrayed in part by the payment f page charges. Under pstal regulatins, this paper therefre must be hereby marked advertisement slely t indicate this fact. 1 Assistant prfessr, Department f Agricultural and Bisystems Engineering, University f Arizna, Tucsn, Ariz. 2 Prfessr, Department f Biresurce Engineering, Ck Cllege, Rutgers University, New Brunswick, N.J. greenhuse r plastic tunnel, as an insulatr fr tree trunks, r as a cver fr frst prtectin. Prtectin frm unsuitable air and leaf tissue temperatures is a majr prblem in the spring and fall f each grwing seasn. In particular, ptential lsses f plant prductin fllws seedling transplant fr early seasn grwth, even when utilizing plasticulture techniques f mulching the sil with plastic film, r emplying rw cvers. Plasticulture techniques include the use f film mulches t cver the sil surface and t warm the plant rt zne, r tunnels frmed f transparent plastic film cnstructed abve each rw f highly valued crps thrughut large fields, in an attempt t mdify the aerial envirnment f the plant. Increased air temperature, humidity, and wind prtectin can be achieved with film-cvered tunnels. The nursery industry uses white plastic film cvered tunnel greenhuses t prtect its nursery stck frm harsh winter cnditins, by preventing large fluctuatins f diurnal air temperatures. In this way, premature cld seasn grwth can be minimized, and damage t the plants reduced (Jhnsn and Rberts, 1983). The lng-lasting aqueus fam has the ptential t act as a superir mulching material when applied directly ver the sil. An experiment was cnducted (fr freeze frst prtectin) t measure sil temperatures and determine the effectiveness f fam as a sil mulch (Chi et al., 1999). In additin, tw trays f lettuce seedlings were mnitred t bserve the mdificatin f plant leaf temperature resulting frm an applicatin f fam. The remval by water spray r rain and ultimate fate f the fam was als discussed. In a review f frst-freeze meterlgy, Perry (1998) nted that fams have successfully averted plant damage, but they culd nt be remved easily. A subsequent experiment (fr frst prtectin) was planned t evaluate the effectiveness f the fam against climatic cnditins that cause frst. Cpper plate surfaces were painted black and instrumented t measure surface temperatures bth with and withut radiatin shields. Three cpper surfaces were established either with a thin fam layer, a thick fam layer, an aluminum fil cver, r unprtected (Fig. 1a). The radiatin shields were supprted slightly abve the cpper surface n wire mesh, in rder t simulate prtected and unprtected freestanding tree canpies, as well as lw prfile trees and plants. The aluminum fil replicate was designed t eliminate radiative heat exchange between the cpper surface and the cld sky, and thus prvide a relative cmparisn fr the ther three treatments. Experimental design Outdr quantitative evaluatins In this paper, tw experiments will be reprted that were cmpleted during January and February 1998 at the Campus Agricultural Research Center f University f Arizna, Tucsn. The first, ne determined the prtectin prvided by fam against freeze and frst, and the secnd evaluated frst prtectin alne. Freeze frst prtectin A plyethylene plastic film-cvered, hp-supprted rw tunnel [1 2 0.8 m (3.3 6.6 2.6 ft, length width height] was installed abve a side-byside test cnsisting f fam applied t the grund as a sil mulch and fam applied t a tray f lettuce plants as a plant insulatr (Chi et al., 1999). The tunnel prvided prtectin frm rainfall, but nt frm radiative heat lss; i.e., lngwave transmissin thrugh the plyethylene was 80% fr a single, nninfrared barrier film (Giacmelli and Rberts, 1993). Thus, radiative heat lss at night culd be significant under the clear, winter desert sky. The sil was tilled and raked smth, and thermcuples were installed with tw replicatins each n three sil treatment areas [30 60 cm (11.8 23.6 inch)]. The treatments included tw depths f applied fam ver sil thin fam [2.5 cm (1.0 inch)], thick fam [5 cm (2.0 inch)] and bare grund as a cntrl. At each f these six lcatins, three thermcuples were lcated t measure the temperature f the sil near the surface [0.25 cm (0.1 inch)] and at a depth f 2.5 cm (1 inch) (see Chi et al., 1999, fr detail lcatins). The air temperature within the tunnel was measured at 12 cm (4.4 inch) abve the bare sil. All thermcuple junctins in the air r within the fam layers were fitted with radiatin shields. Tw trays [30 60 cm (11.8 23.6 inch)] f lettuce (Lactuca sativa L. Black Seeded Simpsn ), cntaining 15 seedlings each, were placed side-byside within the tunnel. At the time f field testing, a clsed plant canpy ex-
isted within the lettuce tray. Thermcuple junctins were attached t selected leaves by inserting the thermcuple within the midrib f the adaxial face f the leaf, abut ne-third the leaf distance up frm the rt zne, and well within the plant canpy. Fam was applied t cver ne entire tray t a depth f 2 cm (0.8 inch). This test began n 29 Jan. 1998, and was terminated n 12 Feb. 1998. Lettuce was chsen fr this study because it grws quickly and can tlerate the cld night air and leaf temperatures cmmn during desert winter nights. The lettuce was germinated and seeded (5 Dec. 1997), and transplanted int 25 mm (1 inch) fam cubes (10 Dec. 1997). The plants were grwn in a greenhuse at 22/ 17 C (72/63 F) day/night air temperatures and natural phtperid cnditins, within a tray fld irrigated with nutrient slutin t a depth f 2 cm (0.8 inch) at intervals f 4 d. Frst prtectin The plastic film tunnel and the lettuce were remved fr this experiment, and the fam layers were expsed fully t daytime slar radiatin and the cld night sky. Fur sets f cpper plates and radiatin shields n insulatrs were placed n the sil (see Fig. 1a), and tw f them were cvered by 4 cm (1.6 inch) and 2 cm (0.8 inch) thick fam layers placed n the screens. A 1.6 mm (0.06 inch) thick, 10 10 cm (4 4 inch) cpper plate with its upper and lwer surfaces painted black was attached t a 25 25 2 cm (1 1 0.8 inch) insulatr. The insulatr and the cpper plate were separated by a radiatin shield (several layers f aluminum fil) t minimize radiative heat exchange and t create an air pcket fr the reductin f cnductive heat transfer. A 30 30 cm (12 12 inch) metal screen [mesh size: 1.25 1.25 cm, (0.5 0.5 inch)] was attached 3 cm (1.2 inch) abve each f the fur cpper plate assemblies. The metal screens were cvered with either 2 cm (0.8 inch, thin fam), 4 cm (1.6 inch, thick fam), r aluminum fil. One screen remained unprtected t serve as a cntrl. Five thermcuples were lcated at the bttm surface f each cpper plate, and their specific lcatins are shwn in Fig. 1b. During the test perid, the nearby Arizna Meterlgical Netwrk weather statin recrded min/max daily air temperatures as 0 and 26 C (32 and 79 F), respectively. Daily average wind speed ranged frm 1.7 t 4.1 m s 1 (5.6 t 13.5 ft/s), and the highest wind speed recrded was 15 m s 1 (49.2 ft/s). Temperatures f the cpper plates were recrded n clear nights t bserve the effectiveness f the fam insulatin t prevent sky radiatin lss. There were 6 rainy days during the perid, and the fam-cvered screens were remved and sheltered. Thus, the fam layers were expsed t the dry utdr envirnment fr 8 d, with nly a minr bserved reductin in fam thickness ( 10%), while lateral (length and width) shrinkage was nt evident. Its size and resilience were abut the same as thse f the fam bserved in the labratry (Chi et al., 1999). Data acquisitin Air, sil and fam temperatures were measured with 24 gage type-t thermcuples. A thermister in each terminal blck (Natinal Instrument SCXI 1100 multiplexer, SCXI 1303 isthermal terminal blck, and AT- MIO-16H-9 multifunctin bard, Austin, Texas) was used as a reference cld junctin temperature, as presented by Chi et al. (1999). All the thermcuple junctins that measured air and fam temperatures were shielded frm Fig. 1. Schematic f field experiments fr frst prtectin. (a) Overall experimental setup. (b) Crss-sectinal view and thermcuple lcatins.
Fig. 2. Air temperature, relative humidity, and wind speed data during the freeze frst experiments. F = 1.8 ( C) + 32. Fig. 3. Sil temperature at 2.5 cm (1 inch) deep; F = 1.8 ( C)+32. Fig. 4. Temperature differences ( T) between prtected and unprtected sils and leaves during the early mrning hurs (0300 t 0600 HR); F = 1.8 ( C) + 32. radiatin. Each experiment was preceded by a calibratin test f the data acquisitin system, with particular emphasis n the thermcuple sensrs. Based n data frm a series f calibratin tests, the accuracy was determined t be ±0.15 C (±0.27 F), and repeatability was ±0.05 C (±0.09 F). The cntacts between the thermcuple junctins and the cpper plates were inspected befre the experiment. T verify the cnsistency f experimental setup and the accuracy f thermcuple readings, a preliminary experiment was carried ut withut radiatin shields (i.e., fam r aluminum fil cvers) during a calm and clear night. Thrughut the test perid, the timeaveraged temperature readings f each thermcuple remained within ±0.15 C (±0.27 F) f the average temperature f all fur cpper plates. The changes in air temperature (inside and utside f the fam layers), sil temperature, and leaf temperature were simultaneusly mnitred in graphic frm n the cmputer screen. Using the data acquisitin system, the average values determined frm 500 scans/s were autmatically stred in a designated cmputer file every 8 min fr later data analysis. Results and discussin During the 2-week test perid, the treatments were cntinuusly mnitred, and fur sets f data, including thse registered n 29 Jan. 2, 5 and 9 Feb. 1998, were used fr analysis, because they typified the pattern f winter weather in Arizna. Daily weather data during the perid are available n an fficial Internet site (University f Arizna, 1999). An unusual amunt f rainfall was recrded in Arizna due t the El Nin effect during the 1997 98 winter, and a ttal f 51 mm (2 inch) f rainfall was recrded during this experiment. As shwn in Fig. 2 (the data cllected frm the nearby Arizna Meterlgical Netwrk weather statin), the wind was usually calm thrugh the night until early mrning. It was partly cludy r cludy frm the night f 29 Jan. until the mid-mrning f 30 Jan. The early mrning air temperature drpped near the freezing pint n the cld, clear mrning f 31 Jan. 1998. The daily minimum relative humidity ranged frm 10% t 30%. FREEZE FROST PROTECTION. Figure 3 presents the temperature fluctuatin f air and sil at a 2.5 cm (1 inch) depth. Each
Fig. 5. Temperature f the midribs f fam-cvered and uncvered lettuce leaves. F = 1.8 ( C) + 32. temperature reading represents the average value frm tw lcatins. The daily fluctuatin pattern was nearly the same as the data cllected at the weather statin, and the temperature readings were 5 t 10 C (9 t 18 F) higher during the day than the weather statin data due t a mderate greenhuse effect caused by the plastic film tunnel. The sil temperature prtected by the thick fam maintained the highest temperature at night, and the thin fam maintained the highest temperature during the day. During the cld mrning (frm 0300 t 0700 HR) f 31 Jan. 1998, the sil temperature difference ( T) between the thick-fam prtected sil and the unprtected sil was 5 C (9 F), and the difference between the thick fam treatment and the air was nearly 10 C (18 F), as shwn in Figs. 3 and 4. The thin fam prvided a reduced insulatin effect during the same perid, with abut a 3 C (5.4 F) temperature difference. On the relatively mild mrning f 30 Jan., the effectiveness f the fam as measured by the temperature difference was reduced. On 1 Feb. 1998, the temperature difference between the fam-prtected sil and the unprtected sil gradually increased (Fig. 4) as the air temperature drpped early in the mrning (Fig. 3). The differences were quite cnsistent during cld and clear nights. The daytime sil temperatures under the thin fam were cnsistently higher than under the thick fam, reaching values between 8 t 9 C (14 t 16 F) greater (Fig. 3). The shrtwave slar radiatin apparently penetrated the thin fam and was absrbed at the sil surface, while the fam acted as a gd insulatr against cnvective heat lss. Hwever, the stred thermal energy was quickly lst after sunset. Radiative lss frm the sil surface t the cld clear sky culd have cntributed t the energy lss. The accumulatin f thermal energy during the day, therefre, did nt significantly alter the sil temperature during the fllwing evening. Temperature f the midribs f famcvered and uncvered lettuce leaves is shwn in Fig. 5. During the cludy and warm mrning f 30 Jan. 1998, the effect f the fam was nt significant. The temperature difference between the prtected and unprtected leaves was negligible (see Fig. 4) due t the clud cver. On 31 Jan. 1998, the temperature differences fr the leaf cvered with fam were 3 C (5.4 F) n average and as high as 4 C (7.2 F) greater than the uncvered leaf. Figure 5 indicates that the leaf temperatures f the prtected midrib were higher than the air temperature, because the fam layer prtected the lettuce leaves frm radiative lss t the cld and clear sky during the mrning hurs f 31 Jan. and 1 Feb. 1998. Such a trend was nt evident n the cludy mrning f 30 Jan. 1998. FROST PROTECTION. The frst prtec- Fig. 6. Air temperature and wind speed data during the frst experiments. F = 1.8 ( C) + 32; 1 m s 1 = 3.3 ft/s; (a) 21 Feb.; (b) 25 Feb.
Fig. 7. Temperature f prtected and unprtected cpper plates. F = 1.8 ( C)+32; (a) 21 Feb.; (b) 25 Feb. tin experiments were cnducted during 6 nights (12, 14, 18, 21, 23, and 25 Feb. 1998), as determined by the apprpriate weather cnditins (wind, temperature, and clud cver) frm the nearby weather statin. The weather cnditins f 21 and 25 Feb. were particularly favrable t bserve the effectiveness f fam t prevent frst. On 21 Feb., there was little wind [generally under 1 m s 1 (3.3 ft/s) in the mrning] and the average air temperature between 0600 t 0700 HR. was 2 C (35.6 F), as presented in Fig. 6a. Only a thin clud cver remained thrugh the evening until the early mrning. On 25 t 26 Feb. (see Fig. 6b), the average wind speed was ver 1 m s 1 (3.3 ft/s) frm 0400 t 0600 HR, while the minimum air temperature drpped nearly t the freezing pint, and the sky was almst clear. Temperatures f the cpper plates and the air shwn in Fig. 7a and b demnstrate that the cpper plates prtected by thick r thin fam layers maintained cnsistently higher temperatures than the unprtected plate. Therefre, it is bvius that the aqueus fam culd be used as a radiatin shield against the cld night sky. The temperature differences were as high as 5 C (9 F), as shwn in Fig. 8a and b. As expected, aluminum fil minimized the radiatin lss, and the plate prtected by the fil maintained the highest temperature fr bth nights. The plate prtected by thick fam maintained a slightly higher temperature than the plate prtected by thin fam. It shuld be nted that the fam was prepared n 12 Feb., and the water cntent must have reached the minimum asymptte befre the experiment, as described by Chi et al. (1999). Nevertheless, bth thick and thin fam acted as gd radiatin shields. In Fig. 7b, the unprtected (n fam cvering) plate exhibited significant temperature fluctuatins during the early mrning perid (0300 t 0700 HR) in cmparisn with the data in Fig. 7a. In particular, the temperature increased significantly [mre than 3 C (5.4 F)] at 0630 HR, while the temperature f the ther plates remained nearly steady. These fluctuatins were prbably due t cntinuusly changing wind and clud cnditins. Clud cver changes wuld mre directly affect the unprtected plate, which was fully expsed t lngwave radiatin lsses and thus mre sensitive t the changing envirnment, than the ther plates that were prtected either by a fam layer r by aluminum fil. If there were a gust f wind, the unprtected plate shuld have readily gained thermal energy by cnvectin. On the ther hand, the cnvective heat transfer n the prtected (fam cvered) cpper surface must have been negligible, because the plate temperatures were nearly equal t the air temperature. It shuld be nted that the temperature readings f prtected plates were generally higher than the air temperature, prbably a result f heat transfer frm the insulatr under each plate, althugh bth cnductive and radiative energy lsses were minimized by several layers f aluminum fil between each plate and the insulatin. In an effrt t quantify the effectiveness f the fam as an insulatr, the fllwing equatin was develped, η = T Fam prtected /T A1 prtected T Unprtected, where η and T represent the effectiveness f the fam and the temperature f the varius cpper plates, respectively. In the abve equatin, the temperature difference, T Al prtected T Unprtected, represents the maximum ptential difference f temperatures due t radiatin heat lss fr the given weather cnditins and is used as a reference base. The effectiveness (η) ranges frm 66% t 80% fr the thin fam and frm 74% t 86% fr the thick fam, as shwn in Table 1, when the temperature f the unprtected plate surface drps under the freezing pint (regins A E in Fig. 7a and b). Overall, the effectiveness f the thick fam was nly 6% t 8% higher than that f thin fam. The average effectiveness f the fam n 26 Feb. decreased 5% cmpared with results n 22 Feb., due, in part, t the degradatin f the fam.
the thin and thick fam cvers, respectively. When cmpared t air temperature, these values increased t 10 and 8 C (18.0 and 16.2 F), respectively. Radiatin heat lsses frm the plant leaf canpy were reduced n clear nights by an applicatin f fam directly t the leaf. Temperatures were frm 3 t 4 C (5.4 and 7.2 F) higher than the air temperature. Leaves were nt damaged after being cvered with the fam fr 14 d. Thermcuples n the cpper plates effectively measured radiatin heat lsses under clear sky and clud cvered cnditins. The aqueus fam prved t be an effective radiatin shield against the cld night sky, and temperature differences as high as 5 C (9 F) were measured between the cvered and uncvered plates. The fam cvered plates were 80% as effective as the aluminum fil cvered plates in reducing radiatin heat transfer. The fam maintained its integrity under dry weather cnditins fr a ttal f 8 d and nights f expsure. In cnclusin, the gelatin-based, aqueus fam can be used as bidegradable sil mulch. The fam can ptentially prtect the aerial prtin f bth lw and high prfile crps frm damage by belw freezing air temperature, and frm frst caused by radiatin heat lss under clear sky night cnditins. The fam can easily adhere t plant canpies, branches and trunks f freestanding tall trees, and unless washed away by water spray, the fam structure will be maintained in typical utdr envirnments. Fig. 8. Temperature differences ( T) f prtected and unprtected cpper plates. F = 1.8 ( C) + 32; (a) 21 Feb.; (b) 25 Feb. Table 1. Effectiveness (η) f fam as insulatr fr specific time perids n 21 and 25 Feb. T Al, T Thin fam, and T Thick fam dente the temperature readings f cpper plates prtected by aluminum (Al) fil, thin fam, and thick fam, respectively. Regin in Figs. 7a and b where T Unprtected < 0 C (32 F) z η Thin fam y η Thick fam A 78.0% 84.0% B 80.2% 85.7% C 67.7% 75.4% D 66.2% 74.2% E 75.1% 81.8% z η Thin fam = (T Thin fam )/(T Al ) y η Thick fam = (T Thick fam )/(T Al ) Cnclusins The fam prtected sil was effectively insulated and generally exhibited temperatures that were 3 and 5 C (5.4 and 9 F) abve that f the bare sil fr LITERATURE CITED Chi, C.Y., W. Zimmt, and G.A. Giacmelli. 1999. Freeze and frst prtectin with aqueus fam Fam develpment. HrtTechnlgy 9(4):654 661. Giacmelli, G.A. and W.J. Rberts. 1993. Greenhuse glazing systems. HrtTechnlgy 3(1):50 58. Jhnsn, J. and W.J. Rberts. 1983. Overwintering structures using well water fr minimum heating, p. 129 137 In: O.S. Wells (ed.) Prc. 17th Natl. Agr. Plastics Cngr. Amer. Sc. Plasticult. Perry, K.B. 1998. Basics f frst and freeze prtectin fr hrticultural crps. Hrt- Technlgy 8(1):10 15. University f Arizna. 1999. AZMET, The Arizna Meterlgical Netwrk. Ariz. Cp. Ext., Univ. Ariz. 18 July 1999. <http:/ /ag.arizna.edu/azmet/>