Characteristics of Weeds Think dynamic with the ability to make serious changes The next few slides will detail some methods of classifying weeds, strategies for success and how/why weeds are weeds
Worst weeds grouped by plant family Family Gramineae Compositae Cyperaceae Amaranthaceae Cruciferae Leguminosae Convolvulaceae Euphorbiaceae Chenopodiaceae # Species 44 32 12 7 7 6 5 5 4 Adapted from Holm et al 1977; World s s Worst Weeds
World s s Worst Weeds by Ranking Weed purple nutsedge (Cyperus rotundus) bermudagrass (Cynodon dactylon)** barnyardgrass (Echinochloa crus-galli galli)** junglerice (Echinochloa colona)** goosegrass (Eleusine indica)** johnsongrass (Sorghum halepense)** cogongrass (Imperata( cylindrica)** water hyacinth (Eichhornia( crassipes) Ranking* 44 32 12 7 7 6 5 5 *Adapted from Holm et al 1977; World s s Worst Weeds ** notice how 6 of the top 8 are grasses
Classification of Weeds Of the 250,000 + known plant species, 1% (2,500) are considered weedy Weeds classified by: Life cycle Habitat Morphology Physiology These classification schemes will be outlined in the next few slides
Life Cycle - Annuals Annuals (germinate from seed, grow vegetatively and set seed in one growing season) In Florida we have two distinct types of annual weeds Summer annuals: : germinate in spring, grow vegetatively in summer, flower and set seed in late summer/early fall Winter annuals: : germinate in fall, grow vegetatively in winter, flower and set seed in late winter/early spring
Summer Annual - Sicklepod Cotyledons at seedling stage April/May Flowering and seed set August/September
Summer Annual - Crabgrass Cotyledons at seedling stage April/May Flowering and seed set June until frost
Winter Annual Annual bluegrass Seedhead formation occurs throughout the winter months in Florida
Common Chickweed Cooler temperatures initiate seed germination, grows throughout the winter months
Life Cycle - Biennials Biennials germinate from seed late in one season, grow vegetatively, overwinter (resting period), and flower/set seed the next spring or summer Some biennials will behave like winter annuals in Florida
Curly Dock Germinates from seed, forms rosette of leaves during the late summer and winter months (above left). Long days in the spring trigger stem elongation and flower production occurs on the elongated stem (above right).
Life Cycle - Perennials Perennials persist over multiple growing seasons through vegetative structures Simple perennials resprout from crown/taproot Creeping perennials reproduce from specialized structures Woody perennials reproduce from woody tissues
Simple Perennial - Dandelion
Creeping Perennial bermudagrass Notice the tough underground rhizomes
Creeping Perennial purple nutsedge Notice the rhizomes and strings of tubers in chains
Woody Perennial Smilax (Greenbrier) Underground corms of smilax. Notice how several corms do not possess an aboveground sprout. This is critical to long- term survival.
Woody Perennial melaleuca
Classifying Weeds by Habitat Could be anything Weeds of corn, weeds of agronomic crops Residential weeds, forestry weeds Aquatic vs. terrestrial weeds Disturbed vs. non-disturbed environments Environmental Protection Agency (EPA) considers site and/or crop to be the defining factors in pesticide registration
Classifying Weeds by Morphology This method uses the physical characteristics of the plant to place into separate groups Dicots vs. monocots Grasses vs. broadleaves Erect growing vs. prostrate Vining,, creeping, bunch type Large seeded vs. small seeded This is the most commonly used method this will be used to study weeds later in lab
Classifying Weeds by Physiology This method is rarely used and distinguishes weeds by the type of photosynthesis C3 vs. C4 photosynthetic mechanisms CAM photosynthesis http://www.colorado.edu/eeb/courses/4140bo wman/lectures/4140-06.html 06.html Provides a good overview of these mechanisms
Why are weeds successful? Strategies (begin and end with the seed) 1. Seed germination over a wide range of environmental conditions 2. Discontinuous germination (dormancy)* 3. Rapid growth rate 4. Tremendous seed/propagule production 5. Self compatible, but able to outcross 6. Short and long distance dispersal 7. Long-term survivability, persistence * Dormancy will be covered in forthcoming slides on weed biology
Genetics Inherent Variability From an ecological standpoint, weeds are primary invaders into an open niche Considered general purpose genotypes, plasticity allows for maximum adaptation (changes in growth in response to changes in environmental conditions) Most crops were once weeds, over the years have an artificially streamlined genetic base (through human selection)
Ecological Survival Strategies r-strategy large number of offspring with high dispersion potential 1. small-seeded seeded annuals such as crabgrass, pigweeds, chamberbitter,, oxalis 2. Pioneering species that get into an area and exploit it as much as possible 3. Generally self compatible (able to self cross) 4. Divert tremendous amount of resources into reproductive development
Ecological Survival Strategies k-strategy strategy annuals larger seeded species* Lower dispersal rate but still a strong ability to exploit an area Divert an equal amount of resources into reproductive development, but fewer, larger seeds Examples include: morningglory,, cocklebur, sicklepod Most crop plants fit in this category corn, soybean, peanut, tomato * Becoming the problem weeds over the last 10-20 years, more difficult to control
Ecological Survival Strategies k-strategy perennials 1. Maintain high levels of sexual and vegetative reproduction 2. Divert more resources into vegetative propagules than seeds 3. Vegetative ability allows for maintained persistence in a given area 4. Examples include hydrilla, nutsedge, bermudagrass, torpedograss
Ecological Survival Strategies k-strategy woody perennials 1. High degree of resources diverted to woody and other persistence type structures 2. Slow growth, but generally not out-competed 3. A reduced degree of sexual reproduction 4. Generally considered climax species
Ecological Survival Strategies The strategies discussed in the previous slides are ecological models of how certain species fit into an overall ecosystem. Generally in an open niche, r-strategy r plants colonize first, followed by k-strategy k annuals, perennials and finally woody perennial trees that form climax vegetation. The type of agriculture production or native ecosystem will dictate the types of weedy species that will be a problem. In annual row crop production (such as corn, soybean), r-annual r weeds are problem; as minimum tillage increased, a shift in weeds to k-k annuals was observed, accompanied by an increase in k-perennials. In natural areas, woody perennials are often the major problem (melaleuca( melaleuca,, climbing fern).
Natural Dissemination* Wind light weight, pappus,, winged Flowing water almost any seed, corky, irrigation water, aquatic weeds Animals (wildlife) hooks, barbs, mucilaginous, excretion (feces) Forceful dehiscence some legumes, oxalis *most of the spread is via the seed and this changes as a function of seed type
Artificial* Dissemination Planting contaminated crop seed, primary means of spread and reason for certification Livestock manure, tropical soda apple spread Equipment mowers, harvesters, hay Humans pant cuffs, shoes, clothing Ballasts of ships, packing crates, nursery, water craft, trailers Introduction as a potential crop or ornamental * Spread via humans, long range spread is mainly through human movement and activities
Concepts to Understand The major weed problems worldwide with most being grasses How weeds are classified Strategies weeds have to be successful, including genetics Ecological aspects of weeds Dissemination characteristics