Classification of weeds

1. Classification of weeds

2. Weed classification groups • Habitat • Life Cycle • Taxonomic Group • Physiology

3. Habitat • Terrestrial • Live on land • Crops, rangeland, turf, forests, etc. • Aquatic • Structurally and physiologically modified to live on, near or around water • Aerial • Not rooted in the soil an use other plants for support • Parasitic: Mistletoe, dodder • Epiphytic: Orchids, Spanish moss

4. Life Cycle • Herbaceous plants • Plants with non-woody aboveground stems • Annuals • Plants that live for one growing season only • Re-establish from seed • Biennials • Plants that require parts of two growing seasons to complete their life cycle • Reproduce from seed • Perennials • Plants that live indefinitely • In addition to seed, some of these plants produce other structures that allow them to overwinter and/or reproduce • Modified stems (stolons, rhizomes,corm, bulb, tuber), leaves (part of a bulb) or roots (tuber)

5. Life Cycle • Woody plants • Plants with woody aboveground stems that persist from year to year • Trees • Woody perennials with a single main stem or trunk • Shrubs • Woody perennials with more than one principal stem arising from the ground

6. Taxonomic Group • Dicots • Plants whose seedlings produce two cotyledons (seed leaves) • Typified by: • Net leaf venation • Flower parts in multiples of 4 or 5 • Called “broadleaves” • Examples? • Monocots • Plants whose seedlings bear only one cotyledon • Typified by: • Parallel leaf venation • Flower parts in multiples of 3 • Called “grasses” or “grass-like plants”

7. Physiology • Photosynthesis is the process by which plants convert CO2 to glucose (carbon fixation) • Plants can vary in how they convert CO2 to glucose (carbon fixation) • C3 plants • First stable product of the Calvin Cycle is a 3 carbon acid • C4 plants • First stable photosynthetic product is a 4 carbon acid • More efficient at photosynthesis under high temperatures • Many weeds exhibit this photosynthetic pathway

8. How weeds interfere with desirable plants

9. Interference • Not all interference is negative • Weeds interfere with growth of desirable plants via three mechanisms: • Competition • Allelopathy • Parasitism • Mistletoe, dodder, broomrape, witchweed

10. Competition • The ability of weeds to compete for light, water and nutrients depends on three main factors: • Timing of weed emergence • The first plant to emerge has a distinct competitive advantage • Why? • Can access resources • Can negatively affect the other plants • Critical weed-free period concept

11. Competition • Growth form • Growth habit and rate of development • Which are more competitive, broadleaves or grasses? • Stem height, leaf area, branching, leaf angle • Root development • Weed density • Increased weed density = decreased crop yield • Are there exceptions to the rule??

12. Allelopathy • Allelopathyrefers to the chemical inhibition of one species by another • The "inhibitory" chemical is released into the environment where it affects the development and growth of neighboring plants • Allelopathic chemicals can be present in any part of the plant • May inhibit shoot/root growth • May inhibit nutrient uptake • May attack a naturally occurring symbiotic relationship thereby destroying the plant's usable source of a nutrient • Has proven difficult to study • The case of spotted knapweed and catechin • 2003 study showed that the plant produces catechin • Found that native grasses were more susceptible to catechinthan European grasses • 2005 study unable to detect catechin in soils infested with spotted knapweed

13. How weeds persist

14. Weed seeds • Large numbers • Weeds can produce thousands of seeds per plant • Small seeded species are usually more prolific Adapted from: Ross, M. A., & C.A. Lembi. Applied weed science. 3rd Edition. 2009.

15. Weed seeds • Can survive and remain viable under adverse conditions • Well-protected by seed coats resistant to breakdown • Resist freezing, drought, fire, animal predation • Inactive seeds cannot be controlled by herbicides • Have adaptations that help with dispersal • Hooks and spines adhere to animals • Floating on water, carried on wind • Are of similar size and shape to crop seeds so they cannot be removed

16. Seed banks • Weed seeds build up in the soil over time • One study • 7 million weed seedlings/ac = 280 million seeds per acre-furrow slice (6”deep) • Weed seed bank dynamics dependent on • Number of seed present • Species composition • Depth of distribution • Seed viability • Seeds can remain viable for years Adapted from: Ross, M. A., & C.A. Lembi. Applied weed science. 3rd Edition. 2009.

17. Seed banks • Dormancy is a temporary condition where viable seeds fail to germinate even when environmental conditions are adequate • Seed dormancy ensures continued germination for many years • Primary dormancy • Seeds are unable to germinate after they first mature • Secondary dormancy • Induced over a period of time by environmental conditions • Quiescence • A period of inactivity cause by the lack of a necessary factor (e.g., oxygen, water, light) • Afterripening • Transition from dormancy to a more readily germinable state • Stratification (exposure to cooler temperatures for a period of time) • Scarification (breaking of seed coat) • Light-regulated

18. Vegetative Reproductive Structures • Major food storage organs • Possess numerous buds capable of starting new plants • Rhizomes, tubers, bulbs, stolons and creeping roots • Serve as overwintering mechanisms • Serve as a second means of propagation • Allow plants to grow more quickly than those from seed • Help with survival after disruption • ½ inch piece of Canada thistle can start a new plant

19. Questions! • Dormancy is a temporary condition where viable seeds fail to germinate even when environmental conditions are adequate. True or False? • Biennial weeds complete their life cycle in ____________ year(s)? • Two • One • Every other • More than two • Which are more competitive, broadleaf or grass weeds? Why?

20. Wild buckwheat • Scientific name: Fallopia convolvulus • Family: Polygonaceae • Life cycle: Annual • Where found: Cropland • Physical description: • Seed: black, three-angled • Cotyledon: linear to oblong • True leaves arrowhead to heart shaped • Alternate arrangement • Growth habit: Prostrate/climbing • Leaves possess small stiff hairs making it rough to the touch • Interesting facts: • Latin name describes resemblance to bindweed • convolvulus=refers to species associated with Convolvulaceae family (bindweeds), meaning “to intertwine”

21. Bedstraw aka cleavers, goosegrass • Scientific name: Galiumaparine • Family: Rubiaceae • Life cycle: Annual • Where found: Range/Cropland • Physical description: • Seed: ball-shaped with stiff, hooked bristles • Cotyledon: ovate to round, apex indented • True leaves linear with barb at the tip • Whorled arrangement, 4-5 leaves at first nodes, then 6-8 • Leaf surface has short stiff hairs that help it cling to other plants • Interesting facts: • Square stems • Latin name describes low, dense growth habit • Galium= milk, aparine= seizer • Rumored to have once been used as a strainer in cheese making