Weed Biology

1. Weed Biology

2. Weed Classifications • Life cycle • annuals • summer annuals • winter annuals • biennials • perennials • simple • creeping

3. Weed Reproduction • Two means • sexual • seed or spores • asexual • vegetative reproduction

4. Weed Reproduction • Sexual • many weeds cross-pollination or outcrossing not necessary • weeds are self-compatible • important weed characteristic

5. Weed Reproduction • Sexual • weeds colonize wide variety disturbed habitats • breeding system in relation to ability to colonize varied habitats is of interest • adaptive value of uniparental reproduction with occasional outcrossing - Baker’s Rule (1974)

6. Weed Reproduction • Sexual • many weeds use inbreeding • produce stable duplicate individuals adapted to local environment • occasional outcrossing for recombination to occupy new or changing microenvironments

8. Weed Reproduction • Asexual/vegetative reproduction • common weed characteristic • most common to perennials • not restricted to perennials • Definitions • ramet is a single unit of clonal growth • genet genetically distinct individuals

9. Weed Reproduction • Types vegetative reproduction: • stolons • above ground shoots grow along surface • produce adventitious roots at nodes • bermudagrass - perennial • large crabgrass - annual

11. Weed Reproduction • Types vegetative reproduction: • rhizomes • underground shoots • produce adventitious roots usually at nodes • johnsongrass - perennial • quackgrass - perennial • yellow nutsedge - perennial

13. Weed Reproduction • Types vegetative reproduction: • tubers • enlarged terminal portions of rhizomes • extensive storage tissue & axillary buds • yellow nutsedge - perennial • Jerusalem artichoke - perennial

15. Weed Reproduction • Types vegetative reproduction: • bulbs • underground modified buds • shoot & fleshy scale leaves • nutrient storage in leaves • wild garlic - perennial

17. Bulbous bluegrass bulbs

19. Weed Reproduction • Types vegetative reproduction: • roots • usually, long, extensive • horizontal sections produce adventitious shoots & new roots • often form buds • Canada thistle - perennial • Leafy spurge - perennial

21. Canada thistle 14 months growth

22. Weed Reproduction • Types vegetative reproduction: • shoots or stems • some spp produce adventitious roots & new shoots near branch tips • tamarisk species • sprout from base or stump • dandelion • locoweed

23. Tamarixparviflorashoots from broken Fragment of stem washed downstream

24. Weed Reproduction • Types vegetative reproduction: • fragmentation • spread/establishment from excised plant part • most underground tissues capable • also from leaves or foliar shoots • purslane - annual • field bindweed - perennial

26. Weed Dispersal • Dissemination/dispersal • scattering of propagules • reinfest site or transport new site • seeds most important mechanism • movement of vegetative propagules also occurs, but less important • human activities often involved

28. Weed Dispersal • Dissemination/dispersal • seeds dispersed in spaceandtime

29. Weed Dispersal • Seeds dispersed in space • adaptations to move away from parents • most fall close to parent • probability seedling mortality (competition) decreases w/distance • optimum distance seed/seedling numbers population establishment • creeping infestation occurs

30. Weed Dispersal • Seeds dispersed in space • wind, water, wildlife, humans • main dispersal vehicles or forces • weeds evolved appendages, coatings, palatability aid in dispersal • fruits or seeds

32. Weed Dispersal • Seeds dispersed in space • wildlife & humans • barbs, hooks • adhere hooves, hair, clothing • palatable fruits/seeds • eaten & excreted • harvested with crops • seed or hay crops • autos & other vehicles

35. Weed Dispersal • Seeds dispersed in space • wind • dust • spores of Equisetum • winged • toadflaxes • plumed • pappus of many Asteraceae • tumbling • diffuse knapweed, Russian thistle

39. Weed Dispersal • Seeds dispersed in space • water • irrigation water • 1920, CO, 81 weed spp., 156 ditches • 1980, NE, 77 weed spp • 2-5X more in canals than North Platte • 1975, WA • 125,000,000/ha non-irrigated • 500,000,000/ha irrigated 5 yr • rivers, streams, creeks • invasive weeds

41. Weed Dispersal • Seeds dispersed in time • dormancy • arrest in seed embryo development under suitable germination conditions • dormancy is dispersal in time

44. Weed Dispersal • Seeds dispersed in time • Harper 1977 • some seeds ‘born dormant’ (innate) • some ‘achieve dormancy’ (induced) • some ‘have it thrust upon them’ (enforced)

45. Weed Dispersal • Seeds dispersed in time • Classification of dormancy • primarydormancy: • innatedormancy is an arrested state of embryo at dispersal • incomplete embryo development • “afteripening” • chemical inhibitors fruit or seed coat

47. Weed Dispersal • Seeds dispersed in time • Classification of dormancy • secondarydormancy • enforceddormancy is the arrested state of embryo maintained in a limited environment • e.g. inadequate O2, temp., pH, moisture • seed will germinate when conditions corrected

48. Weed Dispersal • Seeds dispersed in time • Classification of dormancy • secondarydormancy • induced dormancy • seed shed from parent not dormant; • but limiting environment causes arrested embryo state that will persist even after germination conditions optimal • internal changes within embryo

50. Germination & Establishment • Germination • light requirement • long known seeds need light to germ • light flash mechanism may allow/assist weeds or pioneer plants to exploit disturbed environments