Weed life cycles

1. Weed life cycles

2. Types of reproduction in plants • Vegetative reproduction • Production of new plants from vegetative structures • Clones=daughter plants=ramets • Genetically identical to the parent plant • Sexual reproduction • Production of new plants from seeds • Allows for genetic mixing/diversity in offspring(genets)

3. Five life cycle categories for weeds • Summer annual • Germinate in spring and summer and mature before winter • Problems in spring and early summer planted crops, open sites, winter cereal crops, edges of natural areas • Lambsquarter, redroot pigweed, bedstraw, wild buckwheat, green foxtail, wild oat • Winter annual • Germinate in the late summer, fall, and winter (in warmer regions) • Mature in spring or early summer • Problems in late summer and fall planted crops, open sites, bare spots in pastures • In colder regions, winter annuals may be summer annuals • Shepherd’s purse, field pennycress, downy brome

4. Five life cycle categories for weeds • Biennial • Live for two growing seasons • Seeds germinate and plants grow vegetatively as rosettes the first year • Exposure to cold in winter causes the rosette to bolt in the spring • The biennial then flowers, matures its seed, and dies during the summer or fall of the second growing season • Usually a problem in perennial fields; not in fields that are disturbed in fall • Henbit, houndstongue • Perennial • Survive indefinitely (3+ years) • Flowering doesn’t trigger senescence • Underground structures continue to live

5. Five life cycle categories for weeds • Simple perennials • Start from seed, grow vegetatively • Form a crown of tissue at or below the soil surface on the upper end of a taproot • Root and crown survive indefinitely • Shoots periodically produce flowers and seeds • What is the purpose of seeds? • Found in perennial crops, undisturbed areas, no-till fields • Spotted knapweed • Creeping perennials • Reproduce from vegetative structures and seed • Vegetative reproductive structures are the major means of localized spread, competition, and survival • Creeping roots, rhizomes, stolons, bulbs, tubers • Have large amounts of stored food and numerous buds • Field bindweed, quackgrass, Canada thistle

6. Vegetative reproductive structures • Rhizomes • Elongated horizontal underground stems with nodes, internodes, and modified leaves • Root and new shoot growth always originates from buds at the nodes • Tubers • Thickened underground stems borne on rhizomes • Internodes are shorter than those on a rhizome • Root and new shoot growth always originates from buds at the nodes

7. Vegetative reproductive structures • Bulbs • Modified leaf tissue borne on a small stem plate • Roots and new bulbs develop from this stem plate • Stolons • Horizontal aboveground stems • Leaves produced at the nodes are green • Food storage not a major function

8. Vegetative reproductive structures • Creeping roots • No leaves, nodes, or internodes • Can grow downward and horizontally • Can produce shoots along their length • Tend to penetrate deeper than other vegetative reproductive structures and are harder to control

9. Shoot Regeneration in Weed Species

10. Shoot regeneration • All plants have a growing point from which they can regenerate shoots • Any practice that destroys the plant below the lowest growing point will kill the plant • The position of shoot regeneration on a plant and the resistance of a plant to destruction depends on the life cycle • Annuals • Broadleaves • The bud in the axil of the cotyledons in the lowest point from which an annual broadleaf can regenerate a new shoot • Any action that destroys the weed below that point should kill it • Grasses • The growing point, or crown, is just below the soil surface • Annual grasses need to be damaged under the soil surface for complete kill

11. Shoot regeneration • Biennials • Rosette stage difficult to control • As the seed stalk begins to elongate (bolt) the meristems raise aboveground • Simple Perennials • Crown is 1 to several inches below soil surface • Crown and taproot survive and generate new shoots even after substantial damage • How would you go about trying to control a simple perennial (mechanically?) • Creeping Perennials • Numerous well-protected buds capable of generating new shoots • Large amounts of stored food and many buds • Usually 6 in belowground, sometimes several feet • Severe disturbance usually helps them regenerate • Need repeated action to deplete the plant of energy reserves

12. Growing points

13. Questions! • Summer and winter annuals germinate in spring and summer and mature before winter. True or False? • _______________ weeds are most troublesome in perennial and no-till cropping systems. • Perennial • Annual • Biennial • Centennial • What stage of a biennial weed is most difficult to control? Why?

14. Field pennycress • Scientific name: Thlaspiarvense • Family: Brassicaceae • Life cycle: Annual • Where found: Cropland • Physical description: • Seed: fingerprint pattern • Cotyledon: oval with long petiole • True leaves round to spatulate, then oblong to oval • Forms a basal rosette at first, then alternate when flowering • Early leaves have long petioles; later stem leaves are sessile, clasping around stem • Leaves without hair, in contrast to shepherd’s purse • Growth habit: erect • Interesting facts: • Seed pods are disc-shaped with a distinct notch at tip • Distinct, garlic-like odor when brushed • Latin name describes flat pod • Thlas=to crush or flatten

15. Shepherd’s Purse • Scientific name: Capsella bursa-pastoris • Family: Brassicaceae • Life cycle: Annual • Where found: Cropland • Physical description: highly variable • Seed: small, sticky when wet • Cotyledon: ovate, apex may be indented • Growth habit: basal rosette; flowering stem alternate • True leaves oval, then elliptic to oblanceolate, most becoming pinnately divided • Stem leaves lanceolate • Early leaves have long petioles; later stem leaves are sessile, clasping around stem • Leaves are sparsely hairy • Interesting facts: • Seed pods heart-shaped, flattened • Latin name describes seed pods • caps=small box • bursa= purse, pastor=shepherd