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Using the biology of weeds to leverage weed management • Chuck Mohler • Cornell University Weeds are plants that thrive in disturbed environments • For example, in a farm field • Our crops are mostly annual plants - they live for one season • We kill off natural vegetation & disturb the soil to make conditions suitable for crops • But this also creates habitats for weeds Many ways to be a weed • Annuals – Summer annuals – Winter annuals • Perennials – Stationary perennials • Taproots • Fibrous roots – Wandering perennials • Bulbs & tubers • Rhizomes or storage roots Outline • Two general examples • Managing perennials through exhaustion of roots and rhizomes • Choosing an appropriate tillage regimen for depleting a weed seed bank • A specific example – your choice Wandering perennials • Spread by thickened storage roots or by rhizomes (underground stems) Apical dominance in perennials Tillage Shoot above ground Shoot below ground New rhizomes Old rhizome fragment Management of perennials • • • • • Key is exhaustion of reserves. Time shoot removal relative to growth stage Shallow roots & rhizomes – chop & bury, Deep roots & rhizomes – hit them often Competitive crops, frequently cultivated crops, short season crops Choosing a tillage regimen for managing a weed seed bank Seeds of most weeds are tiny – why? • Disturbed environments are risky • Tiny seeds spread the risk over many offspring • Seedlings can be small because in a recently disturbed environment they have little competition. • Seedlings have limited resources Small seeded species only emerge if near the soil surface Seed longevity Loss per year (%) Species Cultivated Uncultivated Lambsquarters 31 8 Annual bluegrass 26 22 Common chickweed 54 32 Common groundsel High 45 1.0 Seed survival (%) 0.8 Seeds survive better deep in the soil Velvetleaf 0.6 0.4 Pigweed 0.2 0.0 0 2 4 6 Depth in soil (inches) 8 10 Death near the soil surface • Seed predation • Wetting and drying • Freeze-thaw Plowing vs. minimum tillage? • Small seeded species with short lived seeds plow them under – Most will die before they find their way to surface again – Example: hairy galinsoga – Needs to be in the top ¼” to emerge – So if mixed into 8” of soil, the average return time will be 32 years – But few live longer than 2 or 3 years. • Large seeded species with long lived seeds keep them near the surface – Their mortality will be greater at the surface – And most that are tilled under will come back to bother you later – Example: velvetleaf – Emerges well from the top 2” of soil – So if mixed into 8” of soil, the average return time to the emergence zone is 4 years – 80-90% survival below 4” – 0.85x0.85x0.85x0.85=0.52 so 50+% will make it back into the safe-to-emerge zone before they die – <20% survival near the surface Species with small, long lived seeds? • Lambsquarters • Wild mustard Many other examples • Germination cues – Cultivated fallows – Mulches – Stale seedbed • Relative size of crop and weed seeds – Management of crop competition • Plant size distributions – Reduction of weed seed production “Manage Weeds on Your Farm: a Guide to Ecological Strategies” Mohler and DiTommaso, SAN • • • • • Ecology of weeds Cultural control methods Physical control methods Farm case studies Identification, ecology and management of the 75 worst agricultural weeds in the United States A specific example • Most farms have many weeds, but only one or two really problem weeds • Often need to focus on those • http://www.css.cornell.edu/weedeco • http://www.organic.cornell.edu/ocs/index.html