Weed Control Practices

Annual weeds typically germinate, grow and develop to maturity and seed production quickly, and therefore can offer significant competition with crop plants for light, nutrients, and water. They can also produce seeds and increase weed population pressure relatively quickly. Therefore, annual weeds should be terminated early and especially prior to they produce seeds. Because annual weed species do not allocate significant resources to below ground storage organs, they can be terminated with mechanical or physical control tactics such as: plowing, cultivation, hoeing, removal by hand, hay making, mowing or grazing, soil mulching, and flaming. Allowing weed seeds to germinate and then terminating them with light tillage (stale seedbed) strategically uses tillage for weed control. Annual weed germination and establishment can be suppressed with cultural control practices such as crop rotation, rotating crops with different seasonal life cycles, successive planting (double or triple-cropping); the integration of cover crops; and managing for competitive crops with early crop planting and good crop management practices (competitive crop varieties, soil fertility and health management).

Chemical control with herbicides applied at the recommended time, and rotating or varying herbicide chemistry can reduce the evolution of herbicide-resistant weeds. Biological control can also reduce weed populations and may include conserving habitat for weed seed predators such beetles, small rodents and birds by integrating cover and perennial crops on a farm, and avoiding pesticides that can reduce weed seed predatory populations. In addition, in some cases, browsing or grazing animals or specific pathogens of weeds such as bacteria or fungi are sometimes introduced.

Perennial weeds have below ground storage organs that they can regrow from (ex. tubers, rhizomes, stolons, and bulbs); therefore mechanical control strategies such as tillage, cultivation and hoeing can break-up and distribute perennial weed storage organs, facilitating the spread of perennial weeds. Mechanical weed control tactics can be effective if they can bring the majority of the storage organ to the soil surface to desiccate or freeze, thaw, desiccate and die. Perennials typically begin storing reserves for spring regrowth in late summer and early autumn. Therefore, frequent mowing or flaming can deplete a perennial weed's storage reserves, if the tactic is repeated multiple times, particularly during summer before the plant begins replenishing storage reserves.

Applying systemic herbicides that are taken up by the plant and translocated to storage organs is also most effective in late summer and early fall when perennial plants are replenishing storage reserves. Rotating or varying herbicide chemistry can also reduce the evolution of herbicide-resistant weeds.

Cultural control strategies for perennial weed control include crop rotation between perennials and annual crops, planting perennial crops with competitive annual companion crops that are harvested early, and managing for competitive crops with early crop planting and good crop management practices.

Biological control practices as described for annual weeds can reduce perennial weed populations, and be particularly cost-effective in rangelands where other practices are often cost prohibitive. Biological control strategies may include introducing browsing animals such as goats that will eat thorny perennial weeds, or application of weed specific pathogens such as bacteria or fungal spores.