Ed Peachey
March 2022

Weed management in annual cropping systems Successful weed management in annual cropping systems requires a year-round approach, employing a combination of weed control practices that are rotated over several years. Developing these strategies requires knowledge of specific weeds that infest your land. Identify and map major weed species and infested patches within each field. With an established point of reference and occasional observations, you can evaluate weed shifts and adjust crop and weed management strategies before resistant weeds predominate.

Preventing weed shifts and introductions Annual weeds that grow and produce seed quickly often dominate in cultivated fields. Routine cultural practices and repeated use of the same, or similar, herbicides enhance selection for resistant species or weed biotypes. Repeated use of the same herbicide also can modify some microorganisms that degrade herbicides, resulting in shorter soil persistence and poor weed control.

Weeds that survive plowing, cultivation, repeated herbicide treatments, or other routine cultural practices must be eliminated before tolerant species or biotypes become established. Combine a variety of weed control practices or treatments, alternate or rotate herbicides used, spot-treat difficult areas of the field, or remove weeds manually when they first appear.

Clean your equipment when moving from an infested field. Puncturevine seeds have been transported from eastern Oregon and have invaded vegetable production areas in the Willamette Valley (

Field preparation and planting Eliminate perennial weeds before planting by designing a selective-control program in the previous crop, or by controlling the weed during a temporary fallow period. Canada thistle, for example, can be controlled with wiper applications of glyphosate (Roundup) in certain crops if 10% or less of the acreage will be treated (see label). Soil fumigation also might be considered for high-value vegetables if potential losses from soilborne diseases, nematodes, or possibly perennial weeds, such as yellow nutsedge, would justify application costs (see the current edition of the PNW Plant Disease Management Handbook for soil fumigation details and approved materials.)

During field preparation, destroy all weedy vegetation and prepare a reasonably smooth surface for uniform herbicide application. Wet soils or delayed applications after the last soil disturbance often result in erratic weed control.

Effective use of preemergence herbicides Three principles should guide the use of preemergence soil-applied herbicides.

Apply the herbicide before the weed seed germinates. Many herbicides are not effective if applied after weed seeds germinate. The radicle of the seed emerges first, then quickly grows down and away from the herbicide-treated zone. If the herbicide is not active when the radicle emerges, the weed seedling may survive. Herbicides such as Dual Magnum (S-metolachlor), Outlook (dimethenamid-P), and Treflan (trifluralin) provide very poor control if the weed seeds have germinated. This is not the case for all soil-applied herbicides, including Goal (oxyfluorfen) and Chateau (flumioxazin); but even in these cases, the smaller the seedling, the better the efficacy. To ensure proper preemergence herbicide efficacy, till the soil, then apply and incorporate the herbicide before weed seeds have a chance to germinate.

Activate the herbicide. Activation of soil-applied herbicides distributes the herbicide so that it comes in contact with the weed seed. Water usually works best to activate preemergence herbicides, but tillage is required in some situations, as with Eptam (EPTC) because of herbicide volatility or because the herbicide is very insoluble and is difficult to incorporate with irrigation or rainfall (Treflan). Water carries the herbicide down and distributes the herbicide throughout the soil. Most weed seeds germinate, and seedlings emerge from 0.25- to 1-inch depths. If the herbicide is not in contact with the seed when it germinates, the first emerging root may get a foothold, and the seedling will survive.

Preserve the chemical barrier. Properly applied and activated herbicides present a barrier to weed seeds. Too much irrigation water or rain on a soluble herbicide such as Dual Magnum or Eptam will destroy the chemical barrier; the herbicide gets washed too far down and becomes diluted at the soil surface. Incorporating the herbicide too deeply into the soil is another way to diminish the effective chemical barrier. Mechanical disturbance also can destroy this barrier and allow weed seedlings to escape. Goal (oxyfluorfen), Cobra (lactofen), and Chateau are preemergence herbicides that kill some weeds on contact, when weed seedlings grow into the chemical barrier at the soil surface. If the herbicide concentration is lessened at the soil surface because the surface layer has been moved (by cultivation, tillage, or foot traffic), the herbicide barrier will be broken or diminished and weed control will suffer.

Scouting Evaluate the effectiveness of preplant or preemergence weed control treatments as you assess crop emergence, soil moisture, disease incidence, and other factors during the growing season. Note whether gravel or low spots may have caused abnormal weed control or possible crop injury. Map weed problem areas in fields, including patches of perennial weeds. Determine whether additional control measures such as cultivation, application or spot-spraying of a postemergence herbicide, or hand-hoeing individual weeds will be necessary to achieve a quality product at harvest yet minimize the chances of allowing a weed shift.

Postharvest control Soon after harvest, destroy weeds and crop stubble to reduce pests and minimize weed seed production. Weed seed production is often at its peak when short-season crops such as beans are harvested in mid- to late-summer. Harvest procedures do not always kill weeds, and seeds will continue to develop and mature. Steps to control weeds after harvest contribute to an overall weed management strategy that minimizes the amount of seed that will go back into the soil. If weeds are perennials, maintain optimum growing conditions so appropriate herbicides can be applied to actively growing weed foliage for maximum control. Consider factors such as application timing and weed growth stage, herbicide persistence in soil, crop rotations, and label restrictions for subsequent crops when selecting a postharvest herbicide for perennial weed control. After postharvest treatment, consider planting a rotation crop, such as winter grain or a small-grain cover crop that requires significantly different cultural practices.

Herbicide persistence and crop rotations Many herbicides used in vegetables persist or have residual activity in soil. Some of the longer rotational intervals are required following the use of Sandea (halosulfuron), Raptor (imazamox), and Reflex (fomesafen) herbicides. Carryover from persistent herbicides will often have an impact on Brassica crops and sugar beets. Over-application or unusually dry or freezing weather may prolong herbicide persistence. If herbicides persist into the next crop cycle, those crops may be injured, or harmful residue might accumulate in the food product. Managing persistent herbicides requires careful planning of crop rotations according to waiting periods specified on the label.

Accuracy, precision, and food safety Herbicides must be applied at the correct rate and time to selectively control weeds with minimal risk of injuring vegetables. If the label is not followed accurately, illegal pesticide residues may be found in harvested crops and may cause the crop to be removed from the market. Risks of endangering the food supply with pesticide residues can occur when herbicides are applied without label approval for each crop, when excessive quantities are applied, or when harvest occurs before the preharvest interval (PHI) expires. Avoid possible residues in edible crops by plowing the field if a mistake involving a nonregistered pesticide occurs, by verifying mathematical calculations with other knowledgeable persons to ensure accuracy, and by noting the preharvest interval at the time of application to ensure food quality standards are met or exceeded.

Read the label and other information about properly applying and timing each herbicide. Suggested rates in this guide are stated as pounds active ingredient per acre (lb ai/a) or pounds acid equivalent per acre (lb ae/a). Many tank-mixes are listed on herbicide labels. Growers or applicators may accept liability and mix materials unless specifically prohibited on one of the labels. Select products that are compatible and complementary in their weed control.