As temperatures cool down, there’s still time to address your fall plans for weed management in fields that will be no-till planted next spring. While a frost can typically eliminate most of our annual weeds, winter annuals and even our more bothersome biennials and perennials can be difficult for a frost to control.

Controlling winter annuals, such as marestail or horseweed, field pennycress, chickweed and henbit, in the fall can provide many advantages compared to waiting until spring. Winter annuals are smaller and easier to control in the autumn. Systemic herbicides will also be more readily translocated to any belowground plant structures, providing more consistent control compared to other times of the year. Fall weed control may also result in less weed biomass next spring that could interfere with planter operations.

Burndown considerations

Fall herbicide burndown applications can be a great option, but there are some considerations to keep in mind. Closely examine fields after harvest to determine whether winter annuals are present and exposed through the residue cover. Also, be sure to follow herbicide label suggestions that pertain to carrier type, carrier volume, nozzle type and environmental considerations.

Herbicide applications can still be effective after a frost if the foliage of the weeds is still active. Ideally, herbicide applications should be made when the sun is shining and daytime temperatures are above 50 to 55 degrees F and nighttime temperatures stay above 40 degrees. When selecting herbicides, consider the likelihood of resistance to certain herbicide groups and their respective site of action. Widespread populations of marestail are known to be resistant to HG 9 (glyphosate) and HG 2 (ALS) products. Including 0.5 pounds ae (acid equivalent) 2,4-D LVE or 0.25 pounds ae dicamba in glyphosate will increase the consistency of marestail control, even in fields that don’t exhibit any glyphosate resistance.

Other considerations for fall burndown applications include fields with a history of winter annual pressure and high weed densities at harvest. Fields with known resistance that limit herbicide options in the spring should also be considered. Justification could also be made for any factors that may prevent timely springtime applications, such as poorly drained fields or sprayer availability. It should be noted, however, that some winter annual populations may emerge in both the fall and spring, making it difficult to have effective control with a single herbicide application.

Jump-start for spring

Effective control of winter annuals prior to planting is an important first step for weed management in crop fields. For many, starting with a clean field in 2021 will benefit greatly from some work and effort this fall.

Michel is an Iowa State University Extension field agronomist.