February and early March 2024 will go down in the history books as unusually warm and dry. Many winter annual, biennial and perennial weeds have resumed growth earlier than usual. Managers will need to be especially vigilant to treat these “early” weeds in a timely manner this spring.

The primary concern this spring is that temperatures have been warm enough for these species to resume growth but too cool overall to achieve effective, consistent control with herbicides. Herbicide applications for winter annual and biennial control should be targeted prior to bolting, or stem elongation, and flowering. Control of winter annual and biennial weeds with herbicides becomes less consistent as they bolt and flower.

The optimum timing of herbicide application will vary by weed species and field conditions, so site-specific scouting and knowledge of the infestation is important to make appropriate decisions. For example, winter annuals like henbit, common chickweed, and field pennycress generally flower earlier than the winter annual horseweed and biennials like musk thistle and wild parsnip.

Effective burndown treatments should follow herbicide label suggestions for carrier type and volume, nozzle type, and weather considerations. Treatments made on sunny days with warm daytime and nighttime (>40F) temperatures will generally be more successful than applications made during cooler conditions.

When selecting herbicide burndown treatments, choose herbicides labeled for control of the target species and consider the likelihood of resistant biotypes in the field, particularly when treating horseweed. Herbicide group (HG)1 9 (glyphosate) and HG 2 (ALS) resistant horseweed populations are widespread across Iowa. Including 0.5 lb. a.e. 2,4-D LVE, 0.25-0.5 lb. a.e. dicamba (HG 4) or 1 oz. Sharpen (HG 14) or another saflufenacil product labeled for burndown control to glyphosate will increase the consistency of horseweed control.

Check herbicide labels for planting restrictions if treating winter annuals in row crop fields and for haying or grazing restrictions when treating weeds in forages. Most 2,4-D labels have a 7-14 day planting restriction for corn or soybean following application. Ester formulations of 2,4-D have a shorter interval to crop planting after application than amine formulations.

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