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Control of Winter Weeds?

Christy Sprague
Christy Sprague
Assistant Professor of Weed Science
Department of Crop Sciences
(217) 333-4424; csprague@illinois.edu
Ryan Hasty Ryan Hasty
Research Specialist
Department of Crop Sciences
(217) 333-4424; hasty@illinois.edu

Winter weeds, in particular winter annuals and some simple perennials, have been the topic of discussion over the last few years. These weeds have increased in prevalence and growth across most of the Midwestern landscape. Increases in these weeds may be attributed to increased acreage in no-till crop production and, possibly, the use of more herbicides that have little or no residual activity. In addition, the past few winters have been relatively mild, extending the growing season for winter vegetation. This winter vegetation can be a major headache to growers in the spring, since winter annuals can form dense vegetative mats that can physically interfere with planting and tillage, potentially harbor destructive insects, and decrease moisture evaporation from the soil surface. Decreases in soil moisture evaporation equates to wetter soils in the spring that can cause delays in planting and tillage operations.

Winter annual weed growth in the spring
Figure 1. Winter annual weed growth in the spring.

Several winter annual and simple perennial weed species commonly infest corn and soybean fields throughout the Midwest. Some of these species tend to be more problematic in identification and control than others. Currently, winter annuals such as common chickweed, purple deadnettle, henbit, butterweed (cressleaf groundsel), horseweed (marestail), several mustard species, and the perennial dandelion top the list of weeds that concern producers. Identification of these weed species is the first step to controlling them. A few good early-season weed guides are available to help aid in identifying these weeds. The first is the North Central Regional Publication No. NCR 614, Early Spring Weeds of No-Till Crop Production, published by the University of Missouri. The second is a new guide, A Pocket Identification Guide of Early-Season Weed Species.

Until recently, control of winter annual weeds has generally taken place in the spring, with the use of tillage or burndown herbicide applications. In many cases, burndown herbicide applications have been very effective in controlling winter annual weeds; however, there have been instances where winter annual weed control in the spring has not been adequate. These difficulties have arisen anywhere from insufficient spray coverage to timeliness of the burndown application due to uncooperative spring weather. One of the more challenging problems with controlling existing vegetation in the spring is making applications in cool weather when weeds are not actively growing. Overall, making applications when there are increases in temperature can enhance weed control and reduce weed biomass.

Since winter annual and perennial weed control has not been always consistent in the spring, the practice of applying herbicides in the fall to control winter annual weeds has gained widespread popularity over the last few years. In addition to the benefits realized above in reducing winter weed growth, controlling these weeds in the fall prevents them from producing seed, thereby decreasing the soil seed bank and helping to reduce future problems with these species. Additionally, controlling simple perennials (such as dandelions and white cockle) in the fall is much more effective than controlling them in the spring.

Fall herbicide treatments can be extremely effective tools in managing winter annual, biennial, and simple perennial weeds. So, how do you know if fall herbicide applications are suitable for your farming operation? These applications are most effective on fields where these weeds have been a problem in the past. If spring herbicide treatments have been effectively controlling these species and they do not appear to be increasing, there may be little to no benefit to fall herbicide applications in these fields. In addition, even though winter annual weeds may be controlled by fall applications, under certain conditions, a spring burndown treatment may still be needed.

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Department of Crop Sciences
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