Optimum Time for Fall Weed Control

This article was taken from the most recent NDSU Crop and Pest Report.

Now is the time to begin applying herbicides for the control of winter annual weeds, simple perennials such as curly dock and dandelion, biennials such as biennial wormwood, and in some cases cool-season perennial weed species. This is especially true for no-tillage fields, but also for those fields receiving tillage other than moldboard plowing. For fields in which tillage is planned, apply herbicides at least 5 days prior to tillage. Herbicides may be applied within a few days of crop harvest or until the soil is frozen. Based upon research across the Midwest, the most consistently effective control of dandelions is obtained with fall herbicide applications. The best way to drastically reduce or stop seed production of winter annual species is with fall herbicide applications or effective fall tillage. It is always better to apply herbicides in the fall under less than ideal conditions, than to wait until spring to achieve marginal control of these types of weed species, especially dandelion.

The most effective fall herbicide treatment with the most cropping flexibility next spring is an application of glyphosate at 0.75 pound acid equivalent/acre (lb ae/A) [Roundup at 22 ounce/acre {oz/A} or glyphosate products containing 3.0 lb ae/gallon at 32 oz/A] plus 2,4-D ester at 0.5 lb ai/A. The addition of 2,4-D is most important for dandelion control and will antagonize glyphosate's activity on Canada thistle and perennial grass species. Another herbicide option, would be the addition of Valor at 2 to 3 oz/A to the glyphosate plus 2,4-D mixture. Fall applications including Valor will be most beneficial west of the Red River Valley where spring rains are not consistent enough to properly activate Valor. Activation of Valor is almost certain with fall applications in the drier areas of the state. Preliminary studies with fall-applied Valor have shown potential to control or suppress weeds such as kochia, seedling dandelion, canola, and chamomile. However, NDSU and Valent are conducting additional research to determine proper timing of application of Valor and efficacy on spring-emerging weeds. Valor should only be applied in no-tillage fields and any substantial soil movement next spring during planting will reduce the effectiveness of Valor on spring emerging weed species. Read the Valor label and follow the crop rotation guidelines when applying Valor in the fall. Only certain crops can be planted in the spring following fall-applied Valor.

Jeff M. Stachler
Assistant Professor - Sugarbeet Weed Science
NDSU and University of Minnesota