Wet Weather and Weeds

Published: November 3, 2009

As with everything else, the extremely wet conditions we have experienced this fall will have an impact on your winter annual weed populations and also on your plans for managing these winter annual weed populations. Winter annual weeds like henbit, purple deadnettle, and chickweed have already emerged in many corn and soybean fields throughout Missouri, but with the wet weather we have experienced this fall it doesn't look like there will be as many opportunities for fall herbicide applications as there are in most years. However, as I have said in many previous newsletter articles and talks on this issue, our research indicates that applications of residual herbicides made in the early spring can provide similar levels of winter annual weed control as applications of these same herbicides in the fall. In addition, our data indicate that early spring applications of residual herbicides provide better control of emerging summer annual weed seedlings than fall herbicide applications. This is especially the case with our current herbicidal options available in soybeans.

The wet weather may also have an influence on fall or winter herbicide applications that were planned to be made in wheat. Although I have been a proponent of this timing especially for the control of winter annual grass weeds in wheat, most of the herbicide applications that are made in wheat production in Missouri are typically made in the spring anyway. So, I don't think it will hurt to just plan on making these applications in the early spring if necessary. Most of our winter annual broadleaf weeds are not very competitive and it is unlikely that they will have much of an impact until "greenup" of wheat in the spring. If you have winter annual grasses, however, these can be very competitive with wheat and I would try if at all possible to make a herbicide application for the control of these weeds sometime yet this year.

Although it has been really, really wet, and it certainly will be difficult to get across fields without tearing them up, I don't believe all hope is lost just yet. I believe there will still be opportunities for fall or winter herbicide applications yet this season, but these opportunities are fading fast. The problem is, we're getting closer to the time where air temperatures will not be conducive to the application of systemic herbicides like glyphosate and 2,4-D, which are usually included with a residual herbicide in a fall herbicide program. These herbicides need air temperatures to be in the 50's for at least a couple of hours a day over a 3- or 4-day time period in order to penetrate and translocate effectively.

One final thing to keep in mind is that many of these winter annual weeds germinate at two peak periods during the year, usually September-October and February-March in Missouri. With all the rainfall we have experienced this fall, it wouldn't surprise me to see another big flush of these winter annual weeds in the early spring.