Cool, Slow Start to Spring Could Affect Weed Control Strategy

Herbicides may not be as effective in cool weather.

Published on: Apr 18, 2011

Just because you used lower rates on burndown a year ago and got by with it, seeing good burndown weed control, doesn't mean it will work this year in the same manner. These early applications are affected by environmental conditions and how well weeds are growing, implies Aaron Hager, Extension weed specialist at the University of Illinois.

Hager recently issued his opinions on how to control winter annuals. Some are showing up earlier than normal this spring, and are already flowering. Once any type of plant flowers, including these winter annuals, the message is that they're reaching a mature state. Weeds that are maturing require more herbicide to bring down than weeds in a fast growing state. That's especially true of herbicides like glyphosate that must be absorbed and moved through the plant.

"Cool temperatures can slow the activity of many burndown herbicides, Hager says. Translocated herbicides are sometimes slower acting in these situations than contact herbicides. A contact herbicide, such as paraquat, known today as Gramoxone, would be more likely to produce quicker results in cool conditions than one that has to be moved to the roots, or translocated, to work, such as glyphosate.

Use this knowledge to make informed decisions when you're deciding when and how to spray, and what to put into the spray tank, Hager urges. The bottom line is that in cool springs, when weeds aren't growing very quickly, herbicides won't work as well as if it's warm and weeds are growing rapidly. So cool springs are not a good time to try to shave rates on most herbicides and still expect to get the control you've come to expect from any specific product.

"When the forecast calls for several days or nights of cool air temperatures, don't be surprised if symptoms of activity on existing vegetation may take several days to develop," Hager says.

Signs that the herbicide are working and producing damage that will lead to death of weeds differ from product to product anyway. The time for symptoms to appear for glyphosate tends to be slower than some other products.

The timetable for all products will be shoved back in terms of when you see symptoms in cool weather that's not conducive for plant growth, the weed specialist concludes.