Dry conditions are forcing earlier-than-normal post-emergence weed spraying

A weed scientist says the dry weather is forcing many farmers to apply post-emergence herbicides a lot earlier than they had planned.

Dr. Rodrigo Werle with the University of Wisconsin says, “The topsoil as been really dry meaning that the pre-herbicides are not doing what they are supposed to be doing right now.”

Werle tells Brownfield the pre-emergence herbicides might be failing to kill weeds because they needed moisture to activate, and because it is dry, the weeds have the advantage.

Werle says they usually pull the trigger on post-emergence herbicides after the second or third week of June, but they started spraying over a week ago. “We’re telling our growers to be out there, even if they put a good pre-emerge herbicide down, to scout and make sure those weeds are not escaping, and if they are escaping, to not wait too long for that post-emergence herbicide application.”

Werle says farmers should not spray when it is very hot as the herbicides work best when the plant is not stressed.

The University of Wisconsin Weed Science Program can show farmers what they are experiencing so far next Friday, June 11th during a giant ragweed management plot tour at the Rock County Farm in Janesville.  Information can be found online at

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