Turf - Cool Temperatures and Herbicide Activity

We have had an unusually cool May. Broadleaf herbicide activity can be affected by these cold temperatures. The following is an article on the subject.

As most of us know, optimum weed control with broadleaf weed herbicide mixtures is obtained when weeds are actively growing. Conditions that promote active growth are good soil moisture and moderate daytime/nighttime air temperatures. In general broadleaf weed herbicide mixtures work best when applied when daytime air temperatures range between 75 to 85 degrees and nighttime temperatures range between 55 to 65 degrees. April and May have brought us a prolonged early spring weather pattern with many days below 70 degrees and nights below 50 degrees. Due to these cool weather conditions herbicide activity has been very slow to develop.

This has been observed even with herbicide mixtures formulated as esters, which work better in cooler weather conditions. In addition, up until May 12 we experienced dry soil conditions, which may also detract from optimum broadleaf weed herbicide activity if applications were made prior to May 12 on sites that were not receiving supplemental irrigation. Due to these conditions expect the possibility of a significant number of broadleaf weed herbicide failures for applications made prior to May 12 regardless of the product. This possibility may be even greater for tougher to control broadleaf weeds such as oxalis, veronica, ground ivy, and mouseear chickweed. Keep a close watch on sites that have received broadleaf weed herbicide applications under these conditions. If re-treatment is necessary, wait for more moderate air temperatures and visible signs of active regrowth of the weeds before a second herbicide application.

Information adapted from an article by Dr. Steve Hart, Turf and Ornamental Weed Specialist, Rutgers University.