Most herbicides are developed to work under a variety of environmental conditions but the extreme heat we have been experiencing in recent days can reduce the activity of some of them. Many summer annual weeds thrive in hot weather but the characteristics that allow them to do this can reduce the effectiveness of some of the herbicides we use. Weeds adapt to the extreme heat by developing a waxy cuticle on the surface to conserve water. It can be difficult to penetrate this layer with herbicides. An adjuvant can help overcome this. In the middle of the day, weeds will close their stomates or the tiny pours on the leaf surfaces and reduced metabolism. Systemic herbicides do not work well when the weeds have shut down like this. Translocation and water uptake are greatest in the morning and that is the best time to spray. Contact herbicides are most effective on bright and warm days but even they do not work well when the weeds are stressed. Another characteristic associated with heat is the presence of dust. Dust picked up behind tractor tires or from the wind, deactivate or reduces the uptake of some herbicides.

Barry & Juan’s cartoon: Dodder

Tickes/J.Pena

To contact Barry Tickes go to: btickes@ag.arizona.edu.

For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
College of Agriculture, The University of Arizona, Tucson, AZ.