Tuesday, April 12, 2016

Cold Temperatures and Burndown Herbicides

by Bob Hartzler
     Professor of Agronomy Extension Weed Specialist
     Iowa State University


published April 11, 2016
                Integrated Crop Management on line newsletter






The weather forecast appears to be favorable for field activities so people will be anxious to get into the field. A concern for many will be the effect of the widespread freeze on the performance of burndown herbicides. Unfortunately, there is no simple blanket statement that can be made since the plant response will vary depending on weed species, weed size, and the herbicides used.


Postemergence herbicides

A statement found on most postemergence herbicide labels is ‘Apply when weeds are actively growing.’ This is by far the most important consideration in determining whether to apply a postemergence product. Most weeds that emerge in March are adapted to sub-freezing temperatures and will not be killed by frost; however, it takes time for them to recover from these events. Performance of herbicides will be reduced if applied too soon following a frost. How long does it take to recover? Again, no simple answer since it depends on the weed species, severity of the frost, and weather conditions that follow the freeze. Closely monitoring the weeds for evidence of new growth is the best way to determine recovery.