Dry Weather and Weed Control

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

The local weather forecast does not look favorable for rain. Here are some thoughts on what that could mean weed control.

If you have irrigation and your preemergence herbicides have not had water to “activate” them, run the pivot to move the herbicide into the top inch of soil. Preemergence herbicides need to be absorbed by the roots and shoots of weed seedlings as they grow through treated soil; and herbicide only on the soil surface is much less effective.

If you do not have irrigation, physically incorporating with a field cultivator is an option. However, an additional cultivation will further dry the soil. Other implements can be used for incorporation if they thoroughly mix the soil and leave a smooth soil surface. A rule of thumb, a field cultivator will incorporate half the depth of the cultivating tines so set it to run 3 to 4 inches deep. Unfortunately, incorporating is not an option for no-till fields.

If you have a cover crop that has not been killed yet, check the field for soil moisture. Most of the cover crops seeded last fall are large and may have depleted soil moisture. In which case, it may be best to delay termination or planting until rain.

If you applied your herbicides over a week ago and have had not water, there is not much you can do to activate many of them. Root absorbed herbicides such as triazines, HPPD (i.e. Callisto), PPO (i.e. Authority or Valor) and ALS herbicides and may get taken up if activation occurs before the roots develop.

In most situations, you have the option to spray postemergence. So be sure to scout these fields early to find when the weeds begin emerging and be sure to spray based on weed size rather than relying on calendar dates.