Herbicide Application Timing

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

Timing of herbicide application is critical. Whether we are talking about treating small weeds with a postemergence herbicide, spraying residual herbicides at planting (instead of weeks before) to provide in-crop control, or applying preemergence herbicides before weeds start poking through the soil. Maximizing weed control requires an understanding of how herbicides enter the plant.

Some herbicides must be applied to weeds after they are out of the ground because they enter only through leaves and stems (think glyphosate and paraquat) and provide no control of weeds that have not yet emerged.

Some herbicides need to be absorbed by the weed seedlings as they emerge from the soil. The weeds’ roots or shoots absorb the herbicide along with the soil moisture and move to the site of action inside the plant. If these herbicides are applied to weeds that have emerged, they provide little or no control. They may injure some weeds, but they do not provide “weed control”. Think Dual, Command, Zidua and others (see list below).

Finally, some herbicides will provide control of emerged weeds (postemergence control) as well as control of weeds as they emerge from the soil (residual control). Some herbicides in this group can be applied after the crop has emerged (think atrazine in corn) while others must be applied before crop emergence because of the crop injury they can cause (think metribuzin in soybeans).

The following table is refresher:

*Some of these may injury small, emerged weed seedlings but are not recommended for their postemergence control.

**Need to be applied postemergence, but can provide some residual control; however, residual control is inconsistent and generally only for a week or two.

***These herbicides will severely injure emerged crops.