Weed Control In a Drought

Weed control in drought conditions is challenging. The following is some information on the topic.

In a drought, weed control can be challenging. For pre-emergence herbicides to work, water is required for activation. Preemergence herbicides control weeds by creating a chemical barrier that inhibits seed germination or that germinating weed seedlings absorb, killing them. However, to form the barrier, some of the herbicide needs to be solubilized. This solubilization then continues over time for the life of the herbicide until it is no longer effective - a period of 2 to 6 months depending upon the herbicide. Water is required for solubilization of the herbicide. If there is not enough water to solubilize some herbicide initially, weed seeds that do germinate will come through the herbicide barrier. Herbicides that require root uptake by weeds also need to be solubilized to perform properly.

The majority of post-emergence herbicides such as glyphosate work best on weeds that are actively growing. Drought stressed weeds do not absorb herbicides well because stomates are closed and thicker cuticles may have developed. Herbicides also do not move as readily to the site of action because transport mechanisms are not functioning well and activity is reduced because plants have shut down some physiological processes. Ironically, the key to good herbicide performance in a drought is irrigation. By keeping weeds actively growing, your post emergence herbicides will work better. Irrigation will activate preemergence herbicides and make root absorbed herbicides more effective.

Gordon Johnson, Extension Horticulture Agent, UD, Kent County