The drought conditions impacting Florida have implications for your herbicide use. In an unirrigated yard, you may notice your yard has gone dormant and grass is brown while some weeds remain green. Weeds are tough plants and can survive drought conditions while many turf species enter a period of dormancy during drought. This may seem like the perfect time to apply herbicides. However, plants under drought stress do not respond well to herbicides and treatments may not work as expected.
Plants have defense mechanisms that help them survive during drought conditions. For turf, dormancy is a common way that the grass protects itself in cold weather or in periods where there is no water. If your lawn has turned completely brown like it does in the winter, it is protecting itself from the harsh conditions outside. Plants that do not go dormant may instead develop a thicker cuticle, or outer layer, which slows water loss. That same thicker cuticle, though, makes it harder for herbicides to absorb into weeds we’re trying to control.
The type of herbicide you’re using matters. Contact herbicides only affect the parts of plant tissue that they touch and are not moved through the entire plant. These herbicides will not kill underground roots or tubers and will not control perennial or woody species. Systemic herbicides, on the other hand, are absorbed and move through the entire plant to the roots or other underground parts. That movement depends on water, so during drought conditions, systemics lose some of their effectiveness. Both types of herbicides may show reduced effectiveness during a drought period due to these factors.
Weeds in your lawn will respond differently if they are under irrigation.
Irrigated Lawns
Apply between ½ – ¾ of an inch of water during each irrigation cycle. This should be done early in the morning to avoid disease problems due to water sitting on the leaves overnight and reduce loss to evaporation during the heat of the day. Learn more about how to determine how much water you are applying by calibrating your watering system: https://gardeningsolutions.ifas.ufl.edu/care/irrigation/calibrating-your-irrigation-system/. Herbicide efficacy is less likely to be reduced when turf and weeds are not drought stressed. Follow label instructions and avoid applications during temperatures about 85F.
Unirrigated Lawns
Grass is likely to go dormant if it has not already. Going into dormancy keep mowing heights higher and avoid mowing dormant lawns. Herbicide applications will not be as effective and can increase stress in the lawn during an already stressful period. Hand pulling weeds during this time is a safe way to control weeds without damaging your turfgrass.
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