Herbicides in the Heat

So, I am just enjoying my time in my yard, throwing a ball for my dogs, Penny and Ruby, who may or may not bring it back when I spot a small clump of weedy plants in my lawn. Instead of heading straight for my herbicide cabinet. I always ask myself a few questions: 1) is this weedy area problematic enough to warrant treatment, 2) what is the best option to treat the area, and 3) are the conditions right to treat. Question one is entirely up to the individual homeowner based on their desires and preferences for their landscape. Question two can easily be answered by research and/or by asking a local Extension agent. Question three, though, is an often-over-looked thought, as the other two questions always seem to be the leading topics for many homeowners. The timing and conditions must be right for herbicides to produce the desired effect. For information on timing your weed control, please see HGIC 2334, Lawn Weed Control Timing Chart.

Herbicides are carefully developed and heavily researched products that interfere with specific plant processes, whether that be inhibiting certain enzymes or amino acids or, more often, restricting some process of the photosynthetic pathway. With any chemical, it has its own characteristics that aid in its process, including reactions to different environmental conditions. Because of the variability, it is important to thoroughly read the labeling attached to all herbicides to determine if the environmental conditions are appropriate for use.

Grassy areas exhibiting signs of heat or drought stress will not be as tolerant of herbicides labeled for these areas. Adam Gore, ©2022, Clemson Extension

Grassy areas exhibiting signs of heat or drought stress will not be as tolerant of herbicides labeled for these areas.
Adam Gore, ©2022, Clemson Extension

This is especially prevalent during the summer in South Carolina as we regularly experience temperatures in the 90s and stretches of time with little to no rainfall. These herbicides, and our lawn grasses, do not always coexist during these periods of time. When most herbicides are applied during high temperatures (greater than 90°F), multiple problems can occur. Firstly, the weedy plants you are going after may suspend their growth, meaning that the plant processes that most herbicides attack may also be suspended, equating to limited control. Secondly, your desired grass is also experiencing the heat stress, so adding products that the plant still must break down increases the workload and can lead to decline, even if the grass is normally tolerant of the herbicide; this is especially true for cool season grasses like tall fescue. Thirdly, depending on the herbicide, liquids can switch to a gaseous form (volatilization) and move off site, causing damage to desirable plants, an event commonly referred to as drift.

Additionally, during periods of little to no rainfall, soil moisture is reduced, leading to the reduction in the amount of water absorbed by the plant, which also reduces the necessary amount of herbicide being consumed by a weedy plant leading to reduced control. Due to restricted water uptake, it is more difficult for desirable plants to break down the herbicide into nonharmful products.

Ultimately as the applicator, it is up to the homeowner to carefully read the herbicide label to avoid increasing any unnecessary risk to other plants. For more information on avoiding herbicide damage to landscape plants, please see HGIC 2349, Herbicide Damage to Landscape Plants.

If this document didn’t answer your questions, please contact HGIC at hgic@clemson.edu or 1-888-656-9988.

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