Don’t apply pesticides in the heat of summer

man applying pesticides
Applying pesticides at temperatures higher than recommended on the label could cause unwanted damage to landscape plants. Photo by Ashley Andrews, Cooperative Extension.

A client recently came into the Master Gardener office with a problem in his landscape. He noticed that the lower branches of several trees bordering his lawn were showing damage. The leaves were curling and the margins of the leaves looked burnt. He wondered what disease was causing this damage.

By questioning him further, we determined that the broadleaf weed killer he had sprayed on his lawn had turned to a gas in the heat and drifted to the trees adjacent to his lawn. The damage he saw was in fact phytotocity caused by the weed killer.

Phytotoxicity is plant injury generally caused by a chemical applied on or around plants. By far, the most common of these chemicals is some form of pesticide. The symptoms of phytotoxicity on growing plants include stunting or delayed plant growth; misshapen, curled or distorted plants, fruit or leaves; russeting or bronzing of leaves or fruit; dead spots or flecks on leaves; dead leaf tips or leaf margins; and dead areas between the veins of the leaves. Many of these symptoms are similar to symptoms of disease or insect infestation, so how can you tell the difference?

The damage you see might be phytotoxicity if any of the following statements are true: if you see no signs of plant disease or insects that might have caused the damage, if you know chemicals have been applied in the area recently, if the symptoms show only on plants in a certain location in the landscape, if the symptoms do not spread from plant to plant over time, if the damage includes twisted stems and curled leaves.

Using pesticides during the heat of summer is not recommended. Remember, pesticides are substances designed to kill something; herbicides are pesticides that kill plants. Spraying pesticides when temperatures are above 80 degrees F can pose a real hazard to you, your pets and livestock, and the rest of your yard and your neighbor’s yard. At temperatures above 80 degrees F, sprayed pesticides can volatilize, or turn to a gas. The pesticide can then drift to non-target sites, such as your landscape trees, the dog’s water bowl or the neighbor’s prized roses. Spraying during windy conditions can also pose a risk of pesticide drift. Even lower risk pesticides, such as horticultural oils and insecticidal soaps, should not be applied when temperatures are above 80 degrees F. These lower risk pesticides can burn plant leaves if applied at high temperatures.

What can you do?  First, READ THE LABEL. The pesticide label will tell you at what range of temperatures it should be applied. Most pesticides are labeled for application between 60 degrees and 80 degrees F. Remember: when you buy a pesticide, you are agreeing to READ, UNDERSTAND and FOLLOW the label directions. Watch the weather reports. If there will be a cooling trend later in the week, can you postpone your spray application until then? Is it going to be windy, or is there a chance of rain? If so, you should not apply the pesticide that day. Can you spray early in the morning, when temperatures are cooler and winds are generally lighter? This will also help protect insect pollinators such as bees, who are not as active early in the morning.

Melody Hefner is the Urban IPM and Pesticide Safety Program Coordinator for the University of Nevada Cooperative Extension. Have a gardening question? Contact a master gardener at