Can You Apply Roundup in Rainy Weather?

Few herbicides have the scope and the industry recognition of glyphosate. According to the University of Maryland Extension, glyphosate is the second most-used pesticide in the home and garden category. Even if you don't recognize the word "glyphosate" or its chemical compound name, N-(phosphonomethyl) glycine3, you may be familiar with its first patented appearance -- in a product called Roundup. As a nonselective herbicide, Roundup doesn't discriminate among plants. It attacks different kinds of plants with equal vigor, damaging or killing whichever ones it touches. Rainy weather, however, hinders Roundup's effectiveness.

When you spray Roundup on a plant, its leaves and stems absorb the glyphosate, and the chemical moves throughout the insides of the plant, all the way to the roots to kill the plant. If, though, a plant is wet from rain, irrigation water or dew, the water creates a barrier that prevents or hinders the chemical's absorption.

Roundup is most effective when the weather is sunny and warm -- above 60 degrees Fahrenheit. Don't apply Roundup while it's raining or after rain or irrigation water wets plants because the product can wash off. Heavy dew that wets plants may also hinder Roundup's effectiveness.

It's best to hold off applying Roundup if rain is predicted during the six-hour window after it would be applied.

Don't spray Roundup on windy days because its chemicals may drift to unintended plants.

The Roundup trademarked name is given to different formulations of glyphosate, which have different "rainproof" post-application times -- the amounts of time needed for the products to dry on plants before rain interferes with their effectiveness. Different Roundup formulations also have different "visible response" times -- the amounts of time needed before the applied products' results can be seen in the plants; the visible results include wilting, browning, leaf curling and death.

Different Roundup products, their rainproof times and their visible response times include:

Roundup Weed & Grass Killer: rainproof in 10 minutes, visible results in three hours.

Roundup Extended Control Weed & Grass Killer Plus Weed Preventer: rainproof in 10 minutes, visible results in three hours.

Roundup Max Control 365: rainproof in 30 minutes, visible results in 12 hours.

Roundup Poison Ivy Plus Tough Brush Killer: rainproof in 30 minutes, visible results in 24 hours.

Roundup Weed & Grass Killer Plus FastAct Select: rainproof in 10 minutes, visible results in three hours.

Roundup Concentrate Extended Control Weed & Grass Killer Plus Weed Preventer: rainproof in 30 minutes, visible results in 12 hours.

Roundup Weed & Grass Killer Concentrate Plus: rainproof in 30 minutes, visible results in 12 hours.

Roundup Weed & Grass Killer Super Concentrate: rainproof in 30 minutes, visible results in four days.

If you spray plants with Roundup and rainfall happens before the specific Roundup product you used is rainproof on them, then the rain will wash the Roundup off the plants. Making a repeat application of the product may be necessary to kill the plants.

Spray Roundup on dry plants -- no rainwater, irrigation water or dew on the leaves -- until the plants are thoroughly wet but not dripping. Spray all above-ground parts of the plants, including the leaves and stems.

Apply a Roundup herbicide only as that specific Roundup product's label directions recommend. Follow all label precautions, and wear protective clothing, including eye protection, gloves, a hat and clothing that covers your skin.