Herbicide Rain Fastness (January 6, 2016)

When rain is a possibility, people begin to ask how much time they need before or after foliar applied herbicides are applied. In this area, herbicides are more likely to be washed off with irrigation water then they are by rain but the same principles apply. As always, there is no universal answer and it will depend upon many factors. Almost all are rain fast in from 1 to 4 hours. Only 15 minutes is needed for some and others require eight hours. 2 to 4 hours is a good rule of thumb. Some of the most important factors affecting how fast the herbicide gets into the foliage are mode of action, leaf surface characteristics, surfactant use, environmental conditions and age of the plant. Plants that are small and actively growing absorb herbicides faster than those that are larger and more mature. Contact herbicides do not have to be translocated through the plant, work quicker and are not as sensitive to growing conditions. Water-based formulations are more easily washed off than are oil-based products. Herbicides need more time to get into plants that have a thick waxy cuticle or leave hairs. The addition of a surfactant that helps the herbicide spread and stick to the leaves reduces the time needed before rain or irrigation. All of these factors interact with each other and can affect how much time is needed. More important than rain fastness are the growing conditions of the weeds, especially when using systemic herbicides. If the weeds are water stressed or weakened by something else, the herbicide might not work at all.

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To contact Barry Tickes go to: btickes@ag.arizona.edu.