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The Effect of Rain on Herbicides (October 17, 2018)
Foliar Applied Herbicides
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 rainfast 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.

Soil Applied Herbicides
The effect of rain on soil applied herbicides is variable. Those that adhere strongly to the soil such as Prefar and the dinitroaniline (yellow) herbicides including Balan, Treflan and Prowl will most often not be affected by rain of up to 0.5 inch. Heavier rains may help move them down into the soil. Rains in excess of 0.5 inches may leach some herbicides such as Kerb, below the weed seeds and require a second application if the label permits. Volatile herbicides like Goal or Eptam can be evaporated and be lost or cause crop injury.
"Here in Yuma we love the water but hate the rain!”

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For questions or comments on any of the topics please contact Marco Pena at the Yuma Agricultural Center.
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