Applicators of pesticides often question whether an application they have made will be effective if rainfall occurs too soon after the application. But what is too soon? Is it 10 minutes, an hour, 4 hours, 24 hours, etc.? Rainfall occurring after application can have a significant effect on the residual activity and efficacy of pesticides. A pesticide’s rainfastness, or its ability to withstand rainfall, is an important factor affecting the efficacy of foliar-applied pesticides. Generally, it is best to avoid pesticide application when rainfall is likely; however, weather can be unpredictable, so it is best to choose a product with good rainfast characteristics.
Definition of Rainfastness
A pesticide is considered rainfast after application if it has adequately dried or has been absorbed by plant tissues so that it will still be effective after rainfall or irrigation. The degree of rainfastness of pesticides is highly variable. The best source for determining rainfastness for a particular product is to consult its label. Some products contain statements that specifically address the length of time necessary for rainfastness to occur (Figure 1). In many cases, limited or no information about rainfastness is included on the label, and the wording is often vague (Figure 2). Some product labels will expressly prohibit an application if rainfall is expected within a stated timeframe (Figure 3). Others may recommend that a product is not applied within a stated timeframe (Figure 4).