Leaving Pesticides in the Tank: How Long is Too Long?

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Leaving Pesticides in the Tank: How Long is Too Long?

Water testing is the basis for your decision whether or not to condition your spray water. The purpose of conditioning water is to maximize the effectiveness of the pesticide.

Even when water characteristics are ideal, most pesticides break down in water, over time, through a chemical process called hydrolysis. Some break down slowly, while others become inactive within a few hours. The pH of the tank water often has a dramatic effect on the breakdown process. What we need to know is how long products can stay in the tank before undergoing changes that can impact efficacy.

Manufacturers formulate their products to stay in slightly acidic water for twenty-four hours with little appreciable reduction in effectiveness. But this general statement does not hold up for all products.


We express how quickly a pesticide product loses its effectiveness as its half-life; that is, the number of days it takes for half of the active ingredient to break down in water. It is understood that, at that point, the product would not provide the desired efficacy.

If we were to assume that treated Tap water is the perfect mixer because it is safe to drink, we would be wrong. Municipalities create safe drinking water by adding products such as chlorine that disinfect by killing harmful bacteria, and these treatments often shift pH levels into the alkaline range, typically pH 7.8–8.5.

Let’s assume we are using Tap water with a pH of 8.2. Consider how the pesticide products in the chart on below react to changes in the pH.

Assume that you mixed a pesticide product, but weather conditions caused a five-day job delay. Realize that, depending on the water pH, the product in the tank may have broken down and become ineffective.

Some labels bear statements that provide information on stability under specific pH levels.

Here are some examples:

  • Spray preparations are stable if they are pH neutral or alkaline and stored at or below 37.7˚C.
  • Do not let spray mixtures stand overnight.
  • Apply the spray the same day it is prepared, while maintaining continuous agitation.

The University of California at Davis has come up with the following guidelines based on water pH:

  • A pH between 3.5 and 6 is satisfactory for most spraying and short-term (12–24 hours) storage of most mixtures in a spray tank. Not suitable for sulfonylurea urea herbicides.
  • Most products mixed in alkaline water should be sprayed immediately.
  • A pH between 6 and 7 is adequate for immediate spraying for most pesticides. Do not leave the spray mixture in the tank for more than 1–2 hours, to prevent loss of effectiveness.
Maryse Bou Zeid


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