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Vegetable IPM Updates Archive
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The Time Required for Systemic Herbicides to Kill Weeds (February 21, 2017)
Environmental conditions (rain, wind, frost and other factors) can make it necessary to apply herbicides at less than optimal times. Two common questions that arise are 1) How much time is needed for foliar applied herbicides to be rainfast and 2) How long do you have to wait after applying systemic herbicides before you can cultivate the weeds. The first question is fairly easy to answer. The time needed after application and before rain varies from as soon as dry to 6 hours. In almost all cases, 2 hours is adequate and unless it is raining at the time of application or soon after, it is not likely to effect herbicide performance. The second question is more difficult to answer. Systemic herbicides have to move into the plant and to the site of action. How long it takes to do this depends on the physical characteristics of the weeds and herbicide, growing conditions, environmental factors and other factors. It can vary significantly. Two of the most important variables will be how well the herbicide can pass through the leaves and how well the weeds are growing. Desert weeds typically have a thick cuticle on the leaves to conserve water. An adjuvant is commonly required to help the herbicide penetrate the leaves. Once the herbicide has entered the leaves it has to move to the site of action and it is important for the plant to be actively growing for this to occur. Anything that slows the growth will also slow the activity of the herbicide. If the plant is not growing at all, the herbicide may not have any activity on it. It is, therefore, difficult to nail down the time required before the weeds can be cultivated without reducing the effect of the herbicide. A good example is glyphosate. If the weeds are actively growing and an adjuvant has been used, 3 to 5 days is probably adequate. If growing conditions are not good or the weeds are stressed for some reason, it will take longer or not work at all.

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