Avoid using grass clippings as mulch immediately after pesticide application and mowing. They should be composted first. Most widely used lawn-care insecticides and herbicides break down rapidly during composting or become tied to the organic matter in the compost. A better option is to leave the grass clippings on the lawn. The clippings will gradually break down, improving soil conditions and reducing the need for fertilizer. Clippings usually do not contribute to thatch buildup, because they decompose readily. If the clippings form clumps on the surface of the lawn, you may need to rake off the excess, or mow again to evenly distribute the clippings.