It is important to apply weed and feed during the time when weeds are actively growing. The granules must stick to the leafy portions of the weeds, so application should take place when the lawn is damp, but when there is no expected rain for the next couple of days.
The herbicide must have time to absorb into the leaves, so rain or watering during this period prevents the weed and feed from working. If the granules do wash away, it is important not to reapply, as this can cause overfeeding or chemical scorches on the lawn. The application should take place a couple days after mowing the lawn, when the grass is a normal height, approximately 3 to 5 inches.