Pesticides applied to a lawn right before substantial rain or irrigation may not be effective in controlling the target pest. As an example, a herbicide that controls growing broadleaf weeds in a lawn needs to stay in contact with the weeds' leaves after application. Rain or irrigation too soon after the herbicide application removes the product from the weed and greatly reduces its effectiveness. Don't apply pesticides if a rain is forecast within 24 hours of application. This is especially important when spraying fungicides on fruit trees or herbicides on growing lawn weeds.
Wind can drift pesticides to nontarget sites.
An example is a lawn herbicide that blows onto a neighbor's fruit or vegetable garden. The amount of pesticide lost from the target area and the distance it moves both increase as wind velocity increases. Such wind is a gentle breeze that produces constant but light motion of twigs and leaves. Also, keep in mind the wind direction. Once applied, a pesticide may move off the intended site if good management practices are not followed.