Plants often have excessive surface roots when they lack oxygen. Heavy clay or waterlogged soils, or plastic sheet weed barriers prevent oxygen from entering the soil. Shallow roots make the plant more susceptible to drought. Water less frequently but for a longer period of time, and avoid the use of plastic sheet mulch.should not be too deep around the main stem of a plant, often 2-4 inches is best. Much more than this prevents air movement and harbors insects that can cause stem and root rot.