Mulching can reduce environmental stress by providing trees with a stable root environment that is cooler and contains more moisture than the surrounding soil. Mulch can also prevent mechanical damage by keeping machines such as lawnmowers and weedwhips away from the tree's base. Further, it helps to reduce competition from surrounding weeds and turf.
To be most effective in all of these functions, mulch should be placed two to four inches deep and cover the entire root system, which may be as far as two to three times the diameter of the branch spread of the tree. If the area and activities happening around the tree do not permit the entire area to be mulched, it is recommended that you mulch as much of the area under the drip line of the tree as possible (refer to diagram). When placing mulch, care should be taken not to cover the actual trunk of the tree. This mulch-free area, one to two inches wide at the base, is sufficent to avoid moist bark conditions and prevent trunk decay.
An organic mulch layer of two to four inches of loosely packed shredded leaves, pine straw, peat moss, or composted wood chips is adequate. Plastic should not be used because it interferes with the exchange of gases between soil and air, which inhibits root growth. Thicker mulch layers, five to six inches or greater, may also inhibit gas exchange.