Proper mulching as with most things, too much of a good thing can be harmful. Placing heavy amounts of mulch around and up against a trees trunk can cause excessive moisture and decay problems and encourage rodent damage. The practice of over mulching can be very harmful to the trees. Mulch that is deeper than 4 inches may cause the roots to grow upward into the cool moist mulch causing a shallow, vulnerable root zone. Building boxes around tree and filling the boxes with dirt and various types of mulch can also be harmful to your trees. The planting of flowers around new trees (3 to 5 years after transplant) is not recommended as the flowers are taking valuable nutrients away from the trees.
Proper mulching materials include wood chips, shredded bark, ground corn cobs, and composed materials (stone and gravel mulch are not recommended). Place a 3 to 4 inch deep layer of one of the recommended mulch materials over the root zone. Do not allow the mulch to accumulate around the trunk of the tree.
As organic mulch decomposes it will utilize some of the available nitrogen in the soil. To compensate, an annual application of high nitrogen fertilizer such as lawn fertilizer, sprinkled around the drip line of the tree is recommended.