Spring is a great time to mulch your trees, but not if you are using mulch "volcanoes." The practice of creating mulch volcanoes, while a popular landscaping practice, actually is most harmful to trees. Piling mulch at the base of a tree traps moisture around the trunk and root flare, leading to decay and often structural damage. Excessive and improper use of mulch can also rot bark surfaces and prevent deep supporting root development. Further, excessive mulch placed around the bark of trees prevents moisture from penetrating the roots. The best shape is a donut that keeps mulch several inches away from the base of the tree to avoid rot and diseases but encourages moisture to enter the root system.
Trees with mulched root zones are usually larger and more vigorous, develop faster, and have higher rates of survival than plants surrounded by turf grass or bare dirt. Mulches retain soil moisture, reduce erosion, and prevent soil compaction. Mulching under trees but away from the tree trunk results in fewer weeds to compete with tree roots for water. Soil under mulch is likely to stay warmer in winter and warms faster in spring, helping extend the growing season for roots.
Organic mulches are a favorite among professionals, who view wood chips as an effective, attractive mulch for trees. Avoid fine mulch, which can become matted and prevent penetration of water and air. A good mulch bed should extend out at least three feet from a tree's trunk in all directions, though extending to the drip line is preferred. The drip line is defined as the area where the tree branches end. The mulch depth should be 2-4 inches.
Don't keep adding new mulch on top of what is already under the tree. Mulch does decompose, but you don't want to accumulate excessive mulch year after year by adding fresh mulch every spring. If you want the look of fresh mulch, and there is a depth of mulch fewer than three inches under the tree, break up the old with a rake and add only a thin layer of new on top.
--Condensed and edited summary of a 2011 Town Topics article written by a local certified NJ arborist.