Vermont Master Gardeners
Mulching is one of the best things you can do for the health of trees or shrubs.
Benefits of Mulching
Apply mulch starting 6 inches from the base of the tree working out
to the desired diameter. Depth should start at 1 inch at the inner circle,
increasing to no more than 4 inches (2 inches for clay soils) at the outer
edge of the circle. (See diagram below.) Final depth may be reduced if
landscape fabric is placed under the mulch. Also consider using groundcover
plants, such as Vinca or Pachysandra, as a mulch alternative.
Dangers of Improper Mulching
Excessive mulch material piled up against the base of a tree or shrub, a mulch “volcano,” keeps moisture in direct contact with the bark. The moisture penetrates the bark and suffocates the cells of the phloem, which is the layer of living tissue that transfers food up and down the plant. When this supply of food from the leaves is limited, the roots die back, which leads to less water being taken up, and the tree or shrub goes into general decline, leaf drop, and premature death.
Secondary problems, like borers and fungi, move into plants weakened by improper mulching. In sugar maples, the fungal pathogen Phytopthora will move in because of the high moisture around the trunk, creating a canker that girdles the trunk at the base and accelerates the decline of the tree.
Greg Lowenberg provides the following caveat about mulch volcanoes:
Although mulch volcanoes often keep the area near the trunk too moist, sometimes the opposite happens, and thickly applied mulch keeps the soil underneath too dry. If the mulch layer is too thick, light irrigation or rainfall won’t penetrate through to the soil, and feeder and secondary roots may migrate up into the mulch looking for moisture. These adventitious roots grow just above the main roots, and when they reach the drier edge of the mulch volcano, they begin to circle the trunk and may press against the primary roots that spread just under the surface of the ground, eventually cutting off their feeding and anchoring functions and destabilizing the plant. When the upper layer of mulch dries out, these roots become stressed. Water may also have a hard time reaching roots because an impenetrable layer of fungus sometimes forms in a large, deep mulch volcano. In this case, water is shed onto the surrounding ground and won’t reach the plant if its roots have not grown out beyond the circumference of the volcano.
What to Do with Improperly Mulched Trees and Shrubs
Remove excess mulch using a shovel, trowel, or whiskbroom while taking
care not to injure the trunk. A hard stream of water may be used to remove
excess mulch and soil from the trunk and flare. Cut off secondary roots
that may have grown into the mulch. Trunk and flare should be visible.
New mulch can then be applied to the proper depth and distance from trunk.
(See diagram above.)
University of Vermont Extension and U.S. Department of Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin, gender, religion, age, disability, political beliefs, sexual orientation, and marital or familial status.