Heaping mulch around the base of a tree (called volcano mulching) is NOT healthy for the tree and can cause death to the tree. Brochures with information about volcano mulching have been mailed with city water bills.
Thanks to the Ontario County Soil and Water Conservation District for this information about mulching trees:
Help Protect Our Trees
Mulch surrounding a tree looks tidy and makes mowing around the tree easier. However, too much mulch is bad for trees. Mulch piled high around the trunk of a tree is an invitation for unnecessarily early death and replacement of the tree.
Volcano Mulching Consequence
If the tree is planted at the proper depth, adding mounds of mulch above is like adding soil. Tree roots depend upon gas exchange with the atmosphere to thrive.
Volcanoes of mulch encourage the tree to extend roots into the mulch layer instead of rooting more deeply into the soil. Later in the summer, when loose mulch dries out, those roots will be the first to die. Likewise, shallow roots may girdle the tree trunk and impede growth.
Fungus & Rodents
Piles of mulch make an ideal place for decay fungus to thrive. Crown rots and root rots flourish in large layers of mulch. Mice, moles and voles find homes in mulch volcanoes - and find tree roots make tasty meals.
Prevent Volcano Mulching
Mulch is best applied 2 inches deep, and should not touch the trunk of the tree. Think of creating a "shallow doughnut" instead of a "volcano": it will save your money and our trees.