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A layer of mulch around the base of a tree will protect it from damage and reduce competition from turf grass. Grass aggressively competes with tree roots for available oxygen, moisture and nutrients, so maintaining a grass-free zone under a tree will benefit it.

The mulch will also prevent the bark of the tree from being damaged by mowers or string trimmers.

Ideally, the mulched circle would be as wide as the tree’s canopy, though this is not always practical in small lawns with large trees. If you prefer the look of grass growing under the tree, it still is advisable to have least a small mulched ring around the trunk to protect the bark from equipment damage.

However, be careful to spread the mulch in an even layer about 2 to 3 inches deep and do not heap it against the trunk of the tree. Even though you see it frequently, this is a bad practice that can cause problems. The landscape industry has a name for it: volcano mulching, because the pile looks like a volcano.

Mulch piled against the bark will trap excessive moisture, creating favorable conditions for decay. The bark needs to be exposed to the air to function properly.

When mulch is piled up, fungi, bacteria and insects can get into and under the bark and damage the tree’s interior. Insects can also be attracted to bark that is softening or partially decomposing from the excess mulch.

Excessive mulch also provides cover for mice and meadow voles that can eat the bark, cutting the flow of water and nutrients between the roots and canopy and potentially killing the tree.

Even if you spread mulch in an even layer to begin with, it can build up against the bark through routine annual edging. If soil is cut from the surrounding area and piled under the tree, it can gradually mix with the mulch and build up around the trunk.

The situation is even worse when a tree is planted too deeply. Trees that are planted too deep quickly become buried with routine mulching.

A tree trunk that resembles a telephone pole coming out of the ground is likely planted too deep. The trunk flare — where the trunk of the tree widens into the roots — should be visible. In heavy clay soils, I prefer to plant trees high, with the trunk flare 2 to 4 inches above grade.

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