Why a thick layer of mulch around the trunk can harm a tree

But mulch protects the soil and roots first and foremost, not the trunk itself. If it is pressed against the bark in a thick layer, it will no longer be beneficial. On the contrary, the base of the tree may remain moist for months, and a convenient shelter for rodents will appear beneath the pile.

Properly applied mulch is indeed beneficial. It helps the soil retain water more slowly and reduces temperature fluctuations. Over time, the organic material also decomposes gradually. But a thick layer isn’t necessary for this.

Experts at Penn State recommend applying organic mulch in a layer about 5–10 cm thick, leaving the base of the trunk open where it widens and transitions into the roots. On poorly drained soils, the layer should be thinner.

It’s easier to remember this: wider on the ground, not higher on the trunk.

The bark of the trunk is not designed to remain in a moist environment continuously. Mulch gets wet after rain and snow and retains moisture for a long time. If it surrounds the trunk tightly, the bark dries more slowly. Such constant moisture creates conditions for tissue damage and the development of rot and diseases.

It is especially undesirable to cover the area where the trunk begins to widen at ground level. In a properly planted tree, this transition should be visible, not hidden under a cone of bark.

There is a less obvious side to this problem. Roots need not only moist soil but also air. A very thick layer of mulch can slow down gas exchange and keep the soil too wet. This is particularly undesirable in heavy soil with poor drainage.

Additionally, roots sometimes begin growing upward into the constantly moist, thick mulch. Over time, roots may form around the base of the tree.

Therefore, the principle that “thicker is better” does not apply to trees.

A thick layer of mulch around the trunk creates a protected area where mice and voles can hide. In winter, when there is less food available, rodents may damage the bark. Ring damage to the trunk is particularly dangerous as it disrupts the flow of water and nutrients within the tree.

Penn State explicitly identifies providing shelter for rodents as one of the problems associated with mulch piled up around the trunk.

Another common scenario: in spring or autumn, the old bark no longer looks as neat, so a new layer forms on top. After a few seasons, a tall mound builds up around the tree, even though each individual layer seemed small.

Before adding new mulch, first check how much old mulch remains. Often, it’s sufficient to loosen the compacted upper layer and spread it out a bit. Applying new material on top of the old one each year eventually produces the same effect as applying a too-thick layer all at once.

It’s neither a cone nor a hill. Imagine a wide, flat ring around the tree. There’s no mulch right at the trunk, so the base is clearly visible. Further out, there’s a relatively even layer that spreads across the soil surface.

The wider the mulched area, the more beneficial it usually is for the root zone. Ideally, the material can be spread far beneath the canopy rather than concentrating the entire bag within half a meter around the trunk.

For young trees, such a wide ring also reduces competition with grass and protects the trunk from accidental damage from a lawnmower or trimmer.

There is no need to buy new mulch. Gently move the existing material away from the trunk and find where the tree’s base begins to widen. Then distribute the excess mulch further around the drip line, keeping a proper layer on the surface.

Often, the same amount of material is sufficient for both an improperly high mound and proper, wide mulching — it just needs to be redistributed.

But there is an important exception here. If you remove the mulch and the trunk base is still deeply buried in the soil, the tree might have been planted too low in the first place. Do not dig up the soil between the large roots on your own, as this could damage them. In such a situation, it is better to first assess the planting depth and consult a professional if necessary.

If the goal of autumn mulching is to help a young tree survive the winter, then the material is indeed useful. However, it is used to insulate the soil above the roots. A uniform layer helps reduce sharp temperature fluctuations and retain moisture. There is no need to lift the mulch up the trunk for this purpose.

Therefore, before the frost arrives, it’s better to check three things: whether the area around the base is mulched wide enough, whether the layer is not too thick, and whether the moist material is not touching the trunk itself.

If there is already a high pile of bark around the tree, the best autumn care may not be to add more, but rather to spread it out wider and clear the base of the trunk.