Bark piled against trunks can quietly shorten a tree’s life, experts warn

A newly mulched garden might appear neat. However, according to the professionals, there is one popular landscaping practice that could potentially harm the trees without anyone even realizing it. Piling mulch up right against the trunk of the tree, known as a "mulch volcano," traps unwanted moisture in areas that could potentially promote decay and infestation.

As per information provided by the University of Minnesota Extension Service, no amount of mulch should ever be piled against the base of the tree. It is suggested to always create an area of about 6 inches around the trunk without any mulch, which would allow the bark to stay dry and open.

Why the trunk needs breathing room

In reality, mulch is intended to benefit the soil, not the tree itself. As noted in the University of Minnesota Extension publication on mulching soil and gardening, the use of bark and wood chips is beneficial since they contribute to improving soil structure and stability and minimizing nutrient leaching. However, all these advantages can only be achieved when mulch is applied to the soil surface, not when it is pressed against the tree trunk.


In contrast, the trunk flare – the part where a tree's roots branch out – requires ventilation and dryness to operate correctly. When the trunk flare is covered with bark chips, moisture stays there for a prolonged period, resulting in a conducive environment for tree decay and pests.

As recommended by the University of Minnesota Extension, maintaining space between the mulch and tree trunks helps avoid such problems. Even though most homeowners utilize mulch as a decorative tool, experts suggest that appearance should not be prioritized over plant health.

Why “mulch volcanoes” are a problem

Mulch volcanoes have gained popularity in suburbia because of the attractive, finished appearance of the mounded mulch pile. However, warnings from the perspective of professional arborists and gardeners continue to emerge concerning such a practice.

There is nothing wrong with mulch. Indeed, bark mulch and wood chips are thought to be great for trees and shrubs since they take longer to decompose and last longer in the process. Things start going wrong once too many mulches accumulate around the base of the plant.

As per the University of Minnesota Extension's watering guide for mature trees and shrubs, living plant tissues control processes related to growth, blooming, and photosynthesis; hence, they need healthy conditions near the trunk and roots. Wetting mulch right next to the trunk leads to stress in the tissues and, therefore, to damage that cannot be diagnosed until the tree suffers from some negative effects.

How mulch actually helps trees

Properly used, mulch remains among the most valuable resources for creating and maintaining healthy landscape beds. Experts argue that the application of mulch contributes to regulating the temperature of the soil, maintaining moisture levels, and increasing the fertility of the soil.

According to the University of Minnesota Extension, proper mulch choice should be based on such factors as the functionality of the landscape, the types of plants growing in the bed, and the objectives pursued by the garden owner in the long term.

As a result, homeowners need to approach not only the visual but also the functional aspects of updating their gardens' landscape. Among the most common kinds of mulch, wood chips and shredded bark have proven to be very durable and contribute to improving soil fertility.

However, the most beneficial landscape beds created using mulch tend to be much less dramatic than expected. Namely, gardeners should avoid creating cones around the trunk and leave a clear area surrounding it. According to experts, this gap represents a properly made mulch bed.

The simple fix homeowners can make

Fixing this issue will not entail removing the whole mulch. For most cases, the best thing is just to move back the bark from the tree trunk and then level off any raised beds surrounding the tree.

There should be at least 6 inches clear from the tree trunk that is free of mulch to prevent the accumulation of moisture and insects against the bark of the tree. The rest of the mulch will still stay on the plant bed. One other thing that experts say is that people should always check their garden landscape beds from time to time, since mulch can easily migrate over the year.

What gardeners should look for

The best way to recognize an issue is by checking the trunk of the tree. If it appears to sink beneath a thick blanket of bark, then the mulch is positioned improperly. Similarly, a steep mound of mulch around a tree trunk rather than a ring indicates the need for some adjustments.

As per the recommendations from the University of Minnesota Extension, proper mulching involves finding the right balance. The mulch should serve to protect the soil, yet should not completely cover the trunk.

For people wanting to have a perfectly groomed garden, balancing is critical. A good mulch bed should not require climbing the trunk to look right. In fact, the advice is that simplicity is more effective for the life of the tree.