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Mulch zones around new trees and shrubs are extremely helpful in protecting the young plant from mowers, string trimmers and other threats. However, mulch zones are often applied incorrectly, and actually contribute to the decline of the tree or shrub. The most commonly seen example of this is termed a "mulch volcano."
A mulch volcano occurs when mulch is applied too deep and pushed up against the trunk of the tree, as shown in the photo to the right. Mulch volcanoes can kill by:
Girdling Roots. If mulch is piled around the trunk, the roots will start to grow into it. These roots will then grow around the trunk in the mulch. As the root grows thicker, it pushes against the trunk, which is also increasing in size. These roots will eventually strangle the trunk and cause eventual death.
Onset of Disease & Decay. Mulch piled against a tree trunk tramps unnecessary moisture and decaying organic material against the bark. This often leads an the onset of disease and decay around base of trunk which prevents food and energy from traveling up and down the tree. It also creates an ideal environment for fungal and canker development in the trunk, leading to eventual death.
See the examples below for a comparison of a mulch volcano vs. a proper tree mulching. And say no to the mulch volcano!