How to Mulch a Tree—and How Not To

Mulching trees can be helpful, but the way many landscape crews apply mulch does more harm than good. Done properly, mulch has great benefits like retaining moisture in the soil, suppressing weeds, adding organic matter to the soil over time, and adding to the appearance of the landscape.

However, a short drive or walk around your neighborhood will probably reveal many more “mulch volcanos” than properly mulched trees. In fact, so many trees are surrounded by these “volcanos” that many people think that’s normal. Unfortunately, besides being an eyesore, mulching trees in this way actually harms them. Tree biology is quite amazing. The cambium just beneath the bark is the living, growing part of the tree. Because the living tissue of the trunk of the tree exists in such a thin area just under the bark, anything that girdles or damages this tissue threatens the tree.

When mulch is piled high around the base of the trunk of the tree, it holds moisture against this sensitive area. That risks fungal decay and rot. The tree will try to adapt, often growing adventitious roots that weaken and girdle the tree. These roots are unstable and don’t help anchor the trees. This situation invites pest infestations, which are much more dangerous to a tree that has been weakened this way.

So, what should you do? First, an unmulched tree is much better off than one that is surrounded by a “mulch volcano." When in doubt, skip the mulch. If your trees have been mulched in this way, a few minutes of work with a pair of garden gloves can solve the problem. Simply move the mulch away from the trunk of the tree, leaving an empty donut shape around the base of the tree. Make sure no mulch touches the trunk or the root flare. If you choose to mulch, 2”-4” of mulch is ample, avoiding the tree trunk, root flare, and any anchoring roots.