Volcano Mulching

Do not pile mulch high around tree trunks

By: Sandy Feather ©2011
Penn State Extension

Q. It is my understanding that too much mulch around the base of trees is harmful to them, but I see piles of mulch around the trunks of trees everywhere I go. What is up with this "volcano mulching?"

A: I do not know how this fad started, but you are correct that it is not a good practice. I imagine that home gardeners see it in professionally managed landscapes and mistakenly believe it is the proper way to mulch.

Excessive mulch against the trunks of woody plants can cause the bark to rot. It is also a great place for certain insects and small animals such as mice to damage the bark without being noticed. Once the plant shows signs of injury, it may be too late to save it.

Why mulching against the trunk is bad for a tree

The cambium - the thin layer of light green wood just under the bark – is responsible for transporting water and nutrients from the roots to the leaves. It also transports the products of photosynthesis from the leaves to the root system for storage. When the cambium is destroyed, this transport system is disrupted. If damage is confined to one side of the plant, that side of the plant may die. If the cambium is destroyed around the circumference of the trunk - a condition known as girdling - the plant can die altogether. Young trees and shrubs have very thin bark, and are much more vulnerable to the damaging effects of excessive mulch.

Soil issues with excessive mulch depths

Another problem is that excessive mulch limits gas exchange in the soil. We do not often think about it since roots are underground, but they need oxygen as surely as you and I. I have seen declining trees make miraculous recovery once the excessive mulch is removed. Trees often try to survive excessive mulch and being planted too deeply by growing adventitious roots – roots that are growing from stem tissue rather than true root tissue. They are not sufficient to support the tree, and often become girdling roots. These are roots that grow around the trunk in such a way as to “strangle” the tree. Certain species of trees such as maples (Acer spp.), sweet gums (Liquidambar styraciflua) and lindens (Tilia spp.) have a tendency to develop girdling roots, and excessive mulch exacerbates that tendency.

Mulch volcano or was this tree planted too shallow?
Mulch Volcano or not?
This Magnolia appears to have a
"mulch volcano" but it may have been
planted partially above ground.

Yet another downside of excessive mulch is that it may become water-repellent once it dries out completely during hot, dry summer weather. Any rain we get during such weather may simply run off the surface of the mulch rather than benefiting the plants.


Properly applied organic mulches benefit all plants by conserving soil moisture, moderating soil temperatures and controlling weeds that compete for water and nutrients. As an added bonus, they add organic matter to the soil as they decompose. Mulching around the base of woody plants also keeps lawn mowers and weed whackers at a safe distance from their trunks. Young trees and shrubs are especially vulnerable to mower or weed whacker “blight” because their bark is very thin.

The right mulch depth

Two or three inches of mulch provide the benefits without endangering the health of woody plants. Mulch can be placed within a couple inches of the trunk or stems as long as it does not make physical contact. It is ideal to mulch trees out to the drip line (the ends of the branches) when possible.

The right mulch

Some of the best materials to use as mulch include shredded bark, wood chips, mushroom compost, composted sawdust, cocoa bean hulls and leaf mold. Avoid peat moss, whole leaves and uncomposted grass clippings because they tend to mat together, and once they dry out, they are difficult to rewet.

Fine-textured materials such as leaf mold, mushroom compost, cocoa hulls and finely shredded bark look better with herbaceous plants than coarse bark or wood chips because they are more in scale with the plants. Coarser textured bark and wood chips are better suited to woody plants.



Artillery Fungus


Photos of mulches



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