Should I Be Using Mulch Around Trees?

Beware the Dreaded "Mulch Volcano"

mulch volcano around a tree
A "mulch volcano" emerging from an otherwise tranquil lawn. David Beaulieu

Homeowners frequently wonder, "Should I be using mulch around trees? And, if so, are there any rules to follow?"

Using mulch around trees -- particularly young trees -- can be beneficial. However, there are some do's and don'ts to keep in mind when using garden mulch around trees. 

The Don'ts

1. Don't fall into the trap of creating the dreaded mulch volcano, especially with young trees.

"Mulch volcano" is a derogatory nickname that has been coined to describe the excessive use of mulch around a tree.

You have probably seen them on people's lawns, since -- misguided as they are -- they are very common.

A mulch volcano is sometimes (but not always; see photo) the result of folks building circular raised beds around their trees, then filling the raised beds with mulch. The mulch gets steeper and steeper the closer it gets to the tree, which shoots out of the middle like a lava eruption. In such a mulch volcano, the mulch may be two inches high at the perimeter and six inches high up close to the trunk.

But not all mulch volcanoes can be blamed on raised beds. Take a look at the picture on the present page. You can see that this eruption is springing out of the grass, in the middle of a lawn.

Problems with mulch volcanoes include:

  1. Water runs off the sides of the mulch volcano and away from a young tree's base (which is where all of its roots are, for now), thus depriving it of water.
  2. Six inches of mulch is too deep. Much water that would otherwise reach the tree's roots gets trapped in the mulch.

    2. More generally speaking, don't mound up dirt or mulch around the trunks of trees, because:

    1. Such excessive tree mulching can suffocate roots.
    2. It even invites rodent pests and diseases.

    Piling up mulch against tree trunks can cause harm to your trees: it invites diseases and rodent pests, such as voles.

    If you are mulching around a tree, start tapering the height of the mulch down when you get to within about one foot of the trunk, leaving the base of the tree free of mulch. It would even be better to have to weed this one foot than to risk damage to your tree, would it not? Better safe than sorry.

    The Do's

    1. Do apply about two or three inches of mulch around trees, especially young trees.

    Mulching trees keeps down weeds, thus eliminating competition for water. In addition, much water that otherwise would be evaporated by the sun can soak down through a two-inch layer of mulch to the soil around tree roots. Mulching trees also helps keep their roots cooler in hot weather and helps prevent soil erosion in some cases (as when you are planting on a hillside).

    2. If you decide to apply a leaf mulch, use shredded leaves when possible in mulching trees, especially young trees. Bark mulches are more commonly used around trees. However, a shredded-leaf mulch has some advantages, particularly if you shred the leaves first. As an old cheapskate, I love the fact that this type of mulch is free. But I do advise that, when you rake up your leaves in fall, put them through a leaf shredder or leaf vacuum if you already own one; otherwise, shred the leaves by running the lawn mower over them.

    Leaves break down faster than bark, thereby releasing nutrients faster. Young trees, especially, will appreciate those nutrients. Of course, that is a double-edged sword, since faster decomposition means that you will have to replace the mulch sooner.

    If you do not wish to pay to have a load of bark mulch delivered and have your eye on some free wood chips that you see lying around, you may wonder if it is all right to use this material to mulch a tree. There is nothing wrong with using wood-chip mulches as long as you age them first. 

    Another consideration is how to water trees -- an important step in winterizing them.