How to Mulch Around Trees

Mulch volcano around a tree
A "mulch volcano" like this one is to be avoided. 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.

Avoid Mulch Volcanoes and Collar Rot

"Mulch volcano" is a derogatory nickname that describes the excessive use of mulch around a tree. You have probably seen them on people's lawns and in public spaces, since, misguided as they are, they are very common.

A mulch volcano is usually 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 an arrangement, the mulch maybe two inches high at the perimeter and six inches high up close to the trunk. You might also see a mulch volcano springing out of the grass in the middle of a lawn.

There are several problems with mulch volcanoes as well as dirt mounded around trees:

  • 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.
  • Six inches of mulch is too deep. Much of the water that would otherwise reach the tree's roots gets trapped in the mulch.
  • Excessive mulching can suffocate a tree's roots.
  • Deep mulch can invite rodent pests (such as voles) and diseases.

The biggest issue with improper mulching is that it retains moisture at the base of the tree, which will introduce collar rot. Collar rot occurs at the soil line where the plant emerges. The smartest thing you can do when you mulch is not let any mulch touch the trunk of the tree.

Benefits of Mulching Around Trees

Trees, especially young ones, typically benefit most from a 2- to 3-inch layer of mulch around their base. However, for the first 12 inches immediately surrounding the trunk, thin the mulch layer so that it just covers the soil. This will prevent the common problems associated with excessive mulching. Mulch should never touch the trunk of the tree.

Mulching trees keeps down weeds, thus eliminating competition for water and minimizing the number of encounters with a lawn mowers or trimmers that could damage the trunk. In addition, much of the water that otherwise would be evaporated by the sun can soak down through a 2-inch layer of mulch and into the soil around tree roots. Mulching trees helps insulate roots from extreme temperatures, keeping roots cooler in hot weather. It also helps prevent soil erosion in some cases, such as when on a hillside. Mulch works as a slow-release fertilizer, which is especially helpful when you clean up leaves from the ground in the fall.

Types of Mulch for Trees

You can mulch around trees with standard bark mulch, or you can use wood chips or shredded leaves. There is nothing wrong with using wood-chip mulches as long as you age them first. If you decide to apply a leaf mulch, use shredded leaves whenever possible because they break down more quickly than whole leaves, thus releasing nutrients more quickly. This is particularly beneficial to young trees. The only drawback is that faster decomposition means you will have to replace the mulch sooner.

When you rake up your leaves in fall, put them through a leaf shredder or leaf vacuum. Otherwise, shred the leaves by running the lawn mower over them and collecting them in the mower bag.

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