How to Slowly Kill Your Tree, Part 2: Mulch Volcanoes

When planting a tree, dig a hole two to three times wider than the root ball, but only as deep as the ball itself.

Properly Mulch a Tree

When we see a newly planted tree, too often we see excessive amounts of mulch built up around the bottom of the sapling, a situation known as a “mulch volcano.” Though mulch can be great for a young tree’s future, this is the wrong way to apply mulch. 

It’s easy to think more is better, that a tree will be well-protected with a higher ring of mulch beneath it. While it’s true that mulch is important, homeowners routinely apply it incorrectly. Rather than piled high, it should be spread out wide in a flat layer of 2-3 inches that extends several feet out from the tree’s trunk. The shape of the mulch will actually influence the shape of the roots in the first few years as the growing tree expands and the roots grow outward beneath the soil.

Mulch Management

Mulch is typically made of pine bark or other types of wood chips, although it can be made of almost any organic matter, including leaves and grass clippings. The important feature is the porous barrier the mulch creates, suppressing growth of competing plants around the base of the tree, while allowing air and water to pass through and feed the roots below it. The layer of mulch will break down into the soil over time, so it should be reapplied as necessary and maintained for at least the first several years of a tree’s life. 

The major risk of piling up too much mulch around the tree is that it will suffocate the base of the trunk and the roots, since a heaping barrier of mulch blocks sunlight and oxygen from reaching the ground beneath it. This might kill the tree while it’s young. If it survives and the roots do grow, they may grow upward, out of the ground and into the mulch looking for nutrients (this is known as “girdling”). The result is a root system that grows around itself in a sphere, rather than spreading outward from the tree. These tangled roots are unable to reach outward for nutrients and water the way a properly formed root system would. The tree will then be less structurally sound since normal roots spread out 360 degrees around a tree, anchoring it in place. When a tree can’t get enough water or nutrients, the trunk and branches become brittle and, if it survives, the tree will be more prone to break, potentially damaging property or injuring someone. 

A Tree’s Security Blanket

While it seems like a harmless way of making sure the tree gets enough protection in its early years of growth, a mulch volcano can do a lot of damage primarily through suffocation and root malformation. When planting a tree, the long-term strategy is to protect the tree and its roots from damage, and mulch is an important way to do this. A single weak tree, in addition to being a hazard to its immediate surrounding area, is a weak link in the wider tree canopy, and enough gaps will result in less ecosystem services being performed in an area. Mulch management is an easy and effective way for individuals to contribute to a tree’s stable health, as well as that of the people and animals that make up the community around it.