Mulch Volcanos – What Are They, And Why Are They Bad

One of the most common trends in urban landscaping over the last 20-30 years has been the mulch volcano. You know it, heck – you probably think it is the ‘way it’s done’. It is where someone piles up mulch in mound around the tree, deep enough where the trunk is buried.

Look, mulching any plant can provide a lot of benefits. It can help retain moisture, and provide nutrients as the mulch breaks down. It can also help prevent soil compaction. But like so many things in life, too much of a good thing can become bad. And this applies to mulching our trees.

Trunk and root rot

Even though it is common, it is actually a bad thing! Mulch will retain moisture, and high volumes of mulch will be constantly moist below the surface. But this moisture retention, when in contact with trunk bark is bad. As when the mulch is piled high enough, it will actually start to rot the trunk bark. It can even rot the root bark if the soil drains poorly when combined with too much mulch.

Now, the bark of trees is actually similar to our skin in that it is a protective layer that keeps diseases and parasites out. If mulch is piled too high around the trunk, and the bark breaks down, well – now you will have a vector for many diseases and other harmful pests like wood boring beetles. Fungus can eventually enter the tree, and slowly rot it until it dies.

Depending on the amount of mulch, in extreme cases the mulch can begin to hot-compost. This composting will cause the bark to break down even faster.

Root girdling

One other problem that can occur when a tree has a ‘mulch volcano’ is that the roots don’t always grow properly. Being surrounded by the moist mulch can make new roots sprout from the trunk. These adventitious roots will then grow around the tree trunk, not out. Roots go where they find moisture, and if roots are sprouting above the root flare, then they may continue to grow around the mulch volcano rather than extended outwards. Over time, this can result in the girdling of the tree trunk.

Roots that strangle a trunk can be a real problem. As the trunk tries to increase in diameter, it will be restricted at the lower portion. This can create a top-heavy situation that over time could make the tree toppling over more likely.

Proper mulching technique

If you want to mulch around your trees (and you should), then stay away from the trunk! Apply a ring of mulch starting 3-6″ away from the trunk, and make it about 3″ deep. To make the job easier, try sheet mulching so you don’t have grass growing through the mulch. And then, carefully make a small trench to stop the grass from creeping back into the mulch.

Why do people still make them?

If mulch volcanoes are so bad, why do people still do it? Well, there are a few reasons. First, many people initially hire professional landscapers. And, it is much faster to just dump a bunch of mulch around a tree rather than carefully spreading it out in a larger ring. After that, most homeowners just don’t know, and they keep reapplying mulch each year out of a sort of inertia rather than reason and purpose. Also, it has become common enough that most people just think that it is the proper method without thinking critically about it. And once a ‘trend’ sets in, it is hard to break. Personally, I think they look stupid. A large mulch ring looks very out of place, as it isn’t something you would naturally encounter in a forest.

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