Danger: mulch volcanoes kill trees!

mulch volcano around tree
Mulch volcanoes may look attractive, but they can kill your tree. Photo by Connie Oswald Stofko

by Connie Oswald Stofko

Even if you’ve never heard of mulch volcanoes, you’ve seen them. They’re those neat, cone-shaped piles of mulch at the base of trees. They’re kind of pretty.

But like a real volcano, a mulch volcano is dangerous — it can slowly kill your tree.

Nobody seems to know how this trend started. The trend continues, I guess, because people copy their neighbors. And then the mulch volcanoes seem to be everywhere. If everybody is doing it, people think it must be right, and even more people create mulch volcanoes.

Unfortunately, mulch volcanoes can kill trees. The damage happens slowly, but I know you want to help your trees, not hurt them.

Read on to find out why mulch volcanoes cause damage, how you can undo a mulch volcano without damaging roots, and how to properly apply mulch to a tree. (Hint: Your mulch should look like a donut, not a muffin.)

Mulch volcanoes damage, kill trees

You may have been using mulch volcanoes for years without noticing a problem, but the practice can kill your tree slowly.

This short video from the Village of Oak Park Public Works in Illinois is great. It shows you the problem with mulch volcanoes and what to do about them.

stem girdling roots on tree
The roots of a tree are supposed to spread out from the tree, but here the roots are circling the trunk, which cuts off the flow of water and nutrients. Your tree can develop these roots, called stem girdling roots, if you use a mulch volcano. These roots were excavated and painted, which helps us see them better in the photo. Photo courtesy Linda Haugen, USDA Forest Service, Bugwood.org

Damage to roots

Too much mulch blocks the roots from getting sufficient oxygen, according to an article from Penn State Extension by Sandy Feather, Extension Educator. If the underlying soil remains too wet for too long a time, the roots begin to rot.

The excess mulch can also cause the tree to develop adventitious roots – roots growing from trunk tissue instead of true root tissue. These can develop into girdling roots, Feather said. Girdling roots can choke off the flow of water and nutrients to the tree.

Damage to bark

The purpose of tree bark is to protect the trunk, and tree bark likes to be exposed to air and light.

Piling mulch onto the bark creates a dark, moist environment, which rots the bark, allowing diseases and insects to attack the tree, according to a sheet on volcano mulching from Cornell Cooperative Extension in Westchester County.

The mulch volcano can also get hot as the mulch decomposes. (The same thing happens in your compost pile as plant material decomposes.) The heat can further damage the bark and the underlying layer of tissue that transports water and nutrients, according to Feather.

How to mulch trees properly

Mulching around your tree can be good for your tree — if you do it right. Mulch can keep down grass and weeds, prevent damage from lawn mowers, help the soil retain moisture and protect the tree roots against temperature extremes.

But applied incorrectly, mulch can slowly kill your tree.

How to fix mulch volcanoes

This video from the Village of Oak Park Public Works in Illinois shows clearly how to rake away the excess mulch from a mulch volcano without damaging the tree roots.

volcano mulch illustration muffin and donut
Volcano mulch looks like a muffin. The mulch around your tree should look like a donut— a circle around the tree with a hole in the middle where no mulch touches the tree. The tree shouldn’t look like a telephone pole sticking out of the mulch; you should see it flare out at the bottom. Illustration by Connie Oswald Stofko

Donuts, not muffins: properly mulch trees

  • The mulch should look like a donut, not a muffin.
  • Don’t pile the mulch in a cone shape. Spread the mulch out evenly.
  • Don’t let the mulch touch the trunk at all. Have a mulch-free zone for two or three inches away from the tree trunk. That will give it a donut shape.
  • Don’t use too much mulch. Your mulch should be only three to four inches deep. Even if you don’t pile the mulch in a cone shape, spreading the mulch too thick can damage the tree.
  • The trunk should flare out at the base, not look as straight as a telephone pole.

This video from the Morton Arboretum shows the proper way to mulch around trees.

For more extensive information, see The Cornell Guide for Planting and Maintaining Trees and Shrubs here.

7 Comments on “Danger: mulch volcanoes kill trees!

  1. Katherine, if the rocks are up against the tree, that may not give the bark the light and air circulation it needs to keep from rotting, so I do think you should pull them away from the trunk. Otherwise, I don’t know if the rocks would be a problem or not. You can contact the Master Gardeners in your area to find out the answer. You can find the contact information here. I hope that helps.

  2. Agree with Wayne; it’s the “professional” landscapers that promote this practice by their example. Separate question, we have a shallow ring of rocks at the base (and up against) a tree. Is that equivalent to a volcano?

  3. Wayne, I’m glad people aren’t using volcano mulch everywhere, but it seems to be going on in Illinois and Pennsylvania, too. You’ll notice that I’ve linked back to resources from those states. I hope people learn how to mulch in a way that helps the trees.

  4. It’s not just individuals doing this. Most every landscape service I observe does this as well. Here at Dockside Village where I live, the landscapers mulch (to early, of course) and volcano everything. I go out and remove it from the trees next to my apartment.
    I’ve lived in a lot of places, from Anchorage, Boise, San Diego, Albuquerque and now Buffalo. This is the only place I have seen it done.
    If you could get the landscapers to quit doing it, and nurseries would educate people better, maybe we could eventually eradicate this erroneous behavior.

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