Mulch Volcanoes: What They Are and Why You Should Never Have Them

​Mulch is a common landscaping tool used to define garden beds, reduce weed growth, and of course, encircle trees. It comes in many styles and colors, and can provide beneficial water retention and nutrients to trees both young and old. But with great benefits comes great responsibility! Improperly applied mulch, and especially mulch volcanoes, can cause much more harm than good.

​What Are Mulch Volcanoes?

​A “mulch volcano” is a pile of mulch at the base of a tree that’s piled up against the trunk in a mound that often resembles a volcano. They can often spell the end for many unlucky trees, as they break the two cardinal rules of proper mulch application: keep it thin, and never touch the trunk.

​Keep It Thin

​The ideal depth for mulch is 2 to 4 inches, depending on what you’re mulching around. Smaller plants in garden beds will prefer closer to 2 inches of mulch, while larger plants like big shrubs and trees need closer to 4. 

​Mulch volcanoes are often piled much higher than 4 inches – sometimes even up to a foot, or two, or more! Mulch that deep retains way too much water, creating an always-damp environment that can encourage rot and decay.

Deep mulch like mulch volcanoes also retain too much heat during the decomposition process. Thinner spreads of mulch don’t generate enough heat to cause any problems to the underlying root system, and the decomposing leads to rich nutrients for the soil. But mulch volcanoes can create heat wells that can smother young roots.

​Lastly, too much mulch doesn’t allow for airflow. Without airflow, the soil won’t have enough oxygen to support a healthy root system. 

​Never Touch the Trunk

​All the problems deep mulch can cause to roots, even thin mulch can cause to a trunk. Mulch volcanoes come in all shapes and sizes, and some are shallow enough that they don’t exceed the 2- to 4-inch recommendation for mulch thickness. But if the mulch crowds up against the base of the tree, then it’s still a volcano, and it’s still a problem. 

Mulch’s moisture retention, when pressed against the trunk, will hold that moisture against the bark, encouraging decay. Mulch’s decomposition-generated heat, even in a thin layer against the trunk of the tree, can hold damaging heat to a sensitive vascular system. And mulch’s smothering ability can keep airflow from the trunk and bark.

But a mulch volcano up against a tree trunk can also cause a special problem: girdling roots. When there’s a lot of mulch around the base of a tree, it might get confused, and start growing roots out of the trunk (this is called adventitious roots – think of it like roots on an adventure!). 

When these roots reach the edge of the mulch volcano, they won’t just start growing into the air; instead, they’ll curve and follow the edge of the volcano. These roots can create a ‘girdle’ around the tree, and they can restrict the flow of water and nutrients, which can cause a lot of problems for the tree, up to and including complete failure.

​Why Are Mulch Volcanoes Common? 

​With such detrimental consequences for the tree, you may be wondering why mulch volcanoes are so common. Sometimes, they happen by accident – as mulch decomposes, it has to be refreshed. It’s much easier to just spread new mulch on top of the old mulch, rather than raking away the old stuff. A few rounds of this, and suddenly, you have mulch that’s way too thick and crowding the base of the tree. 

But the main reason is a combination of ignorance and convenience: it’s much easier to just dump a wheelbarrow full of mulch around the base of your tree and be done with it, rather than taking time to rake it into an even, thin layer, not to mention pull it away from the trunk. How bad could it be, really? And if you don’t even know that mulch volcanoes kill, it would never cross your mind to go to that extra effort.

​Many people also believe mulch volcanoes are actually the proper way to mulch around a tree, usually because they either saw their neighbor do it, saw it around town (city-maintained trees are notoriously mulch-volcanoed), or even saw a professional landscaping company do it (some companies don’t take the time to mulch the right way). The misconception spreads fast.

​How to Properly Apply Mulch

​So now you know the dangers of mulch volcanoes, and you want to spread your mulch the right way. Whether you're planting a new tree or sprucing up an existing one, it’s really easy! Just follow the two cardinal rules: 

Keep it thin: a 2- to 4-inch depth is all you need. Spread your mulch out evenly around the base of the tree, preferably as far out as the canopy of the tree goes. In the case of young trees and saplings, a few feet from the base will do, and you can expand as the canopy grows. 

Never touch the trunk: rake the mulch away from the trunk to create a well around the trunk flare. Just a couple inches of space between the mulch and the base of the trunk flare should do.

Pro tip: to prevent accidental mulch volcanoes, when it’s time to refresh your mulch, rake through the old layer to loosen it up, and then add a thin layer of fresh material, making sure the total depth of both layers stays between 2 to 4 inches! 

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Author

Emily Casuccio is sister and sister-in-law to Rebekah and Scott Rushing, and has worked with Classic City Arborists for the last 5 years. In addition to handling administrative tasks and customer communication (she’s the voice you’ll hear on the phone!), Emily is an experienced copywriter, editor, and storyboard consultant. To learn more about her, read samples of her work, or contact her, visit her online portfolio.

Sources:
PennState Extension
Arbor Day Foundation