Mulch Volcano Easy to Prevent Difficult to Fix

Mulch Volcanoes: A Hidden Threat to Seacoast Trees

Take a drive through the Seacoast, and you’ll likely notice it: trees with their trunks buried under tall, cone-shaped piles of mulch that look like little volcanoes. To a casual observer, these “mulch volcanoes” might seem neat and professional, a quick way to make a yard look finished. But to gardeners and arborists, they’re a serious red flag. What appears tidy on the surface is actually one of the most harmful practices for trees and shrubs. At Seacoast Gardener, we’ve seen mulch volcanoes showing up everywhere—from residential yards in Rye and Hampton to commercial properties in Portsmouth and even across Southern Maine. In this article, we’ll explain what mulch volcanoes are, why they quietly damage trees over time, and how to mulch correctly so your landscape thrives for decades.

What Exactly Is a Mulch Volcano?

A mulch volcano forms when mulch is piled high against the base of a tree or shrub, rising in a cone that covers the lower trunk. Instead of spreading mulch out in a broad, shallow layer across the root zone where it nourishes the soil and conserves moisture, the material is stacked vertically, smothering the tree’s natural flare. To the eye, it may look tidy—like a crisp cone hugging the trunk—but to the tree, it feels more like trying to breathe with a sweater pulled up over your nose. Trees are meant to show their root flare and keep their bark dry. When buried under layers of mulch, both are hidden, setting the stage for stress and decline.

Why Do People Create Mulch Volcanoes?

If mulch volcanoes are so harmful, why do we see them everywhere? The answer lies in appearance, habit, and a bit of misinformation. For many homeowners, the cone shape has an aesthetic appeal. It looks clean and “finished,” like the tree has been neatly tucked into the landscape. The problem is that looks can be deceiving. There’s also a widespread misconception that mulch should be pressed right up against the trunk to protect or “feed” the tree. In reality, piling mulch this way does the opposite—it weakens the bark and suffocates the roots.

The Hidden Dangers of Mulch Volcanoes

Mulch volcanoes may look harmless, but the problems they create unfold slowly and often invisibly. By the time you notice symptoms, the damage is already deep. Here’s why piling mulch against your trees is one of the worst choices for long-term plant health.

1. Bark Rot and Disease

Tree bark is nature’s armor. Above ground, it’s designed to stay dry and tough, shielding the tree from insects and pathogens. When mulch is piled high against the trunk, that armor stays damp, softens, and begins to rot. This decay becomes the perfect entry point for fungi, bacteria, and wood-boring insects. The infection doesn’t strike all at once; it creeps into the trunk slowly, hollowing out the tree’s strength. From the outside, the bark may look normal for years, but inside, the foundation is weakening.

2. Girdling Roots

When mulch covers the trunk flare, trees can’t “breathe” properly. In response, they often send out new roots into the mulch itself. Instead of spreading wide into the soil, these roots coil tightly around the trunk like a noose. Over time, these girdling roots cut off the flow of water and nutrients through the tree’s vascular system. The tragedy is that the tree may look perfectly fine for a decade, only to suddenly begin declining with no obvious cause—when in fact, it has been strangling itself underground for years.

3. Suffocated Roots

Healthy roots thrive in soil that has a balance of moisture and air. Roots take in oxygen from the tiny spaces between soil particles, which is just as vital as the water they absorb. But a thick, compacted cone of mulch seals off the soil surface like a lid. Air can’t penetrate, water stagnates, and roots suffocate. A tree deprived of oxygen begins to lose vigor, showing up as yellowing leaves, stunted growth, or branches that die back prematurely.

4. Pest Havens

 Mulch volcanoes don’t just smother trees—they also create the perfect shelter for pests. Rodents such as voles and mice love the soft, damp covering because it hides them from predators while giving them easy access to chew on bark. Insects, including borers, ants, and even termites, thrive in the warm, moist environment pressed against the trunk. Once established, these pests quietly eat away at the tree, weakening it from the inside out. By the time you notice chewing marks or sawdust, the infestation is already advanced.

5. Stress and Decline

The final and most insidious danger of mulch volcanoes is cumulative stress. Rarely does a tree die immediately from this practice. Instead, the effects build year after year—rot, girdling, suffocated roots, and pest damage all combine to weaken the tree’s natural defenses. A tree that should have provided shade and beauty for 50 years or more instead limps along for 10 or 15, never reaching its full potential. The end result is not just the loss of a tree, but wasted time, money, and effort that could have been avoided with proper mulching.

Spotting the Root Flare

One of the golden rules of tree care is simple: the root flare should always be visible. The root flare is the slightly swollen base of the trunk where it widens and the first roots extend outward into the soil. It’s the natural transition point between trunk and root, and it should never be hidden.

If you can’t see this flare, it usually means one of two things—your tree was planted too deeply, or it has been buried under mulch. Either way, the tree is at risk. Think of it like your collarbone: if it’s covered by too many layers of clothing, you know something isn’t quite right. Trees give the same signal—the flare should show, not vanish beneath a mound.

The Right Way to Mulch

Mulching correctly is not complicated, but it does require letting go of the habit of building “volcanoes.” Proper mulching is about protecting roots and feeding the soil—not hiding the trunk.

• Keep mulch away from the trunk. Always leave a mulch-free ring (a “doughnut hole”) around the base of the tree, at least 3–6 inches wide. The trunk should breathe freely.

• Apply mulch in a wide, even layer. Spread mulch 2–3 inches deep across the root zone, ideally extending as far as the tree’s canopy (the “drip line”) if possible. This mimics how organic matter naturally falls and breaks down in the forest.

• Refresh, don’t pile. When mulch decomposes, rake and fluff it to restore airflow. If you need to add more, remove any excess first rather than dumping new layers on top.

• Choose the right materials. Shredded bark, wood chips, compost, and leaf mold are excellent choices. These natural mulches enrich the soil and decompose safely. Avoid dyed or chemically treated mulches, which offer little benefit and may harm soil life.

Conclusion

Mulch volcanoes may look like a quick landscaping fix, but they cause lasting harm that quietly shortens the life of your trees. What seems tidy today leads to rot, pests, suffocated roots, and long-term decline. The good news is that proper mulching is simple—and it makes all the difference. Always keep the root flare visible, leave a mulch-free ring around the trunk, and spread mulch wide and shallow across the root zone. With this approach, you’ll protect your trees, strengthen their defenses, and enjoy healthy, thriving landscapes for decades to come.