Mulch Volcanoes Are Killing Your Trees

Drive through any neighborhood in Fort Wayne or Marion and you will see them. Mulch piled up against tree trunks in big cone shapes, sometimes a foot high or more. Landscapers do it. Homeowners do it. Garden centers sell mulch by the cubic yard and people pile it on. But this practice, called mulch volcanos, is one of the most damaging things you can do to a tree. And it is everywhere.

Why People Do It

It looks neat. That is pretty much the entire reason. A thick mound of fresh mulch around a tree makes the landscape look tidy and cared for. Some landscapers pile mulch high because the customer thinks more mulch means better care, and nobody complains about a job that looks impressive.

The problem is that what looks good is actively harming the tree. Slowly and quietly, year after year, until the tree declines and eventually dies. And because the damage happens over 5 to 15 years, nobody connects the mulch to the tree's death.

What Mulch Volcanoes Do to Trees

Bark rot. Tree bark is designed to be exposed to air. When mulch is piled against the trunk, it holds moisture against the bark constantly. This creates conditions for fungal decay to attack the bark and the living tissue underneath (called the cambium). Purdue Extension explains that once decay enters the trunk through compromised bark, it can spread internally and weaken the tree's structure.

Girdling roots. When the root flare (the area where the trunk widens into the root system) is buried under mulch, roots sometimes grow up into the mulch layer instead of outward into the soil. These roots can circle around the trunk and gradually strangle it, cutting off the flow of water and nutrients. The International Society of Arboriculture identifies girdling roots as one of the leading causes of premature tree death in urban landscapes.

Rodent habitat. Thick mulch piled against the trunk provides shelter for voles and mice. These rodents gnaw on the bark during winter, which can girdle (ring-bark) young trees and kill them. Purdue Extension's wildlife guides note that vole damage to young trees is a common and preventable problem.

Root suffocation. While mulch at the right depth improves soil conditions, excessively deep mulch can reduce oxygen exchange in the root zone. Ohio State Extension warns that mulch deeper than 4 inches can create anaerobic (oxygen-deprived) conditions that stress and kill roots.

How to Mulch a Tree the Right Way

Mulching is great for trees when done correctly. Purdue Extension, the ISA, and every major university extension program recommends mulching with these guidelines:

Depth: 2 to 4 inches. That is it. Not 6 inches. Not 8 inches. Not a foot. Two to four inches of mulch provides all the benefits (moisture retention, temperature moderation, weed suppression) without the drawbacks.

Shape: A flat donut, not a volcano. Spread mulch in a wide ring around the tree, but pull it back at least 3 to 6 inches from the trunk. You should be able to see the root flare, that area where the trunk widens at the base. If the trunk goes straight into the mulch like a fence post, the mulch is too close.

Width: As wide as practical. The wider the mulch ring, the better for the tree. A ring extending 3 to 6 feet from the trunk is ideal. The ISA notes that a wider mulch ring reduces competition from grass (which competes aggressively with tree roots for water and nutrients) and provides more of the soil benefits that mulch offers.

Material: Shredded hardwood or composted wood chips. These are the most common and most practical mulch materials in northeast Indiana. Avoid dyed mulch when possible, as the dye does not benefit the tree and can contain unwanted chemicals. Iowa State Extension notes that composted wood chips provide the best combination of moisture retention and nutrient cycling.

How to Fix an Existing Mulch Volcano

If your trees already have mulch piled against the trunks, fixing it is straightforward but important.

Pull mulch back from the trunk. Clear all mulch away from the bark until you can see the root flare. If the root flare is buried below soil level (from years of mulch decomposing and turning into soil), you may need to carefully excavate down to find it.

Reduce the depth. Rake the mulch ring down to 2 to 4 inches. If there is more than that, remove the excess.

Check for girdling roots. While you are clearing mulch from the trunk base, look for roots that are wrapping around the trunk. Small girdling roots can sometimes be cut, but large ones require an arborist assessment. Do not try to cut a root that is tightly pressed against the trunk without professional guidance.

Stop adding new mulch on top of old mulch every year. This is how volcanoes build up. If the existing mulch layer is still 2 to 3 inches deep, it does not need more. Only add mulch when the existing layer has broken down to less than 2 inches.

What to Tell Your Landscaper

If you hire someone to mulch your beds, tell them explicitly: no mulch against the trunk, 2 to 4 inches deep, and pull it back from the root flare. Show them what you expect. Some landscapers default to mulch volcanoes because that is what customers have come to expect, and changing the habit requires clear direction.

A good landscaper will appreciate the instruction. They know mulch volcanoes are wrong. They just do it because nobody tells them to stop.