Volcano Mulching Killing Your Tree? How to Fix an 18‑Inch Mulch Mound

A homeowner discovered an 18‑inch deep “hill” of old mulch and soil around a 12‑foot black maple, with fine roots growing into the mound. Years of annual mulching without decomposition had created a classic “volcano mulch” scenario.

Why Deep Mulch Is Dangerous

Mulch deeper than 2–4 inches starves roots of oxygen, encourages adventitious roots that girdle the trunk, and holds moisture against the bark, inviting rot and pests. The root flare—the transition from trunk to roots—must be visible.

Step‑by‑Step Correction

1. Remove excess mulch by hand (gloved) down to the root flare. Do not use shovels or rakes; they sever fine roots.
2. If the mound is too large, hire a tree service with an air‑spade (pneumatic soil excavator) to blow away mulch without root damage.
3. Reapply fresh mulch 2–4 inches deep, keeping it several inches away from the trunk.

Long‑Term Mulching Rules

• Never exceed 4 inches total depth.
• Replace mulch only as it decomposes—don’t add fresh layers annually for color.
• Extend the mulch ring to the drip line if possible.

The maple should recover once the root flare is exposed and oxygen returns to the root zone.