Mulch is one of the best things you can do for a tree. Done correctly, it retains moisture, moderates soil temperature, prevents competition from grass, and improves soil biology. Done incorrectly — in the pattern called volcano mulching — it slowly kills the tree.
What Volcano Mulching Is
Volcano mulching is piling mulch directly against the trunk and mounding it up — creating a cone shape with the trunk at the peak. It's the most common mulching mistake in residential landscaping, and it's everywhere.
Why It's Harmful
Mulch piled against the bark keeps the trunk chronically moist, creating ideal conditions for fungal decay, bark beetles, and voles. It also encourages girdling roots — roots that grow into the mulch, wrap around the trunk, and eventually strangle it. Girdling roots can kill a mature tree over 10–20 years.
The correct pattern is a donut, not a volcano. Keep mulch 3–6 inches away from the trunk, extend it out to the drip line if possible, and keep depth to 2–4 inches maximum.
The Right Mulch Depth
More is not better. Two to four inches of mulch is sufficient. Deeper mulch restricts oxygen exchange to the roots and can become hydrophobic when it dries out, actually repelling water rather than retaining it.
Best Mulch Materials
Wood chips from arborist work are among the best mulch materials available — they decompose slowly and improve soil biology. Dyed mulches are fine aesthetically but provide less biological benefit. Avoid rubber mulch around trees.