1. Piling mulch into a “volcano”
Mulch heaped up against a tree trunk is one of the most common sights in landscaping, and one of the most damaging. University of Illinois Extension and UConn Extension both warn against it: mulch against the trunk keeps the bark constantly moist and dark, which invites rot, insects and disease, and can encourage roots to grow up into the mulch and circle the trunk.
Do this instead: spread mulch in a wide, flat ring like a doughnut, and leave a bare circle a few inches across around the trunk itself.
2. Making the layer too deep
More is not better. The University of Illinois Extension recommends 2 to 3 inches. Thicker layers can keep water and air from reaching roots, and fine mulches can mat down into a crust that sheds water.
Do this instead: keep it at 2 to 3 inches. Our mulch calculator works out exactly how much to buy for that depth.
3. Adding more every year without checking
Many landscapes get a fresh load every spring whether they need it or not, and the layers build up year after year. University of Illinois Extension notes that annual top-dressing can push mulch far deeper than intended.
Do this instead: poke into the mulch with a trowel and measure. If it’s still 2 inches or more, just rake it to freshen the surface. If it has thinned, add only the difference; the mulch calculator has a field for existing depth so it only counts the top-up.
4. Piling mulch against shrub stems and plant crowns
The same problem as volcanoes applies to shrubs, perennials and young plants. Mulch packed against stems keeps them wet and can lead to rot.
Do this instead: pull mulch back an inch or two from stems and plant crowns.
5. Making the ring too small
A tiny circle of mulch right around a trunk does little for the tree. Tree roots spread far beyond the trunk, and grass competes with them for water and nutrients.
Do this instead: make tree rings as wide as is practical, several feet across at least, and wider for larger trees. The mulch calculator’s “ring around a tree” shape handles the math, including the bare circle at the trunk.
6. Digging woody mulch into the soil
Wood chips and bark work well on the surface. Mixed into the soil, fresh woody material can tie up nitrogen as it decomposes, starving the plants around it.
Do this instead: leave woody mulch on top. To improve soil, mix in finished compost rather than mulch; our compost calculator gives recommended depths for new and existing beds.
The right way, in short
- 2 to 3 inches deep, measured after it settles.
- Wide, flat rings around trees, with a bare circle at the trunk.
- An inch or two of clearance around stems and crowns.
- Check depth before adding more each year.
- Woody mulch on top of the soil, compost into it.
Sources
- University of Illinois Extension: Too much of a good thing, mulch volcanoes slowly kill trees
- UConn Extension: Mulch volcanoes, a growing problem