Is Your Mulch Helping Your Trees — or Hurting Them?
Mulch is one of the best simple things you can do for a tree—when it is kept off the trunk and spread where roots can actually use it.
Mulch is useful because it changes the environment around a tree's roots. It can reduce evaporation, moderate soil temperatures, suppress weeds, and reduce competition with turfgrass.
But the familiar 'mulch volcano'—a steep pile against the trunk—is not the goal.
Think wide, not tall
Tree roots benefit from mulch spread over soil, not stacked against bark. A broad ring covering as much of the root area as practical is more useful than a narrow, deep mound around the trunk. CSU recommends keeping mulch off the trunk and root flare.
Why a mulch ring helps
- It reduces evaporation from the soil surface.
- It moderates soil temperature swings.
- It suppresses competing weeds and turfgrass.
- It creates a buffer that keeps mower decks and string trimmers away from the trunk.
The trunk should still look like a trunk
At ground level, a healthy tree should widen into its root flare. If the trunk disappears straight into a pile of mulch, pull the material back and inspect the base. Bark that is meant to be above ground should not remain continuously buried and damp.
One Colorado caveat
In fire-prone settings, combustible organic mulch close to structures can be a poor choice. Landscape design should balance tree health, water conservation, and wildfire exposure rather than applying one mulch rule everywhere.
Sources & further reading
These Field Notes translate Colorado horticultural guidance into practical homeowner language. For the underlying recommendations, start with the original CSU Extension resources: