Drive any residential street in spring and you'll see the same mistake on tree after tree: mulch piled high against the trunk like a witch's hat. It's called volcano mulching, and it's slowly killing the tree.
The depth rule
Two to three inches deep. Never more.
- Under 2 inches: too thin to suppress weeds or hold moisture. Re-mulch in 6 weeks.
- 2 to 3 inches: the right depth. Suppresses weeds, holds moisture, breaks down at the right pace.
- 4+ inches: water can't penetrate, roots suffocate, fungal problems start.
The volcano problem
The bark of a tree is designed to be exposed to air, not constantly in contact with wet organic material. When mulch is piled against the trunk, it traps moisture against the bark and causes rot. It also invites pests and disease, and encourages roots to grow upward into the mulch — destabilizing the tree long-term.
The correct shape: a donut, not a volcano. Pull the mulch back from the trunk so you can see the root flare (the slight widening where the trunk meets the soil).
The easy test
If you can see the root flare, the mulch is placed correctly. If the trunk is buried, pull the mulch back six inches.
What kind of mulch
Hardwood mulch is the workhorse. Shredded bark from hardwood trees. Breaks down at a moderate pace, neutral pH. Best for general use.
Pine bark or pine straw is more acidic and slower to break down. Best around acid-loving plants like rhododendrons, azaleas, hydrangeas, and blueberries.
Dyed mulch is hardwood mulch with a color coating. Holds appearance longer but doesn't enrich soil the way undyed mulch does.
How much to order
Formula: length × width × depth (in feet) ÷ 27 = cubic yards. A 100-square-foot bed at 3 inches deep needs about 1 cubic yard.