How to Mulch Around Trees (and Avoid Mulch Volcanoes)

Mulch around a tree belongs in a flat ring that stops short of the trunk, not in a cone piled against it. The International Society of Arboriculture (ISA) says to apply mulch near, but not touching, the trunk, in a layer 2 to 4 inches deep on well-drained sites. The sources below agree on the shape and differ on the exact inches, so this guide shows their numbers side by side.

What a mulch volcano is, and why it hurts the tree

ISA defines mulch volcanoes as excessive piles of mulch materials applied around the base of trees. University of Vermont Extension describes the shape: mulch piled high onto the trunk in a cone-shaped mound.

The sources name several kinds of harm:

• Moisture against the bark. Vermont says mulch piled against the bark traps moisture, encouraging decay and providing entry points for insects and diseases. University of Kentucky’s extension service says it keeps the trunk moist, causing the bark to decay.
• A buried root flare. Vermont says volcano mulching can bury the root flare, the part of the tree where the trunk widens into the root system, and that the flare should remain visible above the soil line.
• Roots in the wrong place. ISA says mulch against the trunk can lead to stem girdling roots, which can reduce tree growth or eventually kill the tree. Vermont says applying more than 4 inches can encourage roots that grow from the trunk and can, over time, girdle the tree.
• Rodents. Vermont says volcano mulching can create habitat for rodents like voles, giving them cover to chew and girdle the trunk.

The shape: a flat ring

Vermont’s phrase for the right shape is a donut instead of a volcano: a wide, even ring around the tree. ISA’s guideline says the same thing in plainer words: near, but not touching, the trunk.

How wide

ISA says to extend the mulch to the drip line if practical, and if it is not, to go as far out as you can. Kentucky also says mulch should go out to the dripline of a tree. Maryland says to apply mulch as far out as the spread of the branches, if possible. Vermont says the ring should extend to the tree’s drip zone, the area beneath the tree’s outermost canopy.

For a smaller ring, two sources use the number 3 in different ways. ISA says that, generally, a ring with a 3-foot radius is the minimum for most trees. Vermont reports that many arborists recommend a “3-3-3” method for mature trees, which it gives as 3 feet in diameter. A 3-foot radius is a circle 6 feet across, twice the width of a circle 3 feet in diameter.

How deep, and how far from the trunk

Source	Depth	Gap at the trunk
ISA	2 to 4 in on well-drained sites	Not touching the trunk; pull piled mulch back several inches
University of Vermont Extension	No more than 2 to 4 in; no more than 3 in under the “3-3-3” method it reports	At least 3 in from the root flare
University of Kentucky	3 in at most, or 2 in for finely ground mulch	4 in from the trunk
University of Maryland Extension	1 to 3 in	About 3 in from the base of the trunk
Iowa State	3 to 4 in of wood chips or shredded bark on well-drained sites; 2 to 3 in on heavy soils	At least 6 in from the trunk

Kentucky’s publication was issued in July 2012. Every source in the table keeps the mulch off the trunk. Where a source gives a number, the gap runs from about 3 inches to at least 6.

Fixing a volcano that is already there

ISA’s instruction is short: if mulch is piled against the stems or tree trunks, pull it back several inches so that the base of the trunk is exposed. This guide adds nothing to that instruction.

Adding mulch without building a new one

ISA says buildup can occur if reapplication outpaces decomposition, or if new material is added simply to refresh color.

The sources give the same first step. Vermont says to take stock of the mulch already around your trees, and that you can rake it out and refresh with a sprinkle of fresh mulch. Kentucky says to always check the depth of the mulch on top of the soil before adding more, and that you only need to replace mulch if it has completely broken down.

If you do need more, a ring is a small job. With this site’s defaults of 10% extra and 2 cu ft bags, a circle 6 feet across mulched 3 inches deep takes 4 bags. The calculator treats the ring as a full circle and does not subtract the gap at the trunk, so the count runs a little high.