How deep mulch should be, and the depth that kills trees

Two inches of mulch suppresses most weeds, holds moisture and moderates soil temperature. Three does it slightly better. Four starts to cause problems, and the pile heaped against the trunk that half the landscaped car parks in the country have is actively killing the tree it is meant to protect.

The useful thing is not the number. It is why the number stops working.

What goes wrong when it gets deep

Mulch is porous, but only to a point. Past about four inches it stops air and water moving freely into the soil below, and two things follow.

Roots go looking for the oxygen they are no longer getting, which means upward, into the mulch layer itself. A tree with its feeder roots in the mulch rather than the soil is a tree that dries out the moment the mulch does, which is the opposite of the thing mulch was bought to do.

Water gets held in the layer rather than passing through it. In a wet spring that means the top few inches stay saturated, and roots sitting in saturated material do not get air either.

Against the trunk is the real damage

The pile that touches the bark is a different problem from depth, and a worse one.

Bark above the root flare is not built to stay wet. Held against damp mulch it stays damp permanently, which softens it and opens the way for rot and fungal infection. Rodents nest in the pile over winter and chew the bark while they are in there, and a ring of chewed bark all the way round the trunk girdles the tree and ends it.

The symptoms arrive years later, which is why the practice persists. Nobody connects a tree failing in its eighth year to the way it was mulched in its second.

The shape to aim for

A flat ring, two to three inches deep, going out as far as the canopy if you have the material, with a clear gap of two to three inches between the mulch and the trunk. The root flare, where the trunk widens as it meets the ground, should be visible. If you cannot see it, there is too much material against the tree.

Ohio State and Illinois extension services both describe the same donut shape, and both have been publishing about it for years, which tells you how firmly the volcano habit is stuck.

Depth decides the quantity, not the area

The practical consequence of all this is that depth is the number driving what you buy, and people pick it last.

One cubic yard of mulch covers 162 square feet at two inches, 108 at three and 81 at four. So the same bed can want one yard or two depending on a decision most people make in the car park. Set the depth first, then work out the volume with the mulch calculator, which takes bed dimensions and depth and gives you both the cubic yards and the bag count.

For an existing bed, measure what is already there before ordering. A bed topped up every spring without checking is usually the one that has quietly reached five inches, and the right order that year is none.

Topping up versus replacing

Organic mulch is supposed to disappear. It breaks down into the soil, which is most of its long term value, and that is why the layer thins by an inch or so a year.

That makes topping up to depth the normal maintenance, not a full replacement. The exception is when the surface has matted into a water-shedding crust, which bark fines and finely shredded products can do. Breaking the crust with a rake before adding anything is usually enough to get it absorbing again.

The one case for going deeper

Clearing ground is the exception that proves the rule. A deliberately deep smothering layer, six inches or more over cardboard, is a recognised way to kill turf or weeds before planting a new bed.

That is a demolition technique with a start and an end date, not a mulching depth. Once the bed is planted, the layer comes back down to three inches, and the same depth that killed the grass on purpose will go on to kill the shrubs by accident.