Quick answer: How to Mulch Around a Tree — Mulch is the cheapest thing that happens to a tree: a flat layer over the root zone holds moisture through the dry season, keeps the soil temperature steady and feeds the soil life the tree depends on. Almost every mulch problem is a depth problem or a contact problem — too thick, or piled against the trunk where the bark has to stay dry.
Field note. What we see most on this coast is not too little mulch but too much, in the wrong place: the volcano against the trunk that keeps the bark wet through the fog season and invites rot where the tree cannot show it until the canopy thins.
On the other hand, a thin ring of the wrong material is worse than none at all — matted, water-repellent mulch sheds the rain it was meant to store.
The single most damaging mulch practice on this coast is the cone: a mound of chip pushed up against the trunk until it looks tidy. It kills trees slowly, it is done by professionals as often as by homeowners, and the correct shape takes no longer to make.
Why the cone is harmful
Tree bark above the root flare is adapted to air. It is thick, it sheds water, and it relies on drying out between wettings. Buried in mulch, it stays wet continuously, and the organisms that rot wood work in exactly those conditions. The decay starts at the base of the trunk, which is the part of the tree that cannot be replaced and the part that carries the whole load.
There is a second mechanism: the mulch holds moisture against the flare and the roots that should be at the surface grow upward into the mound, girdling the trunk as they thicken.
The visible result takes years: a thin crown, a trunk that does not taper, and eventually a failure at the base on a windy day.
The correct shape
A ring, not a cone. The layer is two to four inches deep at its outer edge and thins to nothing at the trunk, so that the flare is visible and dry. The cross-section is a saucer, not a hill.
Leave a clear gap of a few inches between the trunk and the start of the mulch. On a young tree that gap is small; on a mature one the flare itself occupies the space.
How wide
As wide as you can afford. The absorbing roots of a mature tree are at and beyond the drip line, not at the trunk, so a ring that extends to the canopy edge and beyond is the ring that does something. A two-foot circle around a forty-foot tree is decoration.
The practical rule used in arboriculture is to mulch out to the drip line at minimum, and further where the ground is available. Where the whole root zone is not available — because of lawn, paving or a boundary — mulch the largest uninterrupted arc you can and do not worry about the rest.
What to do about the grass
Do not lay mulch over turf. The grass survives, the mulch breaks down into a mat, and the two compete. Remove the turf first — by hand, by smothering with card for a season, or by stripping the top inch.
Where the turf is killed chemically, wait for it to die before mulching, or the decaying roots will create a layer that sheds water.
Renewing it
The layer breaks down. Chip loses roughly an inch of depth every two or three years, faster in a wet climate and faster where the material is fine. Top up when the trowel test shows less than two inches, and top up by adding to the outer edge rather than by pushing material toward the trunk.
Never rake the old material toward the centre to make room for the new. That is how a donut becomes a cone over five years.
What it costs the tree if it is wrong
A correctly built ring changes the root environment within a season: cooler soil, more moisture, more fine roots, more fungal activity. A cone changes it too, in the other direction, and the tree spends a decade dying of it. The difference is a few minutes with a rake, which is the whole of the practice.