Quick answer: Choosing a Mulch — 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.

Typical depth: Two to four inches, flat, from the drip line inward — never a cone against the trunk.
Typical timing: After planting or after a clean-up; refreshed in autumn so winter rain carries it down.
What it involves: Keeping the root flare visible, keeping mulch off the bark, and choosing material that breaks down rather than mats.
The standard: ISA and university extension guidance on arboricultural mulching and the root flare.
Our own record: 503 pages and 291 stories published on this site; Uncategorized (109) is the rail this page files under. Counted from our own archive, not a stock table.

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.

Standards and sources: Arboricultural standards (ISA — International Society of Arboriculture) · Soil and root health (UC IPM — University of California pest management)
Prepared by Cambria Pines Tree Service Inc. — ISA-certified arborists working on the Central Coast.

Mulch is sold by the bag or the yard, described by what it is made of, and bought by appearance. The decision that actually matters is what it is meant to stop. Answer that and the material, the depth and the placement all follow.

If the problem is water leaving too fast

You want a coarse layer two inches deep, over the whole root zone, with the surface broken so that water enters rather than runs off. Coarse arborist chip is the material. Fine material at this depth mats and makes the problem worse.

The tell-tale sign that this is your problem: the soil is dry at two inches three days after watering, and the surface is crusted.

If the problem is weeds

You want three to four inches, coarse, over ground that has been cleared first. Depth and particle size are the two variables; the material matters less than either, though fine material at four inches will suppress weeds and shed water at the same time.

Do not mulch over existing weeds. That is the one failure mode that guarantees the problem continues.

If the problem is heat

You want a light-coloured, low-density material — straw is the best of them — because the point is reflectance as much as insulation. A dark material absorbs heat at the surface, and while it still insulates the soil below, it does not lower the temperature of the air just above the bed.

This is the case for a summer mulch in a vegetable garden and the reason straw beats chip there.

If the problem is mud

You want depth and structure, in a place people and animals walk. Coarse bark or chip, four inches or more, over a geotextile if the traffic is heavy. In a paddock or a path, the material needs to be angular rather than rounded so it knits together and does not roll.

This is the one application where a thick layer is correct, because there is no root zone underneath that needs gas exchange.

If the problem is salt

You want a material that sheds rather than absorbs: gravel, coarse bark nugget, pine chip. Keep the depth low and keep a bare, dished strip on the windward edge so the salt moves away. The mechanism is in mulch where the wind carries salt.

If the problem is appearance

Then be honest that it is a design decision rather than a horticultural one, and accept the trade. Bark nugget is the handsomest and the worst at holding water. Gravel is permanent and cannot be changed. Dyed material is a colour, and the dye is irrelevant to the plant.

Appearance is a legitimate goal. It just should not be dressed up as soil improvement.

The three questions before buying

How big is the area, in square feet? At two inches deep, one cubic yard covers about a hundred and sixty square feet; at four inches, about eighty. Getting that arithmetic right is the difference between a completed job and a second delivery.

What is under it? Turf has to come out first; compacted ground needs aerating first; a slope needs a different approach entirely.

What is already growing there? A bed of Mediterranean plants wants gravel and no richness. A bed of woodland shrubs wants chip. A vegetable bed wants compost and straw.

One note on sourcing

Arborist chip is often free, delivered by a tree crew that would otherwise pay to dispose of it. The material is excellent, the price is right, and the only caution is that you do not choose the species. That is why it is the default around trees and the wrong answer in a vegetable bed, where you want to know what is in it.