Quick answer: Mulch Depth for Moisture and Weeds — 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.
Mulch is usually described by material and rarely by depth, which is backwards. Depth is the variable that determines whether the layer holds water or shades out weeds, and past a certain point it does neither and starts doing harm instead.
Two inches: the moisture layer
A two-inch layer of coarse material cuts evaporation from the soil surface by roughly half to two thirds, because it breaks the capillary path that carries water to the surface and because it shades the surface from direct sun. It also keeps the surface temperature several degrees lower, which reduces the vapour pressure gradient that drives evaporation.
What it does not do is stop weeds. Two inches is enough for most annual weed seed to germinate in the material itself, and coarse chip at that depth leaves gaps.
Three to four inches: the weed layer
At three to four inches, the light reaching the soil surface drops below what most weed seed needs to germinate, and the physical barrier stops the ones that do. This is the depth for a bed where suppression is the goal.
The material matters here: fine material at four inches mats and sheds water, so the weed control comes at the cost of the moisture benefit. Coarse material at four inches does both, because the layer stays open.
Six inches and beyond: the problem
Past about four to six inches, depending on the material, the layer stops allowing gas exchange between the soil and the air. Oxygen diffusion into the root zone falls, carbon dioxide accumulates, and the soil biology shifts toward anaerobic. Roots grow up into the mulch layer instead of down into the soil, which makes the plant dependent on the layer and vulnerable when it dries.
This is the mechanism behind the volcano mound — the cone of chip pushed against a trunk. The base of the trunk is in a permanently damp, low-oxygen environment, and the bark that was never meant to be buried starts to decay.
Why the two thresholds differ
Moisture retention is a surface-area effect: it depends on breaking the capillary path and shading the soil, and two inches accomplishes both. Weed suppression is a light-transmission effect: it depends on reducing the light that reaches the soil to near zero, and that takes more material and a coarser particle size.
Which is why a bed that is mulched for moisture and a bed that is mulched for weeds are two different depths, and why a single recommendation for the whole garden is always wrong for part of it.
The technique that makes a thin layer work
Weed first, then mulch. A two-inch layer over established weeds suppresses nothing and gives the weeds a damp environment to grow into. Hand-pull or hoe, water the bed, then apply.
And feather the edges. A mulch layer that stops abruptly at the bed edge loses its depth at the boundary by run-off and by the mower, and the edge is where the weeds get in.
What to measure
Push a trowel into the layer in three places. If it goes through to soil in less than two inches, the layer has broken down or washed and needs topping up. If it goes deeper than four in a bed that is meant to be a moisture mulch, rake some off.
The related question of how much water the layer is actually saving is measuring moisture rather than guessing.