Depth by use
Most garden beds: 2–3 inches. Paths and play areas: 3–4 inches. Newly planted trees: a wide ring, 2–3 inches deep, kept clear of the trunk. More than 4 inches in a planting bed can stay too wet and starve roots of air.
Replenish mulch as it breaks down, usually once a year, and use the mulch calculator to avoid buying far too much or too little.