The short answer is 2–3 inches for almost everything with plants in it — and “almost” is doing real work in that sentence, because mulch depth isn’t one number, it’s a decision. Here’s the use-by-use version, with the reasoning attached so you can bend it correctly.

Depth by use

Use	Depth
Flower & shrub beds (fresh bed)	2–3 in
Annual top-up over existing mulch	~1 in (to reach 2–3 total)
Around trees (donut, not volcano)	2–4 in, kept 3+ in off the trunk
Vegetable garden	2–3 in straw/leaf mulch, often 3–4 in loose straw
Pathways / play areas (wood chips)	3–4 in (paths), 6–12 in where code asks (playgrounds)
Stone or gravel mulch	1–2 in — it doesn’t decompose
Erosion-prone slopes	3–4 in, coarser material that knits

Why 2–3 inches is the sweet spot for beds: it’s thick enough to block light from weed seeds and slow evaporation, thin enough that rain still reaches the soil below and oxygen reaches roots. Push past ~4 in and two bad things happen — the layer mats and sheds water (the “crust” problem), and buried roots get chronically wet and start circling or rotting. University extension guidance is unusually unanimous on this: keep it under ~4 in total, and keep it off the trunks — both sources are on the sources page.

The volcano problem

Mulch piled in a cone against a tree trunk looks tidy and quietly kills trees. Bark isn’t root — it isn’t built to sit damp — so volcano mulching invites rot, fungal cankers, and girdling roots that strangle the flare. And it’s pure waste: the mulch 10 in deep against the trunk is doing nothing the mulch at 3 in deep three inches away isn’t doing better. Pull it back to a flat donut: 2–4 in deep, 3+ in clear of the trunk, ideally out to the dripline where the fine roots actually are.

Refreshing existing mulch

Before ordering for a top-up, measure what’s left, not what you want. If last year’s 3 in is now a matted 1.5 in, you need 1.5 in of new material — not 3 more inches on top (that lands you at 4.5, past the ceiling). Matted old mulch worth breaking up: rake it before adding new, or the fresh layer sits on a waterproof sponge. And a quick sanity check — if your mulch never decomposes, that’s often a sign it’s been too deep, not too good.

Turning depth into an order

Multiply area by depth, done: at the standard 3 in, one cubic yard covers 108 sq ft (162 at 2 in, 81 at 4 in). A 2 cu ft bag covers 8 sq ft at that depth, a 3 cu ft bag covers 12 — so the 200 sq ft bed at 3 in is 1.85 yd³ bulk or 25 × 2 ft³ bags. The mulch calculator runs all of that live, including whether bags or a bulk yard wins on price; topsoil covers the soil underneath if you’re building the bed up.

Frequently asked questions

How deep should mulch be in a garden bed?

2–3 inches is the standard depth for shredded wood and bark mulches around shrubs and perennials — enough to suppress weeds and hold moisture, not so much it suffocates roots. For coarse nuggets or paths, 3–4 in. This is the university extension consensus (see sources), and it's a total-depth rule: old mulch underneath counts toward it.

Is 4 inches of mulch too much?

Around plants, yes — most extension guides cap organic mulch at ~4 in total. Beyond that the layer stays wet, mats down, and sheds rain instead of passing it to roots; deep mulch also invites voles and rot at the crown. 4 in is fine for paths and areas with no plants to smother.

Can you put new mulch over old mulch?

Yes — that's the normal annual top-up — but add only enough to reach ~2–3 in total. If last year's layer is still 2 in thick, you need 1 in of new mulch, not 3. On compacted, matted old mulch, rake it up first or the new layer sits on a sponge. If it never seems to decompose, that's often a sign it was too deep to begin with.

Why shouldn't I pile mulch against the tree trunk?

The 'mulch volcano' — mulch banked up the trunk — keeps bark wet and encourages rot, fungal disease, girdling roots and rodent damage. Extension guidance is consistent: keep mulch 3+ inches back from the trunk in a donut shape, not a volcano. The flare where the trunk widens at the soil line should stay visible.

How deep should mulch be to stop weeds?

2–3 in of shredded mulch suppresses most weeds; weeds that do appear root in the mulch itself and pull easily. Fabric under mulch is a mixed bag — it stops weeds through it but also stops worms and decomposition from feeding the soil, and perennial weeds grow through it in a year or two anyway.

How do I know how many yards I need at a given depth?

sq ft × (inches ÷ 12) ÷ 27. At 3 in, 1 yd³ covers 108 sq ft; at 2 in, 162 sq ft; at 4 in, 81 sq ft. The mulch calculator does this live, including bag counts and bag-vs-bulk pricing.