Mulch depth decides whether it works at all
A mulch has three jobs: suppress weeds, hold moisture, and feed the soil as it breaks down. Only the third works at any depth. The first two have a threshold, and below it you are doing nothing useful.
Under about an inch — 25 mm — light still reaches the soil surface and weed seeds germinate straight through. Worse, a thin mulch is an excellent seed bed for whatever blows in. Two to three inches, 50 to 75 mm, is the working depth where weed suppression actually happens, and it is roughly triple the material most people spread.
The exception is around plant stems and tree trunks, where mulch should be pulled back rather than piled up. Mulch in direct contact with bark holds moisture against it and rots it. The ring of bare soil around a stem looks unfinished and prevents a genuinely serious problem.
| Depth | Coverage per bulk bag (850 L) | Suppresses weeds? |
|---|---|---|
| 25 mm / 1 in | 34 m² · 366 ft² | No |
| 50 mm / 2 in | 17 m² · 183 ft² | Yes |
| 75 mm / 3 in | 11.3 m² · 122 ft² | Yes, well |
| 100 mm / 4 in | 8.5 m² · 91 ft² | Yes, for paths |
Volume is what you buy, weight is what you move
Compost is sold by volume and moved by weight, and the mismatch catches people out. Garden compost runs around 400 kg per cubic metre when moist. A single 850-litre bulk bag is therefore roughly 340 kg — about seven full wheelbarrow loads, and a solid morning of work.
Wet material is heavier still. A bulk bag left uncovered through a week of rain can gain a hundred kilos, which matters if it has to be moved by hand or up steps. If delivery timing is flexible, dry weather is worth waiting for.
The calculator shows weight alongside volume for this reason. The volume tells you whether you have bought enough; the weight tells you whether you can move it before dark.
Mulching over versus digging in
For established beds, spreading compost on the surface and leaving it is both less work and better for the soil than digging it in. Worms move it down within a season, and the soil structure — the fungal networks and the pore spaces that hold air and water — stays intact instead of being broken up annually.
Digging in is worth it in two cases: breaking into ground that has never been cultivated, and incorporating material into the backfill when planting something permanent. Otherwise the surface is the right place.
Timing depends on what the mulch is for. Autumn mulching protects soil over winter and lets worms work through the cold months. Spring mulching holds moisture through summer but should wait until the soil has warmed — a thick mulch spread over cold soil in March keeps it cold, and delays everything you sow into it.
Common questions
How much compost do I need for a 10 × 20 ft bed?
At 2 inches deep, a 200 ft² bed needs about 33 cubic feet — 1.23 cubic yards, roughly 1.1 bulk bags, or 17 bags of 2 cubic feet.
How deep should mulch be?
Two to three inches, 50 to 75 mm. Under one inch it does not suppress weeds at all, and thin mulch tends to become a seed bed for whatever blows in.
Should I dig compost in or leave it on top?
On established beds, leave it on the surface. Worms take it down within a season and the soil structure stays intact. Dig it in only when breaking new ground or backfilling a planting hole.