How Deep Should Mulch Be?
About 3 inches (75 mm) after it settles is enough for most beds. Around trees, extension services suggest 3–4 inches spread wide, with the mulch pulled back a few inches from the trunk. Much deeper layers can restrict air getting into the soil, and mulch piled against trunks invites disease and rodent damage.
What the experts recommend
- A University of Georgia extension horticulturist says a layer 3 inches deep after settling is enough for most plants, and warns that 6 inches of bark may restrict gas exchange in the soil.
- Mississippi State University Extension suggests mulch 3 to 4 inches deep around trees, spread 3 to 6 feet from the trunk, and pulled 2 to 3 inches away from the trunk itself.
- Both warn against "mulch volcanoes": mulch piled up against a tree trunk.
How much mulch each depth takes
Deeper mulch costs more fast: going from 2 to 4 inches doubles what you buy.
| Depth | Volume | Cubic yards | 2 cu ft bags |
|---|---|---|---|
| 2 in | 16.7 cu ft | 0.62 cu yd | 9 |
| 3 in | 25 cu ft | 0.93 cu yd | 13 |
| 4 in | 33.3 cu ft | 1.23 cu yd | 17 |
| Depth | Litres | Cubic metres | 50 L bags |
|---|---|---|---|
| 50 mm | 500 L | 0.5 m³ | 10 |
| 75 mm | 750 L | 0.75 m³ | 15 |
| 100 mm | 1,000 L | 1 m³ | 20 |
Topping up an existing bed
Push a ruler or a finger down through the old mulch in a few places to find how deep it still is. Subtract that from your target and calculate only for the difference. Old mulch that has broken down into the soil doesn't count.
Common mistakes
- Piling mulch against stems and trunks. Keep a small gap.
- Adding a fresh full layer every year on top of the last one, so the total creeps past 4 inches.
- Buying for the target depth with no allowance for settling. Add about 10%.
