How much depth to apply
Two to three inches is the usual target. That is enough to suppress weeds and slow moisture loss, without the problems that come from piling it deeper.
Below about two inches, light reaches the soil and weeds come through. Above four, mulch can shed water rather than letting it through, and the layer beneath can go anaerobic.
Cubic yards = (square feet × depth in inches) ÷ 324
The 324 constant comes from 27 cubic feet per yard × 12 inches per foot. For 600 sq ft at 3 inches: (600 × 3) ÷ 324 = 5.6 cubic yards.
Bags or bulk
Bagged mulch is usually 2 cubic feet, so a cubic yard takes about 13.5 bags. Bulk delivery is almost always cheaper above roughly 2 cubic yards, once you account for the trips and the handling.
Never pile it against trunks
Mulch mounded against a tree trunk — the so-called mulch volcano — holds moisture against bark that is not adapted to it. The result is rot, and it encourages roots to grow into the mulch layer and eventually circle the trunk.
Keep mulch a few inches clear of trunks and stems. The flare where the trunk widens into the roots should stay visible.
Types and how long they last
- Shredded hardwood — knits together and stays put on slopes. Breaks down over a season or two, improving the soil.
- Bark nuggets — last longer but float and wash away in heavy rain.
- Wood chips — cheap, often free from arborists. Best on paths and around established trees.
- Straw — common in vegetable beds, decomposes fast.
- Rubber — does not decompose, and adds nothing to the soil.
Organic mulches need topping up annually as they break down. That decomposition is a benefit, not a defect — it is what feeds the soil.