How Much Mulch Does a Garden Need?

Choose a depth

Horticultural guidance commonly suggests a mulch layer of around 2–4 in (5–10 cm). Thinner layers do less to suppress weeds and conserve moisture; much thicker layers can stop water and air reaching the soil. Keep mulch pulled back from plant stems and tree trunks.

If you are topping up last year's mulch, you may only need an inch or so.

Measure irregular beds

• Break curved beds into rectangles and circles and add the areas.
• For a long, curving border, measure the length along the middle and multiply by the average width.
• Tree rings are circles: area = π × radius².

Convert to bags or bulk

Bulk or bagged?

Bulk mulch is usually cheaper per cubic yard but comes with delivery charges and a pile on the driveway. Bags are convenient for small areas and easier to transport. Put both prices into the calculator to see where the break-even point is for your job.

Worked example: a border and a tree ring

• Border 20 ft × 4 ft = 80 ft².
• Tree ring with a 3 ft radius: π × 3² = 28.3 ft².
• Total 108.3 ft² at 3 in deep: 108.3 × 3 ÷ 12 = 27.1 ft³.
• That is 1.0 yd³ (0.77 m³), or 14 bags of 2 ft³ (27.1 ÷ 2 = 13.5, rounded up).

Which mulch?

• Bark and wood chips: long-lasting and good for paths, shrub borders and around trees; coarse chips break down slowly.
• Compost, leaf mould and well-rotted manure: break down within a season and feed the soil, so they suit vegetable beds and borders that are topped up every year.
• Gravel and slate: do not break down and suit dry gardens, alpines and paths, but are heavy to move — check the weight per cubic yard before ordering.
• Sheet mulches such as cardboard under a layer of organic mulch can help clear weeds from a new bed.

When to apply it

Mulch is most useful when the soil is moist and not frozen. Common advice is to apply it in mid to late spring, after the soil has warmed and been watered by rain, and to top up in autumn. Mulching dry soil traps the dryness in; mulching frozen soil keeps it cold for longer.

Avoid piling mulch up against tree trunks in a cone. Keeping a small gap around stems and trunks reduces the risk of rot and pests.