How Thick Should Mulch Be?

The short answer: 5 cm minimum, 7.5 cm ideal

Mulch should be at least 5 cm (2 in) deep, and the RHS says 7.5 cm (3 in) is ideal.¹ For stubborn weeds, aim nearer 8 cm: the RHS says that is easier and more effective than the bare minimum.³ Coarse wood chip can go deeper, at 8 to 12 cm.⁴ Whatever you use, keep a 10 cm (4 in) mulch-free circle round any woody stem.¹

Thin mulch is the common failure. A light scatter lets light through, so weed seeds still sprout. The RHS says mulches work by blocking the sunlight weed seeds need, and thicker layers block more of it.²

Mulch depth by job

Pick the row that matches what you are doing. Where a depth comes from GardenCalc’s calculators rather than a published source, the table says so.

Weeds

Dig out perennial weeds first. The RHS says low annual weeds do not need removing, because a covering of mulch will smother them.¹ Perennials such as bindweed will push straight through, so lift them before you mulch.

Moisture and ordinary borders

The RHS says mulches help soil hold moisture in summer and let rain soak in during winter.² For most borders, 5 to 7.5 cm does that job. Going thicker on borders with low perennials has no RHS backing, and our mulch calculator treats 10 cm as the upper limit for a border.

Trees and shrubs

After planting a tree, the RHS says to water well, then spread organic matter 5 to 8 cm thick, leaving a 10 cm gap round the trunk.⁵ The Woodland Trust suggests about 10 cm deep over a circle 1 m across, with bark chips, straw or fallen leaves added each winter.⁶

Paths and gravel

None of our sources give a path depth, so treat this as our planning figure. The bark chippings calculator uses 75 mm for a garden path, laid on weed membrane inside edging. For gravel, use the gravel calculator. It does not set a depth for you: you enter the depth, and it says to take it from your supplier’s instructions and your site conditions. The 3 to 5 cm above is our own planning figure. The RHS warns that gravel can mix into the soil if it is not applied thickly, which then encourages surface weeds.²

Play areas

Do not use any depth from this page for a children’s play area. BS EN 1177 sets a way of testing how well a surface absorbs impact, including loose bark and wood chip, and gives each surface a critical fall height.⁷ The depth you need depends on the equipment and on the tested product, so use the depth on the product’s certificate.

Bark, compost, manure or wood chip?

All four work as mulch. They differ in how they feed the soil and how long they last. The RHS names home-made compost or leafmould as usually the best options.¹

On nitrogen, the RHS says there is little evidence that woody mulches cause nutrient deficiency when laid on the surface. Digging woody material into the soil is the risk: microbes can lock up nitrogen for up to two years.⁴ So lay chippings on top and do not fork them in. The RHS also says to avoid fresh broadleaf chippings on newly planted beds, and that woodchips carry a slight risk of introducing honey fungus.^2,4

If you are buying bark for a bed rather than a path, the mulch calculator works out the volume. For paths and decorative borders, use the bark chippings calculator.

Keep mulch off the stems

The RHS says to keep a 10 cm mulch-free circle around any woody stem.¹ Mulch piled against a trunk can stay damp and rot it. The RHS adds that mulch touching the stems of trees or specimen shrubs can make the stem soften.² A flat layer with a bare ring round each plant is the shape to copy. Do not heap it into a cone.

How much bark per square metre

Depth in centimetres times 10 gives litres per square metre. At 5 cm you need 50 litres per m². The RHS gives the same figure and says an 8 cm layer needs about 50% more.³

The coverage rule is: bag size in litres divided by (depth in cm × 10) gives square metres. An 80-litre bag at 7.5 cm covers 80 ÷ 75 = 1.07 m². The RHS puts it more loosely: one cubic metre will mulch 15 to 20 square metres.¹ That is the 5 to 6.7 cm range, so the RHS figure assumes a thinner layer than our 7.5 cm sums.

Worked example: a border

A border 6 m long and 1.5 m wide is 6 × 1.5 = 9 m². At 7.5 cm that is 9 × 75 = 675 litres. Allow 15% for settling: 675 × 1.15 = 776 litres. That is ten 80-litre bags (800 litres) or one 1,000-litre bulk bag. The 15% is our own working allowance; the mulch calculator uses 10 to 20% and the bark chippings calculator adds 10%.

Worked example: a young tree

The Woodland Trust ring is 1 m across and about 10 cm deep.⁶ The circle is 3.14 × 0.5 × 0.5 = 0.785 m². Take out the bare ring round the stem. The RHS says a 10 cm mulch-free circle, which could mean a 10 cm gap all round, so we use the cautious reading, a 10 cm radius: 3.14 × 0.1 × 0.1 = 0.031 m². That leaves 0.754 m², and at 0.1 m deep that is 0.0754 m³, or about 75 litres. One 80-litre bag is only just enough, so buy a second if you want to allow for settling.

Best time of year to mulch

The RHS calls late winter or early spring the ideal time, and for most of the UK that means February to March.³ It also favours mid to late spring, before annual weeds have germinated, and says new plants can be mulched whenever you plant them.² Autumn works too.

The soil matters more than the date. The RHS says to lay mulch over moist soil, after removing weeds, and to delay if the soil is very wet or frozen.^1,2 Mulching bone-dry soil seals the dryness in, so water first or wait for rain.

How often to top up

The RHS advises annual mulching, and says to top up tree mulch at least once a year.^1,5 Organic mulch can simply be topped with another layer, and it needs full replacing only once it has completely rotted down.² Wood chip lasts longer, which reduces the need to re-apply.⁴

A practical check: push a ruler through the mulch in three or four places each spring. If it is under 5 cm, add enough to bring it back to 5 to 7.5 cm. Wood chip is thicker: top it up once it falls below 8 cm, back to 8 to 12 cm. For a quick top-up sum, our mulch calculator assumes about 25 mm a year.

Common mistakes

• Laying 2 cm and wondering why the weeds came through. Stay at 5 cm or more.
• Heaping mulch against trunks and stems. Leave the 10 cm bare ring.
• Mulching frozen, squelchy or bone-dry soil.
• Covering perennial weeds instead of digging them out.
• Digging woody chippings into the soil. Keep them on the surface.
• Using fresh manure at full strength. Halve the rate, or use it well rotted.
• Using a bark depth from a website for a play area. Use the product certificate.

Frequently Asked Questions

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Sources and checking notes

Depths, gaps and timing were checked against the sources below on 8 October 2026. Bag and coverage sums are GardenCalc’s own arithmetic. The path depth is the planning figure used in our bark chippings calculator, and the decorative gravel depth is our own planning figure. Neither is a published standard. No prices are quoted on this page; the calculators show current ranges.

1. Royal Horticultural Society, How to mulch with organic matter.
2. Royal Horticultural Society, Mulch.
3. Royal Horticultural Society, Organic matter: how to use in the garden.
4. Royal Horticultural Society, Woody waste: using as mulch.
5. Royal Horticultural Society, How to plant a tree.
6. Woodland Trust, How to care for your trees.
7. British Standards Institution, Impact attenuating playground surfacing: methods of test for determination of impact attenuation (catalogue entry for BS EN 1177:2018+A1:2023, the current edition).