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Mulch is one of the most useful materials in a UK garden, but its benefits depend on two things: how deep you lay it and where you place it. A layer that is too thin will not suppress weeds or hold moisture; a layer piled against a plant stem can soften bark and invite disease. This guide explains the depth and placement rules that protect plants without burying them.
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The correct depth for biodegradable mulches
The RHS advises that biodegradable mulches, such as garden compost, leaf mould, well-rotted manure or composted bark, need to be at least 5cm (2in) thick, and ideally 7.5cm (3in). This depth is enough to block light from weed seeds and reduce evaporation from the soil surface, while still allowing rain and air to reach the soil below. Thinner layers may look neat but will not do the job; thicker layers are not necessarily better, because very deep organic mulch can stay excessively wet and may smother low-growing plants or create conditions that encourage stem rot if it touches woody growth.
This depth applies to loose, biodegradable materials. The RHS does not give a specific depth for non-biodegradable mulches such as gravel, slate or pebbles, nor for sheet mulches like woven landscape fabric. For those, follow the manufacturer’s guidance and use enough to block light and stay in place, but do not assume the same 5–7.5cm rule automatically applies. A 5cm layer of gravel behaves very differently from 5cm of composted bark.
Placement: the stem-free zone
The single most important placement rule is to keep mulch away from woody stems. The RHS is explicit: if mulch is in direct contact with the stems of trees or specimen shrubs, it can cause the stem to soften, making it vulnerable to diseases. The recommended practice is to leave a mulch-free circle of about 10cm (4in) around any woody stem. This collar allows air to circulate and keeps the bark dry, reducing the risk of rot and fungal infection.
For newly planted trees and shrubs, the RHS goes further: keep a vegetation-free circle at least 1.2m (4ft) in diameter around the plant for its first three years. Within that circle, you can apply mulch, but still leave the 10cm collar immediately around the stem. The larger circle is about reducing competition from weeds and grass, not about piling mulch higher; the mulch depth remains the same 5–7.5cm across the circle, tapering to nothing at the stem.
For single trees and specimen shrubs, the RHS recommends mulching to the radius of the canopy. This means the mulched area should extend as far as the branches spread, not just a small ring around the trunk. The depth remains 5–7.5cm, and the 10cm stem-free collar still applies at the centre. This wider placement helps conserve moisture over the main root zone, which often extends beyond the canopy edge.
How to apply mulch without burying stems
Start with moist soil, after removing perennial weeds. Do not mulch frozen or waterlogged ground. Tip or shovel small piles of mulch onto bare soil between plants, then spread it with a rake or fork to an even depth. Use your hands to work around delicate emerging plants and under shrub branches. A narrow spring-tined rake is useful, but a garden fork works too.
As you spread, create the stem-free collar by pulling mulch back from the base of each woody plant. Check the gap with a ruler or marked stick so the mulch-free collar is about 10cm wide. For herbaceous perennials, avoid burying the crown. You can temporarily cover the crown of an emerging perennial with an upturned pot while you mulch around it, then remove the pot. This prevents the crown from being stepped on or buried.
Low-growing plants need particular care. The RHS warns against smothering low growing plants with mulch. If a plant is naturally prostrate or has foliage close to the soil, either mulch very lightly around it or leave it unmulched. Ground-covering Mediterranean plants such as Thymus species may be best left unmulched altogether, because mulch can hold too much moisture around their stems and foliage, which they dislike.
If you use a permeable woven landscape fabric, cut slits for planting and ensure the fabric does not touch woody stems; the same 10cm gap applies. The fabric itself can be covered with a thin layer of bark or gravel for appearance, but do not let that covering build up against stems.
Decisive checks after mulching
Once you have finished, run through these checks before you consider the job done:
- Depth check: Use a ruler or stick to confirm the biodegradable mulch is between 5cm and 7.5cm deep across the bed, not just in the spots where you tipped it.
- Stem check: Look at every woody stem. There should be a clear ring of bare soil at least 10cm wide around each one. If mulch has crept back, pull it away with your hands.
- Crown check: For herbaceous plants, make sure the crown or emerging shoots are visible above the mulch, not buried.
- Moisture check: The soil beneath should have been moist when you applied the mulch. If it was dry, water the bed before mulching next time; mulch over dry soil can slow re-wetting.
- Weed check: Perennial weeds should have been removed before mulching. If any remain, they will grow through; hand-weed rather than hoeing close to permanent plants to avoid root damage.
Common mistakes and how to avoid them
Piling mulch against trunks. This is the most common error. It creates a damp, dark environment where bark stays wet and pathogens can enter. Always maintain the 10cm collar.
Using too thin a layer. A 2cm sprinkle of bark looks tidy but will not suppress weeds or conserve moisture effectively. Aim for the full 5–7.5cm for biodegradable materials.
Burying low plants. Mulch spread too enthusiastically can cover the foliage of low perennials or alpines. Work from the back of the border forward and use your hands around small plants.
Mulching dry or frozen soil. Mulch over dry soil can prevent rain from reaching the roots; mulch over frozen soil will not settle properly. Wait until the soil is moist and workable.
Using fresh woody chips. Fresh woody material can cause nitrogen competition when incorporated into soil and may be unsuitable near young plants. Use a suitable aged mulch and keep it on the surface; a few weeks of storage is not a universal maturity test. Grass clippings are nitrogen-rich and should not be treated as equivalent to woodchips.
Maintaining the right depth and placement over time
Biodegradable mulches rot down, so the depth will gradually reduce. Check the bed each spring and top up with fresh material to restore the 5–7.5cm layer, but always re-establish the stem-free collars after topping up. Do not simply add new mulch over old mulch that has crept against stems; pull the old mulch back first. Non-biodegradable mulches do not rot, but they can be displaced by wind, birds or foot traffic, so check that the depth is still adequate and that no material has moved against stems.
If you see white fungal mycelium in organic mulch, do not be alarmed: this is usually harmless saprotrophic fungi breaking down the material, and there is no need to remove it.
Useful products for this job
The links below open Amazon UK product searches. Check the exact item, seller, current price and compatibility before buying.
- Well-rotted garden compost or composted bark mulch: Provides the biodegradable mulch layer at the correct 5–7.5cm depth; improves soil as it breaks down. Check the material is well-rotted and free from weed seeds, and avoid fresh woody chips that may temporarily reduce nitrogen availability. Search Amazon for Well-rotted garden compost or composted bark mulch.
- Permeable woven landscape fabric: Useful for new beds where you want to plant through a sheet mulch; must be permeable to let rain and irrigation reach roots. Confirm the fabric is water-permeable, not waterproof, and that you can cut clean slits for planting while keeping a 10cm gap around woody stems. Search Amazon for Permeable woven landscape fabric.
Related garden guides
Sources and further guidance
- Mulches and Mulching | RHS Advice
- How to mulch with organic matter | RHS Advice
- Trees and Shrubs: Planting Guide | RHS Advice
Guidance researched in September 2026. The feature image is an AI-generated illustration. Product guidance is based on published information, not hands-on testing.