Mulch Around the Trunk: Leave 10 cm Clear, Says the Royal Horticultural Society

Mulch helps retain moisture in the soil and limits competition from weeds, but the way it is applied matters too. The Royal Horticultural Society advises keeping a layer of mulch across the soil while leaving around 10 cm clear around the trunk, as damp material in direct contact with bark may encourage rot.

Why should mulch not touch the trunk?

Mulch can stay wet for a considerable period. If it is piled directly against the base of a tree or woody shrub, the bark remains continually in contact with this damp material. This can soften the bark and make it more susceptible to deterioration.

For that reason, it should not look like a heap shaped into a cone around the trunk. Instead, spread the mulch over the soil as a ring, leaving an unmulched area immediately around the base.

How deep should a mulch layer be?

The Royal Horticultural Society recommends a layer at least 5 cm deep and, in its various guidance, suggests depths of up to roughly 7.5 to 10 cm depending on the type of use. For domestic application, a 5 to 8 cm layer is a practical guide that falls within these recommendations.

Suitable mulching materials include well-rotted compost, composted leaves, straw and shredded bark, depending on the intended purpose. As well as helping the soil retain water, mulch can help suppress the growth of unwanted plants.

How do you create a mulch ring around a tree?

Start by spreading the material evenly across the soil around the plant. Then pull the mulch back from the trunk base to make a clear circle of approximately 10 cm. This gap helps prevent mulch from sitting directly against woody stems.

There is also no need to build a mound of material beside the trunk. An even layer over the root area works more effectively as mulch and avoids concentrating moisture around the plant's most vulnerable part.

Which plants need particular care with mulch around the trunk?

This precaution is particularly important for trees, shrubs and other plants with woody stems. The same advice applies in gardens where leaves, bark and other organic materials are added to the soil regularly.

If mulch has already built up against the trunk, simply draw it back carefully to create the clear strip. In this way, mulch around the trunk no longer forms the familiar “volcano” and instead performs its role as a protective layer over the soil, without keeping the base of the plant constantly damp.