Garden bark

Bark goes down on a garden as a loose layer over the soil. It can cover the bare ground between plants, sit under shrubs, or fill a space where a lawn used to be.

What a layer of bark does

The Royal Horticultural Society (RHS) counts bark among the mulches, the loose coverings gardeners lay over soil. In its advice, a mulch helps the soil hold water in summer and lets rain soak in during winter. It also keeps weeds down by blocking the light their seeds need, and shields plant roots from heat and cold.

Bark breaks down slowly. As it rots, the RHS says, it feeds the soil and improves its structure, so the layer needs topping up once it has gone. Processed conifer bark and wood chippings are both on its list of the best materials for mulching.

Choosing a bark or chip

Bark and chipped wood come in bags from garden centres or in bulk from compost suppliers. The RHS says chips from broadleaf trees such as oak and beech make a good mulch, and so do chips from conifers such as pine and larch.

Fresh wood can hold natural chemicals that harm young plants. Conifer material is more likely to carry them than broadleaf. The RHS suggests stacking fresh chips in a heap to weather for three to four months, which makes them safe even round new plants. Mature, settled plants are unlikely to be affected either way.

Some wood belongs elsewhere. The RHS advises against shredded treated timber, because older treated wood can contain arsenic or chromium. Chipboard and fibreboard hold synthetic resins, and the RHS sends those to the council recycling centre. It also says to leave out the base of any tree that had honey fungus or Phytophthora root rot, and any part of a tree with a stem disease such as verticillium wilt or coral spot.

How deep, and how much to order

The RHS asks for a layer at least 5cm deep, and ideally 7.5cm, for a mulch that rots down. Wood chips are coarser and pack down less, so it suggests laying them deeper, at about 8 to 12cm.

For ordering, the RHS guide to mulching borders gives a rule of thumb. One cubic metre of mulch will cover 15 to 20 square metres of ground. A standard bag of around 70 litres, or two buckets, covers about one square metre. Bulk bags and loose loads save on plastic packaging, and the RHS suggests sharing a delivery with a neighbour.

When and how to spread it

The RHS gives mid to late spring and autumn as the best times to mulch a bed. Its border guide prefers late winter or early spring, while the soil is still damp from winter rain. Wood chips can go down at any time of year, though not onto dry soil.

Gardeners dig out perennial weeds first and lay the bark on moist soil that isn't frozen. The RHS says to leave a ring of about 10cm clear round the stem of every tree and shrub, because bark piled against a stem stays damp and can lead to rot. Low plants and small bulbs should be left uncovered.

Some beds do better without a woody mulch. The RHS suggests keeping it off annuals, vegetables and soft fruit, whose roots may sit near the surface. Fertiliser can be scattered on top of bark in late winter, and rain washes it down to the roots.

Bark under shrubs and in place of grass

For a garden that needs less work, the RHS suggests planting shrubs through a weed suppressing membrane and covering it with bark or gravel. It may then be several seasons before the bed needs weeding or a fresh layer. The sheet should let water through, so that rain reaches the roots.

The RHS calls a lawn one of the most intensive features in a garden to look after. It lists bark alongside gravel, permeable paving and decking as ways to replace one.

Bark in peat-free compost

Composted bark, most often from pine, is one of the materials blended into peat-free potting compost. The RHS says bark is stable and porous, so it lets air into a mix.

The RHS advice is to look for the words peat-free on the bag. Words such as "organic" or "environmentally friendly" can appear on compost that still holds peat. The Responsible Sourcing Scheme, run by the industry, grades bagged compost and other growing media from A to E on their environmental impact, and its ratings can be searched online.

Bark under play equipment

Bark chips, wood chips, sand and pea gravel are the loose surfaces RoSPA Play Safety names for playgrounds. It says these soften falls well. They can also scatter, so someone has to keep them raked and topped up, and they don't always suit spots that need a clear gap under the equipment, such as beneath a roundabout.

RoSPA Play Safety says no law requires an impact absorbing surface. Its view is that one is standard practice, recommended by RoSPA, the British Standards Institution and other safety experts. It recommends that suppliers give written assurance that their product has been tested to BS 7188 and EN 1177, and that it is laid over the areas set out in EN 1176. Low equipment is the exception. Where a new item is below 600mm, RoSPA Play Safety asks only for a ground surface with some give in it.

Two kinds of bark are most used in the UK, according to guidance written for RoSPA Play Safety by Melcourt Industries, a bark supplier: granular bark, mostly from pine, and a more elastic bark, mostly from spruce. In a 300mm layer of granular bark, the lower 200mm tends to bind into a firm, free draining base, while the top 50 to 100mm stays loose. The layer wears down with play, and RoSPA Play Safety says to keep spare bark to top it back to its original level.