Why Mulching Is Worth the Effort

Mulching is one of the kindest things you can do for a tree, and it costs little more than time. On the woodland floor, leaves, twigs and fallen branches settle into a loose layer that keeps the soil cool and damp, feeds worms and fungi, and slowly returns nutrients to the roots. A mulch ring in a garden does much the same job, which matters most where soil is compacted, sandy, or baked dry by a south-facing wall.

The benefits add up quickly. Moisture is conserved through summer dry spells, weed competition drops, and soil temperature swings become gentler — important for the fine surface roots that do most of a tree's feeding. A mulch layer also keeps strimmers and mower blades at a distance, a surprisingly common cause of bark wounds on young trees.

The Mulch Volcano: A Mistake That Costs Trees Dear

The most damaging mulching error is also the easiest to make. Piling material up against the trunk — a cone-shaped mound sometimes called a mulch volcano — traps moisture against bark that is meant to be dry. Fungi and bacteria take hold, the bark softens and dies back, and cankers can develop. Rodents and voles often move in beneath the warm, loose cover and gnaw the bark through winter.

Burying the root flare is just as serious. That flare is the point where the trunk widens into the buttress roots, and it needs air. When it is smothered for years, trees may send out new roots into the mulch itself, and these can eventually girdle the trunk. The damage is slow, often invisible until the tree is beyond saving.

• Keep the root flare clear. You should be able to see where the trunk widens before it enters the soil.
• Never let mulch touch bark. A gap of 10–15 cm between trunk and mulch is plenty.
• Do not dig mulch into the soil. Lay it on the surface and let worms do the incorporating.

Choosing the Right Material

Almost any well-rotted organic matter will do, but some materials suit trees better than others. Arborist woodchip — the mix of chipped branches and leaves you get from a tree surgeon — is close to ideal, as it breaks down slowly and supports the fungal life trees depend on. Composted bark, leafmould and garden compost are all excellent. Fresh manure, however, is too rich and can scorch young roots, so let it rot down first. Grass clippings should be spread thinly or composted, as thick layers mat down, smell and shed water. Cocoa shell is attractive but best avoided where dogs are about.

• Woodchip or bark: long-lasting, good for structural soil improvement.
• Leafmould: free, mild, perfect for woodland planting and underplanting.
• Garden compost: nutritious but quick to vanish; top up more often.
• Avoid: fresh manure, thick sawdust, and anything carrying weed seeds or perennial roots.

How to Apply Mulch: The Donut Method

Think of a ring doughnut, not a pyramid. Water the tree thoroughly first if the soil is dry, then spread the mulch evenly around it, starting 10–15 cm out from the trunk and working outwards. For a young tree, aim for a ring at least a metre across; for a mature specimen, extend it as far as the canopy spreads, or as far as your patience and the lawn allow.

Depth matters as much as width. Around 5–8 cm is right for most soils; on heavy clay, stay nearer the shallower end. More than 10 cm and you risk smothering roots, encouraging surface rooting, and keeping the soil too wet through winter. Leave the surface slightly uneven rather than smoothed flat, so rain percolates instead of running off.

Timing, Topping Up and Woodland Gardening

Early spring is the best moment to mulch, while the soil is still moist and the growing season is getting under way. An autumn application helps insulate roots and muffle winter wet, though it is rarely needed on established trees. Check the depth before adding more — many trees are over-mulched rather than under-mulched, and a layer built up over several years may simply need raking out, not topping up.

If you garden under trees, mulch is your best friend. A generous layer of leafmould or woodchip over dormant bulbs and shade-tolerant perennials such as hellebores, wood anemones and ferns suppresses weeds, feeds the soil and lets you plant without disturbing roots. Work carefully with a hand fork, and never dig deeply where tree roots run.

Newly planted bare-root whips benefit most of all: a clear, weed-free mulch ring through their first three summers can mean the difference between a tree that romps away and one that sulks. Check it as the tree settles, refresh when the layer thins, and remember the golden rule — bark should stay dry, roots should stay cool, and the two should never meet.