Why Leaf Mould Deserves a Corner of Your Garden

Every October and November, our streets, parks and gardens shed a fortune in free soil conditioner. Those drifts of oak, beech and sycamore leaves are not litter to be bagged up and sent away — they are the raw material for leaf mould, one of the finest things you can add to a British garden soil.

Leaf mould is not a fertiliser in the usual sense. It holds few nutrients compared with farmyard manure or garden compost, but what it does brilliantly is improve soil structure. Dig it in and heavy clay begins to crumble into crumbs; spread it on sandy ground and it holds onto water far better through a dry July. Sieved finely, it makes a superb seed-sowing medium that is low in nutrients and just damp enough for germination. Best of all, it costs nothing but a little patience. A year from now, that pile of wet leaves will be dark, crumbly and sweet-smelling, and your borders will thank you.

Just as importantly, a leaf pile left in a quiet corner is a wildlife hotel. Hedgehogs, toads, beetles, worms and overwintering queen bumblebees all tuck themselves into damp leaves, and blackbirds, robins and wrens will work the edges for grubs. It is hedgerow ecology brought right into the garden.

Which Leaves to Keep and Which to Treat Differently

Most deciduous leaves break down well, but they do not all behave the same way. Thin, soft leaves rot fastest and give the best results.

• Quick to rot (about 12 months): oak, beech, hornbeam, ash, lime, birch and hazel. Oak and beech are the traditional champions and give a lovely crumbly mould.
• Slower (18 months to 2 years): sycamore, horse chestnut, walnut and sweet chestnut. Horse chestnut leaves are often marked by the leaf miner moth, which does no harm to the finished mould.
• Best kept out of the leaf bin: evergreen leaves such as holm oak, laurel, rhododendron and Leyland cypress, plus conifer needles. These are leathery and waxy, and will sit for years. Shred them and add them to the ordinary compost heap in small amounts, or use them as a rough mulch under acid-loving shrubs.

A useful trick is to run the mower over the lawn when leaves have fallen. The chopped mixture of grass and leaf collects in the box and rots far faster than whole leaves ever will.

Two Simple Ways to Make It: Wire Bins and Damp Sacks

You do not need anything clever. The traditional method is a wire bin: knock four sturdy stakes into the ground in a square roughly a metre across, then wrap chicken wire around them, leaving the base open to the soil. Open-ground bins let worms and soil fungi move in from below, which speeds everything up. Fill it loosely rather than pressing the leaves down, and top it up as more fall.

If space is tight, use sacks instead. Fill old compost bags or hessian sacks with damp leaves, punch a good few drainage holes in the sides with a fork, and tie the tops loosely so air can still move. Do not use sealed black bin bags — without air, the leaves turn to a slimy, sulphurous sludge rather than crumbling mould. Tuck the sacks behind the shed or under a hedge and forget about them.

You can also simply heap leaves in a quiet corner and cover them with a piece of old carpet or a tarpaulin weighted at the corners. It is untidier, but it works perfectly well.

Keeping the Pile Damp, Aerated and Alive

Leaf mould is made by fungi rather than the heat-loving bacteria of a compost heap, so it needs a different kind of care. The two essentials are moisture and air.

• Water in dry spells. A pile that dries out stops working. If autumn is dry, give the bin a good soak with a watering can every couple of weeks. Aim for the dampness of a wrung-out sponge.
• Turn it once or twice. Fork the pile over in spring and again in midsummer. This mixes the outer dry leaves into the middle and lets air in.
• Do not add kitchen scraps. Keep leaf bins for leaves alone. Food waste attracts rats and turns the pile into something quite different.
• Weigh down the top. A layer of heavier material stops the wind scattering your work back across the lawn.

That is genuinely all the maintenance required. The fungi, woodlice, millipedes and worms do the rest.

When Is It Ready, and What Does Ready Look Like?

Patience is the whole art. Leaves collected this autumn will be usable as a rough mulch by next autumn, but the finest, most crumbly mould takes two years. You will know it is ready when the original leaves are no longer recognisable — the heap has become a dark brown, soft, faintly earthy material that smells of woodland floor after rain.

If you find half-rotted leaves in the mix, simply sieve them out and return them to the bin for another year. A garden sieve or a piece of mesh over a wheelbarrow makes short work of this, and what passes through is the good stuff.

Putting Leaf Mould to Work in the Garden

Rough, unsieved leaf mould is ideal as a winter mulch. Spread a five-centimetre layer over empty borders, around shrubs and along the base of hedges, keeping it clear of stems and crowns. Worms will pull it down into the soil over the coming months, feeding your plants from below.

The sieved material is more precious. Forks of it dug into a vegetable bed transform heavy clay, and a handful in the planting hole gives new shrubs and roses a fine start. Mixed half and half with garden compost, it makes an excellent potting mix for larger containers. For sowing seeds, use it on its own or with a little sharp sand — its low nutrient level suits seedlings far better than rich compost.

So before you reach for the bin bags this autumn, give the leaves a second life. Rake them, bag them, keep them damp, and let the fungi get on with it. Next year's soil is already falling from the trees.