Making Leaf Mould for Free Soil Improver

Why leaf mould is worth the wait

Come October, most of us are out there with a rake, cursing the drifts of leaves blocking the light and smothering the lawn. It is tempting to bag the lot and leave it at the kerb for the council. But those fallen leaves are one of the best free resources a garden can offer, and turning them into leaf mould takes almost no effort at all — just patience.

Leaf mould is not the same as compost. It is broken down mostly by fungi rather than bacteria, in a cool, slow process that produces a dark, crumbly, sweet-smelling material. It is low in nutrients but superb at improving soil structure, holding moisture and encouraging the soil life that keeps plants healthy. Sieved finely, it makes an excellent seed-sowing mix. Used as a mulch, it mimics what happens naturally on a woodland floor. And it costs you nothing but a few bin bags and a corner of the garden you were not using anyway.

Which leaves to keep and which to avoid

Not all leaves break down at the same speed, so it pays to know what is falling on your patch.

• Quick to rot (roughly a year): sycamore, horse chestnut, lime, cherry, ash, willow and most soft, thin leaves.
• Slow (allow two years): oak, beech, hornbeam and sweet chestnut. These are rich in tannins and lignin, but the resulting leaf mould is particularly good stuff, so they are well worth the extra wait.
• Very slow, best kept separate: conifer needles, holly, laurel and other thick evergreen leaves. Shred them or add them in small quantities, and expect a long haul.
• Best avoided: leaves from diseased trees and shrubs, such as those with blackspot or heavy leaf miner damage. A hot compost heap would deal with most of it, but a cool leaf mould bag will not, so keep them out.

If you have a mix, which most gardens do, just bung it all in together. The quicker leaves will get things going and the tougher ones will catch up eventually.

The bin bag method, step by step

This is the simplest system going, and it works brilliantly in even the smallest garden.

• Collect the leaves when they are dry-ish. Damp leaves are fine, but sodden ones tend to turn sludgy. A dry autumn day after a light breeze is ideal.
• Use old compost bags or strong bin bags. Reusing bags you already have is the whole point — there is no need to buy anything.
• Punch holes in the sides and base. A garden fork stabbed in half a dozen times on each side is plenty. This lets air in and excess water drain out, which is what keeps the process working rather than stagnating.
• Fill the bags loosely, leaving room to tie the tops. Do not cram them solid — the contents need air.
• Add a splash of water if the leaves are bone dry, then tie the tops loosely rather than knotting them tight.
• Stack the bags in a shady corner behind a shed, under a hedge or along a north-facing wall. Weigh them down with a brick or tuck them behind something solid so they do not blow about.
• Label them with the date. A scrap of paper and a marker saves a lot of guessing next autumn.

If you have more leaves than bags, a simple wire cage — four stakes and a length of chicken wire — does the same job on a larger scale and can be turned occasionally if you feel energetic. You do not need to turn it, though. Leaf mould is a patient business.

Waiting, checking and fixing trouble

Put the bags out of sight and largely out of mind. Check them after six months by opening one up. You should see the leaves darkening and beginning to stick together. After twelve months, the quicker leaves will have broken down into a rough, crumbly mulch. Give the slower ones another year.

It is ready when you can no longer recognise individual leaves, the colour is a rich dark brown, and it smells earthy and woodland-like rather than sour. If a bag turns sludgy and whiffs of stagnant water, the contents have gone anaerobic. Tip it out, mix in some dry leaves or a little shredded cardboard, and restack it in a thinner layer so air can get in. It will recover. If a heap seems stubbornly unchanged, it is probably too dry — water it and give it a stir.

How to use your finished leaf mould

This is where the rewards come in. Spread it as a mulch around shrubs, perennials, roses and fruit bushes in autumn, letting the worms work it in over winter. Dig it into vegetable beds to improve drainage on heavy clay and water retention on sandy soil. Sieve it and mix it roughly half and half with garden compost and a little sharp grit for a fine seed-sowing mix. It is also perfect for woodland plants that resent rich feeding — ferns, hellebores, rhododendrons and blueberries all love it. A handful tucked around the base of newly planted trees and shrubs helps them settle in beautifully.

Leave some leaves for wildlife

Before you tidy every last leaf, remember that a good layer of them is one of the best wildlife habitats your garden can offer. Hedgehogs, frogs, toads and countless insects overwinter in damp leaf litter, and a pile left under a hedge or behind a border is a small act of kindness that pays you back in slug control and pollinating visitors next spring. So sweep the paths and lawn, fill your bags, but leave the leaves under the shrubs and in the wilder corners. Your soil, your plants and your garden's residents will all thank you for it.