Why leaf mould beats anything you can buy

Every autumn, Britain's streets, parks and gardens shed a fortune in free soil improver. Leaf mould is what you get when fallen leaves are left to rot down into a dark, crumbly, sweet-smelling material that has almost nothing in common with the slimy bags of leaves you started with. It is not a fertiliser — its nutrient content is low — but as a soil conditioner it is hard to beat. It holds moisture, opens up heavy clay, gives body to sandy soil, and feeds the soil life that does the real work in any kitchen garden or smallholding.

If you have ever walked into an old broadleaf wood and scuffed your boot through the top few inches, you have seen the stuff. That crumbly brown duff under the trees is leaf mould, and it is exactly what you want to spread on your borders and beds.

The best part: it is free. All it asks for is a corner of ground, a bit of effort on a crisp afternoon, and patience.

Which leaves to collect — and which to leave

Not all leaves rot at the same speed, so it pays to know what is falling on your plot.

• Quick to break down (a year or less): ash, birch, lime, cherry, willow, sycamore, hornbeam.
• Slower (often 18 months to two years): oak, beech, sweet chestnut and horse chestnut. These make superb leaf mould in the end, but the wait is longer. Oak and beech are best kept in their own bin if you have the space.
• Avoid in bulk: evergreen leaves such as holly, laurel and conifer needles. They are tough, acidic and very slow. A few swept up with other leaves will not hurt, and conifer needles can go in a separate bin for two or three years, but keep them out of the main mix.

Skip leaves from trees that were clearly diseased — black spot on roses, for example — and keep walnut leaves out of the mix, as they can suppress growth while fresh. Otherwise, collect whatever you can lay your hands on.

Bags, bins and wire frames

You do not need anything fancy. The simplest method is to stuff the leaves into old compost bags or black bin bags, punch a few drainage holes in the bottom and sides with a fork, add a splash of water if the leaves are dry, and tie the top loosely. Store them behind a shed or under a hedge and forget them.

A wire bin is even better if you have the space. Make a cylinder about a metre across and a metre tall from chicken wire or weldmesh, held in place with a few stakes. Pile the leaves in as they fall, tread them down gently every so often to squeeze out air, and water them if there is a dry spell. A bin this size holds a surprising amount, and because it breathes, the leaves rot faster than in bags.

If you have a lot of leaves, build two or three bins side by side. Fill one this autumn, leave it, and start filling the next one next year. That way you always have a bin quietly maturing while you get on with other jobs.

Getting it to rot down faster

Leaves left to their own devices will rot, but you can hurry things along.

• Shred them. Run a mower over them on the lawn, or tip them into a dustbin and go at them with a strimmer for a minute. Smaller pieces rot in roughly half the time.
• Mix in a little nitrogen. A handful of grass clippings, a scattering of chicken manure pellets, or a diluted liquid feed speeds up the fungal and bacterial work. Do not go mad — too much and you get a slimy, smelly mess.
• Keep them damp. Leaf mould needs moisture. If the pile dries out completely, rotting stops, so water it in dry spells.
• Turn occasionally. Once or twice a year is plenty. It is not essential, but it mixes the outer, drier leaves into the middle where the action is.

Resist the temptation to pile leaves into your compost heap in bulk. They are slow to break down and will cool the heap, slowing everything else. Keep them separate if you can.

What to do with the finished stuff

Check your bags and bins after a year. If the leaves have turned into a dark, crumbly material with a pleasant earthy smell and only a few skeleton leaves left, it is ready. Sieve it if you want a finer product.

• Mulch borders: spread a 5cm layer around shrubs, perennials and fruit bushes in spring or autumn.
• Improve vegetable beds: fork it into the top few inches, or leave it on the surface for worms to pull down.
• Make seed compost: sieved leaf mould mixed with sharp sand and a little garden compost makes an excellent seed-sowing mix, especially for woodland plants, ferns and hellebores.
• Top-dress the lawn: a thin scattering brushed into the grass in autumn does wonders.
• Pot up: mix it half and half with garden compost for a free potting medium.

Anything still clearly recognisable as a leaf after a year is not ready — put it back and give it another six months.

Making the most of autumn

Gather leaves when they are dry if you can, as they are lighter to handle and rot better. A leaf rake and a big tarpaulin make short work of a lawn. Ask neighbours and nearby friends if they want their leaves taken away — most keen gardeners will bite your hand off. If you are on an allotment, put a note on the noticeboard or in the shed: leaves wanted, all welcome.

One last trick: spread a few leaves directly under hedges and around the base of trees and let the worms do the work where they fall. It saves carting them about, and the soil life underneath will thank you.

Set aside an hour or two this autumn, fill a bin or a few bags, and then leave them be. Next year you will have a free supply of the finest soil improver money cannot easily buy.