Why leaf mould deserves a corner of your plot

Every autumn, gardeners up and down the country face the same job: raking up leaves before they smother the lawn and clog the gutters. Most of it ends up in the green waste bin or the bonfire. That is a shame, because those fallen leaves are one of the best free soil improvers you can lay your hands on.

Leaf mould is what you get when leaves are left to rot down slowly, mostly through the action of fungi rather than the bacteria that drive a hot compost heap. The result is a dark, crumbly, sweet-smelling material that holds moisture beautifully, improves soil structure and makes an excellent mulch. It is low in nutrients compared with compost, so it feeds the soil rather than the plant, which is exactly what heavy clay and thin sandy ground both need.

Best of all, it costs nothing but a bit of wire, a few stakes and some patience. If you have room for a simple bin in a quiet corner, you can turn this year's nuisance into the potting mix you will be proud of in two years' time.

Which leaves are worth collecting

Not all leaves rot at the same speed. As a rough guide, thinner leaves break down faster, while thick, leathery or waxy leaves take far longer.

• Quick to break down (a year or so): sycamore, lime, horse chestnut, ash, cherry and most ornamental fruit trees.
• Slower (up to two years): oak, beech, sweet chestnut, hornbeam and walnut. These make superb leaf mould if you are prepared to wait.
• Best avoided: evergreen leaves such as laurel, holly and rhododendron, plus conifer needles. They are tough, acidic and can sit around for years. Use them sparingly as a mulch under acid-loving shrubs instead, or leave them out altogether.

Skip any leaves from trees that have suffered disease that year, and never collect from pavements where there is heavy traffic pollution. Leaves from your own lawn, borders and driveways are ideal, and mixing several kinds together gives you a more balanced end product.

Building a simple wire bin

You do not need anything fancy. The classic leaf mould bin is a cylinder of wire netting held open by stakes, and it takes about twenty minutes to make.

• Cut a length of galvanised wire netting roughly 3 metres long and 1 metre high, with a mesh of about 25mm. Smaller mesh keeps the leaves in better, especially in windy weather.
• Drive four wooden stakes into the ground in a square about 1 metre apart, leaving them a little proud of the netting height.
• Wrap the netting around the outside of the stakes and tie it to each one with garden wire, folding the cut ends inwards so they cannot catch your hands or a hedge trimmer later.
• Leave the top open to the rain and let the bin sit on bare soil, not on paving or gravel. Worms and soil fungi need to get in from below.

If you have a lot of trees, make two or three bins side by side so you can have one filling, one rotting and one ready to use. A lid is not necessary, but a sheet of old carpet or a piece of tarpaulin weighted down with bricks helps keep the middle from drying out in a hot summer.

Filling and managing the bin

Collect leaves when they are damp but not sodden. This is the single most important point: dry leaves will sit in the bin for years and barely change. A rainy October day is perfect raking weather, and a light sprinkling from a watering can will help if the autumn has been dry.

As you add each barrow load, tread it down firmly. Compacting the leaves reduces air pockets, keeps the mass damp and speeds up the fungal breakdown considerably. Aim to fill the bin right to the top, then leave it alone.

Whole leaves work perfectly well, but you can speed things up by running the lawnmower over them first and adding the chopped leaves and grass cuttings together. The grass adds nitrogen and the smaller pieces rot faster. Do not put large quantities of leaves on your ordinary compost heap, though: they break down by a different route and will slow the whole heap to a crawl.

After that, the work is mostly waiting. Check the bin occasionally in dry spells and water it if the top few inches have gone crispy. Otherwise, resist the urge to fiddle.

How long it takes, and judging when it is ready

Leaf mould is not a quick crop. Expect around twelve months for a bin of mixed soft leaves, and two years for oak, beech and other tough customers. A bin started this autumn will be crumbly and useable the autumn after next if you leave it undisturbed.

You will know it is ready when the leaves have lost all trace of their original shape and the material is dark brown, soft and faintly earthy, like a light, fibrous compost. If you can still recognise a sycamore leaf, give it another six months. Some gardeners end up with two grades: a coarse, partly rotted leafmould at the bottom and a finer, darker product in the middle. Both are useful, just for different jobs.

Putting your leaf mould to work

Once it is ready, leaf mould is wonderfully versatile and you will find you never have quite enough of it.

• Mulch: spread a 5cm layer around shrubs, perennials and fruit bushes in spring or autumn to lock in moisture and suppress weeds.
• Soil conditioner: fork it into vegetable beds and borders to improve structure, especially on heavy clay or hungry sand.
• Seed sowing: sieved leaf mould mixed with roughly equal parts loam and sharp sand makes a lovely, low-nutrient seed compost that encourages strong root growth.
• Autumn top dressing: spread it over the lawn and let the worms pull it down.
• Potted ferns and woodland plants: blend it with garden compost for a moisture-retentive mix that suits shade lovers.

So next time the leaves come down, reach for the rake rather than the bin bag. A few stakes, a roll of wire and a little patience will repay you with a free supply of the finest soil improver a garden can have.