The best free soil improver you will ever make

Every autumn, Britain's streets and gardens disappear under a golden carpet of fallen leaves. Most of it gets raked onto the lawn, shoved into the wheelie bin or left to smother the grass. With a little patience, though, those same leaves become one of the finest soil conditioners you can lay your hands on, and it will not cost you a penny.

Leaf mould is not compost. It is made by fungi rather than the heat-loving bacteria that work in a compost heap, and the process is slower and cooler. What you get at the end is a dark, crumbly, sweet-smelling material that holds water like a sponge, opens up heavy clay and gives sandy soil some much-needed body. It is gentle enough for seedlings and perfect for pots. All it asks for is a few sacks and a corner of the garden where you can forget about it.

Which leaves are worth collecting

Not all leaves are equal, and a few are best left alone.

• Slow but superb: oak, beech, hornbeam and sweet chestnut are rich in tannins and take up to two years to break down, but produce the finest, most crumbly leaf mould of all.
• Quick and easy: sycamore, lime, birch, alder, ash and horse chestnut rot down in a year or less, making them a good choice if you want results sooner.
• Keep separate: conifer needles, holly, laurel and other thick evergreens are waxy, acidic and painfully slow. Bag them on their own and use the result around rhododendrons, camellias and blueberries instead.
• Leave it: avoid leaves from trees with serious disease, and skip anything slimy, smelly or fouled by dogs.

If you have a mix of the quick and slow types, simply combine them. The faster leaves get the whole heap going and the slower ones add structure, giving you a usable mulch in year one and something finer in year two.

Sacks, cages and the damp-and-wait method

The simplest system is the black bin bag, and it is the one I would recommend to anyone starting out. Rake your leaves onto a dry day when they are crisp rather than sodden. A leaf blower on reverse suction makes short work of a large lawn, but a plastic rake and a bit of elbow grease is just as effective.

Fill old bin bags loosely, leaving room for air, then punch half a dozen holes in the sides with a garden fork. If the leaves are bone dry, give them a quick soak from the watering can before tying the tops loosely. Add a handful of garden soil or a scoop of last year's leaf mould to introduce the fungi that do the work. Stack the bags behind the shed, in shade, and leave them.

A wire cage works just as well for larger quantities. Drive four stakes into the ground in a square, wrap chicken wire around them and tip the leaves in. The open sides let in air and rain, though you may need to water it in a dry summer.

Speeding things up and avoiding common mistakes

The single biggest error is letting the pile dry out. Leaf mould should stay as damp as a wrung-out sponge. If you can hear the leaves rustling when you push a fork in, add water.

• Shred leaves by running a mower over them, or chop them roughly in a bin with a strimmer, to halve the waiting time.
• Layer in a few handfuls of grass clippings between the leaves to add nitrogen, but go easy, as too much turns the whole thing slimy and smelly.
• Turn the pile once or twice a year if you are keen; if you are not, no harm done.
• Do not dump large quantities on your compost heap. Leaves break down far more slowly than kitchen waste and will slow the whole bin down.

Check the bags after twelve months. The contents should have shrunk by half and turned into a coarse, crumbly mulch. Leave it another year and it becomes dark and fine enough to sieve for potting.

Putting your leaf mould to work

Leaf mould contains very little in the way of nutrients, so treat it as a soil conditioner rather than a fertiliser and keep using your compost or a balanced feed alongside it.

• As a mulch: spread 5–8cm around roses, fruit bushes, herbaceous perennials and woodland plants in autumn. It suppresses weeds, locks in moisture and slowly feeds the soil beneath.
• For clay or sand: dig the coarser grade into vegetable beds every few years. It improves drainage on heavy ground and water retention on light ground.
• In pots: mix one part sieved leaf mould with one part garden compost and one part grit or sharp sand for a lovely free potting blend.
• For seed sowing: the traditional mix is two parts sieved leaf mould, one part loam and one part sharp sand. It is low in nutrients, which suits seedlings perfectly.
• On the lawn: sieve a thin 6mm layer over tired turf in autumn and let the worms pull it down.

Start a batch this autumn and by the time the leaves fall again you will have a free supply of the best soil improver money cannot buy. Patience, damp sacks and a shady corner: that is the whole secret.