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Why fallen leaves are worth more than a tidy lawn

By mid-October, most British gardens are wearing a carpet of gold, copper and rust. It looks lovely for a fortnight, and then it turns into a brown, soggy mat that smothers the grass and hides the spring bulbs. The temptation is to rake it all up, bag it, and put it out for collection. Please don't. Fallen leaves are one of the finest soil conditioners you can get hold of, and they cost nothing but a little patience.

There is a good reason leaves behave differently from grass clippings or kitchen scraps. They are rich in carbon and poor in nitrogen, so they break down mainly through fungal action rather than the bacterial heat of a compost heap. That makes them slow — but it also means the end product is wonderfully crumbly, dark and friable, holding moisture and improving soil structure for years. Gardeners call it leaf mould, and it is worth its weight in gold.

Sorting the good from the slow

Not all leaves rot at the same speed, and knowing which is which helps you decide what to do with each batch.

• Quick to break down (about a year): sycamore, lime, cherry, ash, poplar, elm and horse chestnut.
• Moderate (eighteen months to two years): oak, beech and hornbeam. Slower, but they make superb quality leaf mould.
• Very slow (two years or more): holly, evergreen oak and conifer needles. These are acidic, so use them sparingly and reserve them for acid-lovers such as rhododendrons, camellias and blueberries.

Leaves carrying tar spot or the horse chestnut leaf miner will still rot down, but keep them out of any mix intended for seed sowing. If you have a hot compost heap, they are better added there, where the heat will deal with any fungal spores.

Leaf mould: the slow, patient method

The simplest approach is a wire cage. Drive four stakes into a corner of the garden — somewhere out of sight behind a shed or hedge is ideal — and wrap chicken wire around them to make a cube roughly a metre across. Fill it with leaves, firm them down as you go, and water them if the pile is dry. An open top lets the rain in; a sheet of old carpet on top during a dry spell keeps the moisture in.

If space is tight, use bin bags instead. Fill them with damp leaves, punch a handful of holes in the sides with a fork, tie the tops loosely and tuck them behind the shed. Either way, leave the leaves alone for twelve to twenty-four months. You will know it is ready when it is crumbly, dark brown and smells of woodland floor rather than rot. Sieve the coarser material and return it to the cage for another year.

Composting leaves: quicker results with a bit of balance

If you would rather have something usable by next autumn, add leaves to your compost heap as the brown, carbon-rich ingredient. Layer them with roughly half as much green material by volume — grass clippings, vegetable peelings, annuals pulled from the borders, or a few handfuls of comfrey leaves. Turn the heap every few weeks to keep it aerated, and it will heat up nicely.

Speed matters here, and shredding is the single best trick. Run the lawnmower over a thin layer of leaves on the grass and tip the chopped results into the heap or the leaf cage. Shredded leaves rot in roughly half the time, and a light dusting of soil or old compost between layers introduces the fungi that do the work.

Putting leaf mould to work in the garden

• Spread it as a mulch around woodland plants: ferns, hostas, hellebores, wood anemones and bluebells all adore it.
• Sieve it finely, mix with garden compost and a little grit, and it makes an excellent seed-sowing mix.
• Use it as an autumn lawn dressing, spread thinly and brushed in with a stiff broom.
• Dig it into vegetable beds or top-dress borders and fruit bushes for a slow, steady feed.

Leave some for the wildlife

A completely tidy garden is a poor habitat. Leaf litter shelters hedgehogs, frogs, toads, beetles and the pupae of moths, and blackbirds and robins rummage through it for food all winter. Rake the lawn by all means, but leave the leaves under hedges, in the corners behind shrubs and around the base of mature trees. A generous layer there will feed the soil and the wildlife at the same time.

One last word of caution: if you are tempted to burn the surplus, check the pile first. Hedgehogs, frogs and slow worms often tuck themselves into a dry heap of leaves, so move it a day or two before you light anything, and ideally compost it instead. Autumn's debris is next spring's soil — it is a shame to waste it.