Using Fallen Leaves to Build Leaf Mould

Why leaf mould is worth the wait

Every autumn, British gardens produce the raw material for one of the best soil improvers you can get — and most of it ends up in the council green bin or, worse, in a smoking pile at the bottom of the garden. Fallen leaves are free, plentiful and quietly transform into leaf mould: a dark, crumbly, almost black material that smells of woodland floor on a damp morning.

Unlike compost, which is broken down mainly by bacteria and heats up as it works, leaf mould is the product of fungal decay. That makes it low in nutrients but superb for soil structure. Dig it in and it opens up heavy clay, helps sandy soils hold water, and feeds the soil life that keeps plants healthy. It is also, conveniently, exactly what British woodland plants have evolved with. Bluebells, wood anemone, primroses, wild garlic, ferns, hellebores and foxgloves all expect a carpet of slowly rotting leaves above their roots. Give them leaf mould and they will look at home.

Which leaves rot down best

Not all leaves are equal. Thin, soft leaves break down quickly; thick, leathery ones can sit there for years. Sort them roughly as you collect, or at least be prepared for different waiting times.

• Quick (roughly a year): oak, beech, hornbeam, birch, hazel, ash, alder and lime. Oak and beech are the classic pair and make excellent leaf mould.
• Slower (up to two years): sycamore, horse chestnut, sweet chestnut, walnut, London plane and most large ornamental leaves.
• Best avoided: evergreen leaves such as holly, laurel, conifer and leylandii. They are waxy and tough, and a pile of them will still be a pile of them three years from now.
• Bin or burn instead: leaves carrying disease — rose black spot, horse chestnut leaf miner, ash dieback. Don't spread those back around the garden.

Acorns, conkers and twigs will slow the whole process down, so shake them out where you can. Shredding helps enormously: run the lawnmower over the lawn once leaves have fallen, or over a pile on the patio, and the chopped pieces will rot in about half the time.

Building a wire bin or using a bin liner

For most gardens, a simple wire bin is ideal. Drive four wooden stakes into the ground in a square about a metre across, wrap chicken wire around the outside, and leave one side openable for turning. Site it somewhere out of the way — behind the shed, against a north-facing fence — because it is not a thing of beauty and it does not need sun.

Fill it in layers as the leaves fall, pressing each layer down lightly but never compacting it. Keep leaf mould separate from your compost heap. Compost relies on bacteria and a good mix of greens and browns; a pile of pure leaves will slow your compost down and produce something quite different.

If space is tight, the bin liner method works well. Fill a heavy-duty black sack with damp leaves, punch a few drainage holes in the base and sides, tie the top loosely and tuck it behind a shed or hedge. Check it every few months and add a splash of water if it has dried out. Two or three sacks will quietly produce enough leaf mould for a small border.

Keeping the pile damp

This is the single thing most people get wrong. Leaves need to be damp, not merely present. A dry pile will sit unchanged for years; a damp one will be crawling with worms and fungal threads within weeks.

• Water the pile with a can if autumn turns dry, and again during any long summer spell.
• Cover the top with a piece of old carpet, a flattened cardboard box or a tarpaulin weighed down with bricks to hold moisture in.
• Turn the heap once or twice a year if you have the energy — it speeds things up but is not essential.
• Leave the base in contact with the soil so worms and soil organisms can move in freely.

When it's ready, and how to use it

After twelve months you will have a coarse, dark mulch. After two years, you will have fine, friable crumbs you can run through your fingers. Sieve it if you want the finer grade for sowing.

Uses worth knowing:

• Mulch: spread a 5cm layer around woodland perennials, shrubs, ferns and spring bulbs in autumn or early spring.
• Soil conditioner: fork it into the top 15cm of borders before planting.
• Seed sowing: sieved leaf mould mixed with sharp sand and garden loam makes a lovely low-fertility seed mix, ideal for woodland wildflowers.
• Lawn dressing: a thin scatter brushed into the grass improves the turf.

Leaves, wildlife and the woodland bed

Before you sweep up every last leaf, remember that a certain amount of litter is habitat. Hedgehogs, frogs, slow-worms, beetles and moth pupae all shelter in damp leaf layers over winter, and birds will rummage through them for grubs. Leave deep piles under hedges and in quiet corners — and leave fallen leaves in place beneath trees and shrubs, where they will feed the soil directly.

A leaf bin at the end of the garden, a mulch of your own leaf mould around the bluebells and wood anemones, and a few untouched drifts of leaves under the hedge together make a garden that works a little more like a British wood. All it asks of you is a rake, a bit of wire and some patience.