Autumn's Most Generous Gift

Every October and November, Britain's woods and gardens shed a fortune in slow-release soil improver, and most of it ends up in the green bin or on the bonfire. Leaf mould is simply fallen leaves left to rot down in a cool, damp heap or bag, broken apart mostly by fungi rather than the bacteria that drive a hot compost heap. The result is a dark, crumbly, sweet-smelling material that holds water beautifully, improves almost any soil and makes a superb ingredient in potting mixes.

It costs nothing but a little effort and a year or two of patience. If you have trees nearby — a street lime, a garden oak, a hedgerow of beech — you already have the raw material.

Which Leaves to Use, and Which to Avoid

Not all leaves behave the same way in a bag, and knowing the difference saves a good deal of frustration later.

• Quick to break down (roughly 9–12 months): oak, beech, hornbeam, alder, ash, birch and field maple. These are the gold standard — thin, low in lignin and quick to soften.
• Slower (18–24 months): sycamore, horse chestnut, lime, walnut and sweet chestnut, whose thicker leaves take far longer to crumble.
• Best kept separate: holly, laurel and conifer needles. They rot down very slowly and produce a distinctly acidic mould, useful in its own right around rhododendrons, camellias and blueberries, but not what you want for seed sowing.

Avoid collecting from busy roadside verges, where leaves come mixed with grit and grime. Otherwise, mix your leaves together freely — a blend of species rots more evenly than a single one.

Gathering and Storing Your Haul

Speed matters more than neatness here. Whole leaves can take two years; shredded leaves can be ready in one. Run the lawnmower over a thin layer on the grass with the collector box on, and you will have a half-shredded, airy mix that fungi can get to work on straight away.

For storage, you have three reliable options:

• Bin bags: the classic. Fill black bags, punch a dozen holes in each with a fork, tie the tops loosely, and tuck them behind a shed or under a hedge.
• A wire cage: four stakes and a length of chicken wire, about a metre square, in a shady corner. Cheapest of all and the easiest to turn.
• A bin or builder's bag: ideal if you have an old dustbin or tonne bag going spare. Add a few drainage holes in the base.

Whichever you choose, damp the leaves as you go if they are dry, and tread them down to exclude air pockets. Aim for the consistency of a wrung-out sponge, never a swamp.

Let the Wildlife Move In

A pile of leaves left in a quiet corner is one of the best pieces of habitat you can offer a British garden. Hedgehogs, frogs, toads and slow worms all use leaf mould and leaf piles as winter shelter, while beetles, millipedes and a whole kingdom of fungi live in it year-round. Blackbirds and song thrushes will work patiently through the edges for grubs.

This is one good reason to keep at least one bag or cage undisturbed between October and April. Empty and turn your stores in late spring or early summer, once everything has woken up, rather than in the depths of winter.

Patience, Then the Payoff

Check your bags after twelve months. You should find dark brown, crumbly material with an earthy, woodland smell and only a few recognisable leaf skeletons. Sift out any twigs and use that coarser mould as a mulch or soil conditioner. Leave it another year and the texture becomes fine and friable — this is the leaf mould worth sieving for potting.

If it looks dry, water it. If it smells sour and slimy, it is too wet: mix in some dry leaves or a handful of shredded paper and turn it. Very little else can go wrong, and the heap asks nothing of you in between.

Putting Leaf Mould to Work

• Soil improvement: spread 5–8cm over beds in autumn and let the worms pull it down. It transforms heavy clay and helps sandy soil hold water.
• Mulch: a 5cm layer around shrubs, perennials and woodland plants conserves moisture and suppresses weeds.
• Potting mixes: blend one part sieved leaf mould with two parts loam and one part grit for a free-draining mix for established plants.
• Seed sowing: sieved leaf mould is naturally low in nutrients, which is exactly what seedlings want. Mix with equal parts sharp sand for a fine sowing compost.
• Lawn top dressing: a thin autumn dressing brushed into the turf improves the soil beneath without pushing lush, soft growth.

Start a bag or a cage this autumn, forget about it, and you will have a season's worth of free, home-made soil conditioner waiting when the sap rises again.