Why Bother With Leaf Mould?

Every autumn, a smallholding produces a free harvest that most people rake up and burn. Fallen leaves are one of the best soil conditioners you can lay your hands on, and they cost nothing but a bit of effort and patience. Leaf mould is not a fertiliser in the usual sense — it is low in nitrogen, potassium and phosphate — but what it does brilliantly is improve soil structure, hold moisture and feed the worms and fungi that keep everything else ticking over.

On heavier clay, leaf mould opens up the soil and lets air in. On light, sandy ground, it acts like a sponge and stops nutrients washing straight through. Sieved, it makes a superb seed-sowing mix and a gentle potting compost for young plants that would sulk in something richer. Around fruit trees and soft fruit bushes, a mulch of leaf mould suppresses weeds, keeps roots cool in summer and slowly builds a crumbly, woodland-style topsoil that orchard trees adore.

Gathering Leaves: Which Ones and When

The best time to start is as soon as the leaves begin to fall, usually from late September through to early December depending on where you are. Sweep or rake them up while they are still dry if you can — damp leaves clog rakes and make the job twice the work. A plastic leaf rake, a tarpaulin and a bit of help from the family turns a tedious chore into an hour's work around the orchard.

Not all leaves break down at the same speed, so it pays to know what you are dealing with:

• Quick to rot (roughly one year): sycamore, ash, lime, horse chestnut, willow and most fruit tree leaves. These are the ones to use if you want results soon.
• Slow (up to two years): oak, beech, sweet chestnut, holly and conifer needles. Excellent mould eventually, but they need patience and a separate bin so they do not hold up the rest.
• Avoid altogether: leaves from diseased trees — black spot on apples, for example — and anything covered in a thick layer of road grit or oil. Compost the diseased ones properly or bin them rather than spreading the problem around your beds.

If you keep chickens or geese, let them scratch through a pile of leaves for a week before you bag it up. They will shred it, add a little manure and save you some turning later.

Wire Bins, Hessian Sacks and Other Containers

You do not need anything fancy. The classic approach is a simple wire bin made from a metre or so of galvanised chicken wire formed into a cylinder and held with four wooden stakes driven into the ground. A bin roughly a metre across and a metre high will swallow a surprising volume of leaves — expect it to shrink to perhaps a tenth of its original bulk as it rots, so do not be shy about filling it right to the top.

Wire bins have the advantage of excellent airflow, which is why leaves in them rot faster than in a solid container. If you have more leaves than bins, hessian sacks are the traditional alternative. Fill them loosely, tie the necks loosely and stand them somewhere out of the way — behind a shed, along a hedge or under a tree. Old feed sacks, builder's bags and even bin bags with a few holes punched in them all work, though plastic tends to go anaerobic and smelly if you are not careful.

Whatever you use, site it somewhere with dappled shade rather than full sun. Leaf mould wants to stay moist, and a bin baking in direct summer sun will dry out faster than you can keep up with.

Keeping It Damp and Turning It

Dampness is the single most important factor in making good leaf mould. The heap should be like a wrung-out sponge — moist right through, but never waterlogged. In a wet British autumn you will rarely need to do anything, but a dry spell in spring or a hot summer will stop the process dead. If the leaves look dusty or rustle when you dig in, give the bin a good soak with a watering can, then cover it with an old piece of carpet or a tarpaulin weighed down with bricks to hold the moisture in.

Turning is where the real acceleration happens. Once every two or three months, fork the heap from one bin into another, or simply pull the wire cylinder off, mix the contents and rebuild it. This introduces air, breaks up mats of compacted leaves and lets you check the moisture level throughout. A heap turned regularly can be ready in a year; one left alone will take two.

Tread the leaves down lightly as you build the heap, but do not compact them fiercely — you want air pockets, not a solid brick. Adding a few handfuls of garden soil or finished compost between layers introduces the microorganisms that get things moving, and a sprinkle of urine-diluted water (a traditional trick) gives the heap a nitrogen boost.

How to Tell When It's Ready

Leaf mould is ready when the leaves have lost all trace of their original shape and structure. The colour should be a dark, rich brown, and the texture like coarse crumbs or fine potting compost. Rubbed between your fingers it should feel soft and slightly spongy. If you can still recognise an oak leaf, it needs more time.

When you judge it done, sieve the best of it through a quarter-inch riddle for potting and seed mixes, and use the rougher material as a mulch straight away. Any leaves that have not broken down can go back into the next bin to carry on rotting.

Putting It to Work on the Smallholding

Use the fine, sieved leaf mould as the base of your own seed compost — roughly equal parts leaf mould, sharp sand and garden soil, with a little sieved garden compost for fertility. For potting on shrubs and perennials, mix it half and half with your normal compost to lighten the mix and encourage strong roots.

Around the orchard, spread a generous mulch of coarse leaf mould in a ring out to the drip line of each tree in spring, keeping it clear of the trunk itself. Soft fruit respond well too — a mulch under raspberries, currants and gooseberries keeps the roots cool and the fruit clean. On woodland plants such as rhododendrons, blueberries and ferns, leaf mould is close to ideal, mimicking the forest floor they evolved in. Even the vegetable beds benefit: dig it into the top few inches in autumn and let the worms do the rest over winter.

It takes a year or two of waiting, but leaf mould is the sort of quiet investment that pays back season after season, and it all starts with a rake and a few wire bins in the corner of the orchard.