Using Leaf Mould to Feed Garden Soil

Why leaf mould earns its place in a cold-weather garden

When the first hard frosts strip the trees, most of us are thinking about protecting tender plants rather than planning next year's soil. That is a shame, because autumn leaves are the cheapest soil improver you will ever get your hands on. Leaf mould is not a fertiliser in the usual sense — it holds very little nitrogen, phosphorus or potassium — but what it does superbly is build structure. Spread it on heavy clay and it opens the soil into crumbs that drain in winter and hold moisture in summer. Work it into light sandy ground and it acts like a sponge, keeping nutrients where roots can reach them.

There is a fungal story here too, and it is the reason leaf mould behaves so differently from compost. Leaf mould is made mostly by fungi rather than bacteria, and the resulting material is rich in the dark, crumbly humus that supports mycorrhizal fungi. Those fungi partner with the roots of most shrubs and perennials, effectively extending the root system and helping plants shrug off drought, poor soil and cold. A garden that receives a regular dressing of leaf mould simply has a livelier soil food web.

Which leaves are worth collecting

Not all leaves rot at the same speed, and a little sorting pays off.

• Best: oak, beech, hornbeam, alder and field maple. These break down into a fine, friable material that is a joy to handle.
• Good: most other deciduous leaves, including birch, rowan and lime.
• Slow: horse chestnut, sycamore and sweet chestnut — they work, but they want an extra year.
• Avoid: conifer needles, which are acidic and slow, and leathery evergreens such as laurel, holly and cherry laurel, which can take three years or more.

Leave walnut and eucalyptus out as well if you have them; both contain compounds that slow decomposition and can inhibit other plants while fresh. And never add leaves from diseased trees to a mix you intend to spread around the same species.

Bag it, bin it, or build a cage

The simplest method in a small garden is the bin liner. Fill black bags with damp leaves, stab a few drainage holes in the sides, tie the tops loosely and tuck them behind a shed or hedge. Rain keeps them moist; the holes let air in and excess water out. It is not elegant, but it works, and a stack of bags takes up almost no room.

If you have more space, a wire cage is better for larger volumes. Make a cylinder about a metre across from chicken wire or rigid mesh, held with a few stakes, and simply tip the leaves in. Tread them down as you go — it is surprising how much a cage will hold — and water them if the autumn has been dry. Whichever route you take, aim for damp but not sodden: if you squeeze a handful and a drop or two appears, that is about right.

The two-year timetable

Leaf mould is a patient business. Collected this autumn, leaves will be recognisable and coarse after twelve months — perfectly usable as a mulch, though not yet as a sowing medium. After two years the material turns dark brown, crumbly and earthy-smelling, and that is the point at which it is genuinely worth digging in. Some leaves take three years, particularly sycamore and horse chestnut.

The trick to avoiding a gap in supply is to start a new batch every autumn and label the bags with the year. Once you have three batches at different stages, you will always have something ready to spread.

Speeding things up

You can shave a year off the process without any special equipment.

• Run the mower over the lawn once leaves have fallen. Chopped leaves rot far faster than whole ones, and the shreds can go straight into the bags.
• Mix in a few grass clippings, or a handful of soil from the vegetable bed, to introduce the microbes that get things going.
• Turn the pile once or twice if you are using a cage — this alone can knock months off the wait.
• Keep the bags out of full sun; steady, cool, damp conditions suit the fungi better than a hot, drying spot.

Using leaf mould around frost-hardy plants

Leaf mould is gentle, so it suits even the most delicate things in the garden. For soil improvement, spread a 5cm layer over beds in late winter and let the worms work it in — no digging required. As a mulch, a layer of the one-year material around the crowns of hellebores, hardy geraniums, ferns and spring bulbs will insulate roots against hard frost as well as suppress weeds, and it will not burn foliage the way fresh manure can.

Two-year leaf mould makes an excellent ingredient in a homemade seed mix — roughly one part leaf mould to one part garden compost and one part grit or sharp sand. It also works nicely as a top dressing for container-grown shrubs and alpines, and as a conditioner in the planting hole of new bare-root hedging in winter. Keep a bag or two aside for emergencies. A handful worked into the surface of a cold, wet border in February does more good than most of us realise.