How to Improve Soil with Leaf Mould

Come late October, most British gardens are drowning in leaves. The temptation is to bag them up, drive them to the tip and forget the whole business until next year. But those fallen leaves are one of the cheapest and most generous soil improvers available to any kitchen gardener. Given nothing more than a damp corner and a year of patience, they break down into leaf mould: a dark, crumbly, sweet-smelling material that feeds the soil rather than the plant, and which no bag of bought compost quite replicates.

Why Leaf Mould Deserves a Place in the Kitchen Garden

Leaf mould works differently from compost. It is low in nutrients — much of the nitrogen leaches away during the slow fungal breakdown — so it does not feed plants directly. What it does brilliantly is improve soil structure. Dig it into heavy clay and the stuff begins to crumble into a workable loam; fork it through sandy soil and it holds water like a sponge. It is also a magnificent mulch, smothering weeds around hungry crops such as courgettes, beans and brassicas, and when sieved it makes an excellent seed-sowing medium mixed with garden compost and a little sharp sand.

Best of all, it costs nothing but an afternoon with a rake.

Which Leaves Are Worth Keeping

Not all leaves behave equally. Thin, papery leaves break down fastest and make the finest mould.

• Best: oak, beech, hornbeam, field maple, lime and sycamore. Oak and beech are the classic pairing — slow, but superb.
• Good and quick: apple, pear, cherry and other fruit-tree leaves, plus most soft hedgerow trimmings.
• Slow: horse chestnut and walnut, which are leathery and can take a year longer than the rest.
• Avoid in bulk: evergreen leaves such as holly, laurel and rhododendron, and conifer needles. They are waxy, acidic and can take three years or more. A sprinkling of pine needles is fine; a whole bin of them is not.

If you garden on a modest plot, do not agonise over the mix. Sweep up what falls, keep the evergreens out, and let the rest get on with it.

Bag It or Bin It: Two Simple Ways to Store Leaves

The golden rule is damp, dark and contained. Bone-dry leaves simply sit there for years; sodden ones turn to slime. Aim for the moisture level of a wrung-out sponge.

• The bin-bag method — ideal for small gardens. Fill black bin bags with damp leaves, punch half a dozen holes in each with a fork, tie the tops and stack them behind the shed. Weigh them down with a brick so they do not blow about. Tidy, cheap and it takes up almost no room.
• The wire bin method — best if you have the space. Make a cylinder of chicken wire about a metre across, held in place with a few stout stakes. Fill it with leaves, tread them down firmly to squeeze out air pockets, and water it if the weather turns dry for a fortnight. A second bin started a year later gives you a rolling supply.

Whichever route you take, keep the heap out of direct sun and scatter a handful of garden soil or a few comfrey leaves between layers if you want to nudge things along.

What Happens During the Waiting Year

Leaf mould is a fungal process rather than a bacterial one, which is why it takes longer than compost and why it rewards being left alone. Expect the volume to drop by half to two-thirds. After twelve months you will have a coarse, crumbly mulch with a few identifiable leaf shapes still in it. Leave it a second year and it becomes a fine, dark material you can rub between your fingers — this is the grade to sieve for seed sowing.

You can hurry it slightly by turning the pile once in spring and keeping it moist through dry spells, but there is no shortcut worth chasing. Cold winters slow it down; mild, damp ones speed it up.

Putting It to Work

• Spread a 5cm layer over bare beds each autumn as a soil-conditioning mulch.
• Mulch around raspberries, blackcurrants, rhubarb and fruit trees in late winter.
• Dig it into the top 15cm of clay soil a few weeks before sowing roots and salads.
• Mix equal parts leaf mould, garden compost and sharp sand for a fine potting blend.
• Top-dress hungry crops in mid-season and water it in well.

A Few Final Notes

Skip leaves collected from busy roadside verges, where exhaust residue lingers, and avoid anything from a tree you know has been sprayed. Keep bulk leaves out of your normal compost heap too — they slow the whole thing down and you will get better results keeping the two systems separate.

Start a bin this autumn, even if it is only three bin bags behind the shed, and this time next year you will have a free supply of crumbly, sweet-smelling mulch ready to fork into your beds. It is perhaps the least effort you will ever spend on your soil, and the garden will thank you for years afterwards.