Why leaf mould earns its keep

Every autumn, the trees in your street, your hedge and your back garden hand you a free supply of the finest soil conditioner money can't easily buy. Leaf mould is what you get when fallen leaves are left to be broken down by fungi rather than bacteria, and the result is a dark, crumbly, sweet-smelling material that does wonderful things to soil structure.

It is worth being clear about what leaf mould is not. It is not a fertiliser. Compared with garden compost or well-rotted manure, it is low in nitrogen and phosphorus, so it won't feed hungry crops on its own. What it does contain is humus — stable organic matter that holds moisture in sandy soils, opens up heavy clay, encourages earthworms, and supports the fungal networks that healthy roots rely on. Spread it, dig it in, or use it as a seed-sowing medium, and you are building soil that gets better every year.

Which leaves are worth collecting

Not all leaves break down at the same speed, and knowing the difference helps you plan. As a rough guide, thin and soft leaves rot quickest, while thick, leathery or waxy leaves take far longer.

• Quick (often ready in about twelve months): ash, birch, cherry, lime, willow, poplar, elm and most fruit tree leaves.
• Slower (two years or more): oak, beech, hornbeam, sycamore, horse chestnut, sweet chestnut and London plane.
• Best kept out of your main batch: conifer needles, holly, laurel and other evergreens. They are slow, acidic and leathery.
• Handle with care: leaves from trees with known problems, such as horse chestnut leaf miner or rose black spot. They will still rot down, but keep them in a separate batch and don't use the result for seed sowing.

Don't agonise over a mixed bag. A blend of species rots perfectly well, and a bit of oak in the mix simply means a slightly longer wait.

Bin, bag or wire cage?

You have three sensible options, and all of them work. Bin bags are the easiest if space is tight: punch a dozen holes in the sides and base with a fork, fill with damp leaves, tread them down, tie the top loosely and tuck the bags behind the shed. A wire cage — four sturdy stakes with chicken wire wrapped around, roughly a metre square — gives more air and is easier to turn, but takes up more room. A lidded plastic bin or a purpose-made leaf bin sits somewhere between the two.

Whichever you choose, a few practical habits make a real difference:

• Dampen the leaves as you add them. Dry leaves can sit unchanged for years.
• Tread each layer down firmly. A bin that looked full will shrink to a fraction of its volume within weeks.
• Sprinkle in a handful of garden soil or old leaf mould as you go — it introduces the fungi that do the work.
• Add a very thin layer of grass clippings if you want to speed things up, but keep it thin or the whole batch turns slimy.
• Stand the container on bare soil if you can, so worms and soil life can move in from below.

One thing to avoid: don't tip bulk leaves onto your compost heap. The fungal process is slower than the bacterial one in a hot heap, and a thick layer of leaves will simply sit there and stall everything else.

Keeping it damp and letting fungi work

The single most common reason leaf mould fails is dryness, especially during a hot summer. Check your bags or cage every few weeks. If you squeeze a handful and it holds together without dripping, the moisture is about right. If it crumbles, water it — a full watering can over the top of a cage, or open the bags, water and retie them.

Turning is not essential, but it speeds things up. Give a wire cage a fork-over once or twice a year and shake bagged leaves about a bit. White threads running through the leaves are a good sign, not a problem — that is the fungus at work. Because this is a fungal process, the pile will not heat up the way a compost heap does, and you don't need to worry about green-to-brown ratios.

How long it takes, and how to use it

After a year you will have coarse, partly broken leaves — perfectly good as a mulch. After two, most leaves are unidentifiable and the material is dark and crumbly: ideal for mulching borders, fruit bushes and trees. After three years you get the finest grade, friable and dark, which is the stuff to ring-fence for seed sowing and potting mixes.

• Mulch: spread a 5–8cm layer around shrubs and perennials in autumn or spring. Worms will pull it down into the soil.
• Soil improver: fork a 5cm layer into vegetable beds and tired borders.
• Lawn dressing: sieve it and mix with loam and sharp sand for an autumn top dressing.
• Seed and potting compost: use the three-year grade, sieved and mixed roughly half and half with garden compost, plus a little grit or loam.

Start a batch this autumn, label it with the date, and forget about it. There are few jobs in the garden that reward patience so generously, and fewer still that cost nothing but a few bags and a bit of floor space behind the shed.