Why leaf mould is worth the wait

Every autumn, streets, parks and gardens are carpeted with fallen leaves. Many people rake them into the green waste bin or burn them, but those leaves are one of the best free resources a gardener can have. Leaf mould is the dark, crumbly, sweet-smelling material you get when leaves are left to break down slowly by fungi. It is not the same as garden compost, which is made mostly by bacteria and heats up as it rots. Leaf mould is cooler, quieter and slower, but the result is a superb soil conditioner.

It holds moisture, opens up heavy clay, gives body to sandy soil and encourages earthworms. It is low in nutrients, so it will not burn plants or upset soil balance. Think of it as a conditioner rather than a fertiliser. Best of all, it costs nothing but time and a few bags or bins.

Which leaves make the best leaf mould

Not all leaves break down at the same speed. Thin, soft leaves rot fastest, while thick, leathery or waxy leaves take much longer. You can still use the slow ones, but mix them with faster material or shred them well.

• Quick to break down: ash, cherry, lime, poplar, willow, elm and most fruit tree leaves. Usable in about a year.
• Slower but excellent: oak, beech, hornbeam and alder. May take two years or more, but the quality is outstanding.
• Very slow: sycamore, horse chestnut, plane and sweet chestnut. Shred them or mix with grass clippings.
• Avoid or use sparingly: laurel, holly, rhododendron and other thick evergreens. Pine and conifer needles take two to three years, though they make a useful mulch for acid-loving plants.
• Leave out: leaves from diseased trees, especially roses with black spot, and any contaminated with oil, paint or dog waste. Walnut leaves contain juglone, so avoid using their leaf mould around sensitive vegetables.

If you have a mix, do not worry. A blend of fast and slow leaves makes good leaf mould. Sieve out tougher ones later for a finer product.

Choosing a container: bags, bins and wire cages

The simplest method is to stuff damp leaves into black plastic bin bags. Punch a few holes in the sides and bottom for air and drainage, fill the bag, tie it loosely and leave it behind a shed or under a hedge. This works well in small gardens. Woven polypropylene bags breathe even better. A reusable bin with a lid also works: drill holes around the sides and fill it with leaves.

For larger gardens, a wire cage is ideal. Make a cylinder about a metre across using chicken wire or sturdy mesh, held in place with a few stakes. Fill it with leaves and leave it open to the rain. The leaves will settle and shrink dramatically. You can also use a spare compost bin, a cardboard box with the bottom removed, or a wooden pallet bin. Site it out of the way but easy to reach.

How to speed up decomposition

Leaf mould will happen on its own, but a few simple tricks can shave months off the process.

• Shred the leaves. Run over them with a lawnmower, or put them in a bin and use a strimmer briefly. Smaller pieces break down faster.
• Keep them moist. Leaves need to be as damp as a wrung-out sponge. Water them as you fill the bag or bin if dry. If sodden, mix in dry leaves.
• Add a nitrogen boost. A few handfuls of grass clippings or comfrey leaves can kick-start activity. Do not overdo it, or the heap may turn slimy.
• Turn occasionally. Every few months, empty the bag or bin and mix the leaves. This introduces air and evens out moisture.
• Be patient. After one year, you will have a coarse mulch. After two years, a fine, crumbly leaf mould ready for potting mixes and seed sowing.

Using leaf mould in the garden

Leaf mould is wonderfully versatile. Use it as a mulch around trees, shrubs, roses, fruit bushes and perennials. Spread a layer 5–8cm thick in autumn or spring, keeping it clear of stems and trunks. It suppresses weeds, conserves moisture and slowly improves the soil. Fork it into vegetable beds before planting to boost water retention and structure.

For a homemade potting mix, sieve two-year-old leaf mould and blend it with garden compost and loam. A classic mix is one part leaf mould, one part garden compost and one part loam. This is excellent for seedlings, tomatoes, and woodland plants such as ferns, hostas and hellebores. Leaf mould also makes a fine autumn lawn dressing, mixed with a little sand and brushed into the grass. Acid-loving plants like rhododendrons, camellias and blueberries appreciate a mulch of conifer-needle leaf mould.

Common problems and easy fixes

Leaf mould is forgiving, but a few issues can crop up.

• Slimy, smelly leaves: too wet and compacted. Mix in dry leaves or shredded cardboard, turn the pile and leave the container open to the air.
• Dry, unchanged leaves: too dry. Water them, cover the container to keep moisture in, and turn.
• Leaves blowing away: dampen them before filling bags or bins, and weigh down the top with a brick or cardboard.
• Slow decomposition: shred the leaves, add grass clippings, and check that the container is not sealed too tightly. Fungal activity needs air.
• Weeds or seeds sprouting: leaf mould is not hot enough to kill all weed seeds. Avoid adding leaves from weedy areas, or use the mould as a mulch rather than in seed compost.

Once you start making leaf mould, you will never look at a drift of autumn leaves the same way again. It is free, it is easy, and it turns a garden nuisance into a garden treasure. Start a bag or bin this autumn and your soil will thank you.