Using Leaf Mould to Improve Garden Soil

Why Leaf Mould Deserves a Place in Your Allotment

Every autumn, your allotment site probably has more fallen leaves than you know what to do with. Instead of treating them as a nuisance to be bagged up and carted away, think of them as a free harvest. Leaf mould is one of the finest soil improvers you can make, and it costs nothing but a little patience. Unlike compost, which is rich in nutrients, leaf mould is primarily a soil conditioner. It improves structure, water retention and drainage, and it feeds the soil life that keeps your vegetables healthy. After a year or two in a sack, those dry, rustling leaves transform into a dark, crumbly, sweet-smelling material that will make your raised beds and kitchen garden plots sing.

Which Leaves to Collect and Which to Avoid

Not all leaves are created equal. The best leaves for making leaf mould are those that break down reasonably quickly and don't harbour pests or diseases. Oak, beech, hornbeam and field maple are excellent choices. Horse chestnut and sycamore are fine too, though they take a little longer. If you have a lot of them, shredding helps. Avoid collecting leaves from evergreen trees and conifers, such as pine, spruce or cedar, as their needles are acidic and waxy, taking several years to rot down. Also leave rose leaves alone if they show signs of black spot, and skip any leaves that are slimy or smelly. A mix of different leaves is perfectly fine and often breaks down more evenly than a single type.

• Good choices: oak, beech, hornbeam, field maple, ash, birch, fruit tree leaves.
• Use with patience: horse chestnut, sycamore, walnut (though walnut leaves can inhibit growth, so compost separately for two years).
• Avoid: conifer needles, evergreen leaves, diseased rose leaves, and any leaves treated with weedkiller.

How to Make Leaf Mould in Sacks

The simplest method for allotment growers is the sack method. Collect leaves when they are damp but not soaking wet. If they are dry, give them a quick watering. You can speed up decomposition by running a lawnmower over them a few times, or by chopping them with a spade in a wheelbarrow. Then pack them into old compost sacks, bin liners or woven polypropylene bags. Punch a few holes in the bottom and sides for drainage and airflow. Fill each sack loosely, leaving room for the leaves to settle, then tie the top loosely or fold it over. Store the sacks behind a shed, under a hedge, or anywhere out of direct sun. Keep them moist but not waterlogged. A lid or a piece of old carpet on top helps retain moisture. That's it. Leave them alone for at least twelve months, ideally two years. Check occasionally. If they dry out completely, add a little water. If they smell sour, turn them out and mix in some dry leaves or a handful of garden soil.

Using Leaf Mould as Mulch

After a year or two, your leaf mould will be dark, crumbly and earthy. Use it as a mulch around perennial vegetables, fruit bushes, rhubarb, asparagus and soft fruit. Apply a layer about five to eight centimetres thick, but keep it a few centimetres away from stems and trunks to avoid rotting. Leaf mould mulch suppresses weeds, conserves moisture and slowly improves the soil beneath. It's especially useful on allotment plots with light, sandy soil that dries out quickly, or heavy clay that bakes hard in summer. Unlike manure, leaf mould won't burn plants and won't encourage excessive leafy growth. It's gentle, slow and safe. You can also spread it over bare soil in autumn as a winter blanket, letting worms pull it down into the earth.

Leaf Mould in Potting Mixes and Seed Sowing

Leaf mould is a superb ingredient for homemade potting mixes. Because it is low in nutrients, it won't scorch young roots. For a general potting mix, combine one part sieved leaf mould, one part garden compost and one part coarse grit or sharp sand. For seed sowing, sieve the leaf mould to remove lumps and mix it with an equal amount of sieved compost. Some growers use leaf mould alone for seed sowing, but it can crust over, so a little grit helps. For raising seedlings in modules, a mix of two parts leaf mould to one part grit works well. Just remember that leaf mould has almost no feeding value, so once seedlings have their first true leaves, you'll need to pot them on into a richer mix or start feeding with a liquid organic fertiliser.

Troubleshooting and Getting the Best Results

Leaf mould is forgiving, but a few things can go wrong. If your sacks become waterlogged, the leaves will turn into a smelly sludge. Fix this by emptying them, mixing in dry leaves or shredded cardboard, and repacking loosely. If the leaves are still recognisable after two years, they need more time, more moisture, or more air. Turning the heap or shaking the sacks can help. You can also mix leaf mould with spent mushroom compost or garden compost to stretch it further. Don't worry if you find earthworms, woodlice or fungi in your sacks — they are all part of the process. The best leaf mould is made from a mixture of leaves, kept damp, and left alone. Once you have a supply, you'll wonder why you ever raked leaves into the bin. Your soil, your carrots, your brassicas and your roses will all thank you.