Why Bother With Leaf Mould?

Every autumn, Britain's gardens and allotments produce a free harvest that most people rake into bags and cart off to the tip. Fallen leaves are not rubbish. Left in a heap for a year or two, they rot down into leaf mould, one of the finest soil conditioners you can lay your hands on. It is not a fertiliser — it holds precious little in the way of nutrients — but it does something arguably more valuable. It opens up heavy clay, gives body to thin sandy soil, holds moisture through dry spells, and feeds the soil life that makes everything else work.

There is a second reason to make a proper bin for it. Leaves do not belong in your ordinary compost heap. They are dry, low in nitrogen, and slow to break down, and a big dump of them will cool your compost down and slow the whole operation just when you want it ticking over. Give leaves their own space and both jobs get easier.

Choosing a Spot and Gathering Materials

Leaf mould is made by fungi, not by the heat-loving bacteria that run a compost heap, so the bin needs no insulation, no turning schedule, and no lid. What it does need is air, moisture, and a bit of patience. A simple wire frame does all three jobs.

You will need:

• Four sturdy wooden stakes, roughly 1.2 metres long, ideally 5cm square. Tanalised timber or chestnut paling posts last longest.
• Roughly 4 metres of wire mesh per bin — chicken wire or, better, weldmesh with 25mm holes, which holds leaves while letting air move freely.
• A hammer or maul, a pair of heavy gloves, and stout cable ties or galvanised staples.

Site it somewhere out of the way but reasonably accessible — behind the shed, along a boundary fence, or on a spare corner of the plot. A little shade is fine and even helpful, as it stops the heap drying out. You want level ground, because the bottom of the heap needs contact with soil so worms and fungi can move up into it.

Building the Bin

A useful size is a cube about a metre on each side. Any smaller and the heap dries out too quickly; much bigger and the middle sits stagnant for years.

Drive the four stakes in at the corners of your square, leaving about a metre above ground. Knock them in truly vertical — a leaning bin sags and spills. Then wrap the wire round the outside of the posts, securing it to each stake as you go. Leave the top completely open and, if you like, leave one side as a removable panel held with a couple of hooks, so you can get a fork in when the time comes to empty it.

Line the inside of the wire with a couple of sheets of damp cardboard if you have them. It stops leaves blowing out through the mesh in the first gusty week and rots down happily alongside them. Do not line the bin with plastic or put a lid on it. Airflow is the whole point.

Filling the Bin: The Details That Matter

Rake leaves up when they are dry and crisp, but get them into the bin within a week or so. Leaves left sitting damp in a plastic sack go slimy and anaerobic, and that smell never quite leaves the finished product.

As you fill, tread the heap down every 20cm or so. This halves the volume and keeps the leaves in contact with each other, which is exactly what the fungi need. A loose, airy pile of leaves simply sits there looking untroubled for three years.

A few additions speed things along:

• A thin scattering of garden soil or old compost every few layers, to introduce organisms.
• A light dusting of a nitrogen-rich organic fertiliser, or a sprinkling of grass clippings, to balance the carbon.
• Water if the autumn has been dry. The heap should feel like a wrung-out sponge.

Mix your leaves where you can. Sycamore, lime, ash, birch and willow break down in a year. Oak, beech and hornbeam are tougher and can take two. Conifer needles and horse chestnut leaves are slow and acidic, so keep them to their own bin or use them sparingly as a mulch under acid-lovers such as blueberries and rhododendrons.

How Long, and What To Do With It

Leave the heap alone. Check it once or twice over winter and water it if it has dried out. After twelve months, the bottom of the bin will have turned into a dark, crumbly material with a pleasant woodland smell. Fork it out and sieve it if you want a finer grade.

One-year leaf mould is coarse and perfect as a mulch: spread a 5cm layer round fruit bushes, roses and perennials to suppress weeds and hold moisture. Two-year leaf mould, especially if sieved, is a wonderful ingredient in seed and potting mixes, mixed roughly one part to two of your own compost. It is also the best lawn top dressing going — brush a thin layer into the turf in autumn and the worms will do the rest.

A Few Things That Go Wrong

If your heap has sat unchanged for years, it is almost certainly too dry. Soak it with a hose and fork it over to get some air in. If it smells sour and slimy, it has gone anaerobic from being too wet and compacted — turn it, mix in some dry leaves or shredded cardboard, and it will right itself.

Finally, do not expect next spring's crop from this autumn's leaves. Leaf mould is a slow, quiet sort of gardening, and that is part of its charm. Build the bin now, fill it well, and by the time you are sowing next year's broad beans you will have a free supply of the best soil conditioner money cannot easily buy.