Why leaf mould deserves a corner of every plot

Autumn delivers a free harvest that most gardeners shovel into the green bin without a second thought. A single mature oak can drop a quarter of a tonne of leaves in a season, and every one of them is a small parcel of valuable organic matter. Left to rot quietly in a wire bin, those leaves become leaf mould: a dark, crumbly, sweet-smelling material that is one of the finest soil conditioners you can make.

It is worth being clear about what leaf mould is and is not. It is not a fertiliser, because it contains very little in the way of nitrogen, phosphorus or potash. What it does contain is lignin, humus and fungal life, and that combination does something no bag of feed can do: it opens up heavy clay, helps sandy soils hold water, and feeds the soil food web that keeps borders thriving. Spread a couple of inches over a bed in spring and you will notice the difference in the way water soaks in, and in how easily your fork slides through the ground the following autumn.

Sorting the good leaves from the slow ones

All leaves rot eventually, but they do not all rot at the same speed. If you have room for more than one bin, it pays to separate the quick customers from the sulkers.

• Fast rotters (roughly a year): ash, lime, elm, willow, birch, apple and other fruit trees. These break down into usable mulch within twelve months.
• Slow rotters (up to two years): oak, beech, sycamore, horse chestnut, sweet chestnut, poplar and London plane. Oak and beech also turn beautifully acidic, which makes them a prize for rhododendrons, camellias and blueberries.
• Best left out: holly, laurel and most conifer needles. They are thick, waxy and slow, and conifer needles in quantity can make the heap too acidic for general border use. A handful here and there will not hurt, but do not make them the bulk of the pile.

Mixing fast and slow leaves in one bin is perfectly fine if space is tight. Just be prepared for the heap to be ready in stages, and use the crumbly material at the bottom while the top is still recognisably leafy.

Building a bin that does the work for you

A wire bin is the classic solution because it lets air and rain in while keeping the leaves corralled. You do not need anything clever. Four wooden posts driven about 45cm into the ground at the corners of a square metre, wrapped in chicken wire or a sturdy plastic mesh, will hold far more than you would think. Line the sides with a couple of layers of cardboard if you want to slow the drying wind, but leave the top open to the rain.

There is one trick that halves the waiting time: shredding. Run the lawnmower over a layer of leaves on the lawn, collecting as you go, and you will end up with a bag of chopped fragments that rot far faster than whole leaves. Do not shred leaves you intend to keep separate for acid-loving plants, as the mix gets muddled, but for general-purpose leaf mould it is a huge time saver.

If you are short of space, a few bin bags with a handful of holes punched in the sides work surprisingly well. Fill them, tie them loosely, tuck them behind the shed and forget about them.

Keep it damp, keep it aired

Leaf mould is made by fungi and worms rather than by the heat-loving bacteria of a compost heap, so it does not need turning in the way a compost pile does. What it does need is moisture. A heap that dries out will sit there stubbornly for years, still looking like leaves. In a dry autumn, water the heap as you build it, and give it a good soak during any prolonged dry spell. Covering the top with a piece of old carpet or a folded tarpaulin helps hold moisture in without sealing out the air.

Once a year, ideally in early autumn, fork the heap from one bin into another. This mixes the dry outer edges into the damp centre, breaks up matted layers, and gives you a chance to see how far the process has got. It is also a good moment to squirt in a little water if things feel dusty.

When it is ready, and where to put it

After twelve months you should have a coarse, crumbly mulch with a few stubborn leaf skeletons in it. After two years, the material becomes fine and dark, closer to a potting compost in texture. Sieve the oldest stuff if you want a fine grade for seed trays, though be aware it holds few nutrients and is best mixed half and half with garden compost for that purpose.

Where to use it? Everywhere. Spread a 5cm layer around roses, fruit bushes, shrubs and herbaceous perennials in spring, keeping it clear of stems. Dig it into vegetable beds in autumn as a soil conditioner. Top-dress containers. Mulch woodland plants and acid lovers with the oak and beech batch. It is gentle enough to use at almost any time of year, and there is no such thing as too much of it.

Start a bin this autumn and you will wonder why you ever bagged the stuff up and gave it away.