Why Leaf Mould Deserves a Place in Every Garden

Come October, most of us are out there with a rake and a mounting sense of resentment. The lawn disappears under a blanket of sycamore, oak and beech, and the whole business feels like a chore with no end. But those fallen leaves are one of the finest free resources a garden can offer, and turning them into leaf mould is about as close to alchemy as gardening gets. Unlike compost, which needs a careful balance of greens and browns and a good deal of turning, leaf mould asks almost nothing of you. Pile the leaves up, keep them damp, and wait. A year or two later you will have a dark, crumbly, sweet-smelling soil conditioner that will transform the structure of your beds, improve water retention on sandy ground and open up heavy clay.

It is worth understanding what leaf mould actually is. It is not compost in the usual sense. The breakdown is carried out mostly by fungi rather than bacteria, which is why it happens slowly and coolly, and why the end result is so rich in humus. That humus is what your soil has been crying out for.

Which Leaves to Use and Which to Avoid

Not all leaves are created equal, and a little sorting will save you time later. Thin, quick-rotting leaves are the gold standard. Oak, beech, hornbeam and field maple all break down reasonably quickly and produce excellent mould. Sycamore and horse chestnut are thicker but still perfectly usable, though they will take longer.

There are a few to treat with caution:

• Evergreen leaves such as holly, laurel and conifer needles are waxy and leathery. They will rot eventually, but they can take three years or more. Keep them in a separate bag and be patient.
• Leaves from diseased trees, particularly those affected by black spot on horse chestnut or rose blackspot, are best kept out of the mix and disposed of rather than composted.
• Leaves from walnut trees contain juglone, which can inhibit growth in some plants. Use them only after a long, thorough decomposition.
• Weeds, grass clippings and kitchen scraps do not belong in a leaf mould pile. Adding them turns it into a compost heap and slows the fungal process down.

In practice, most of us simply sweep up whatever falls and mix it all together. That works perfectly well. The important thing is to get the leaves collected before they are rained into a sodden mat on the lawn.

The Bin Bag Method: Simplest of All

If you have limited space or simply want the least fuss, the bin bag approach is hard to beat. Fill a black plastic refuse sack with damp leaves, shake it down, and tie the top loosely. Then puncture the sides a few times with a fork to let air in and excess water out, and tuck the bags behind a shed or under a hedge.

Do not fill them too tightly. You want the leaves to remain moist but not waterlogged, so if the foliage is bone dry when you bag it, give it a light watering first. Aim for the consistency of a wrung-out sponge. Leave the bags for twelve months, and you will open them to find a dark, crumbly material at the bottom and partly broken leaves at the top. Give the contents a turn into a fresh bag, or simply use what is ready and re-bag the rest for another six months.

Black bags work better than clear ones because they hold warmth and exclude light, which discourages weeds from germinating in the mix. Store them somewhere out of the way but not somewhere they will bake dry in full sun.

Wire Frames and Open Bins

For larger gardens, or if you would rather not accumulate a small mountain of plastic, a wire frame is the traditional solution. Four stakes driven into the ground with chicken wire wrapped around them makes a simple, effective bin roughly a metre square. Fill it with leaves, press them down lightly, and leave it alone.

The advantages of an open frame are that air circulates freely and you can see what is happening. The disadvantage is that leaves dry out at the edges, so check the pile during dry spells and water it if the top few inches feel crisp. Turning the heap once or twice a year speeds things along, though it is by no means essential. If you have the room, build two or three frames side by side so you can turn one batch into the next as it matures, keeping a production line going.

Some gardeners line the frame with an old piece of carpet or a tarpaulin weighted at the corners. This keeps the moisture in and the leaves from blowing about on windy days, and it does seem to hurry the process along.

How Long, and How to Tell It Is Ready

Patience is the main ingredient. A year will typically give you a coarse mulch, useful for spreading around shrubs and over bare soil in autumn. Two years produces something finer, crumbly enough to work into the top layer of the soil. Three years and you have a material close to a light potting compost in texture, ideal for sowing and for mixing into seed trays.

You will know it is ready when the original leaf shapes have largely disappeared and the material feels soft and spongy in your hand, smelling of woodland floor rather than rot. Anything still recognisably a leaf needs more time.

Putting Your Leaf Mould to Work

Leaf mould is remarkably versatile, and you will quickly find you cannot make enough of it.

• As a soil conditioner, dig or fork it into beds in autumn to improve structure and boost the soil's ability to hold water and nutrients.
• As a mulch, spread a five-centimetre layer around perennials, shrubs and fruit bushes to suppress weeds and feed the soil slowly.
• As a seed sowing medium, mix well-rotted leaf mould with garden compost and a little sharp sand for a fine, free-draining mix.
• As a lawn dressing, sieve it and brush it into the turf in autumn to improve the soil beneath the grass.

Best of all, it costs nothing but a bit of gathering and a year or two of waiting. Start a batch this autumn, and by the time the leaves fall again you will wonder why you ever raked them onto the bonfire.