How to Make Leaf Mould: Free Seed Compost from Autumn Leaves

Every autumn the best free ingredient in gardening falls on the lawn, and most of it is bagged up and driven to a recycling centre. Leaf mould is what happens when you do almost nothing to fallen leaves except keep them damp and keep them together. There is no turning, no activator, no balancing of greens and browns, and no heap to manage — only patience, because the process is cold and slow. What you get after two years is a dark, sweet-smelling, crumbly material that costs nothing, holds its own weight in water, and makes the best homemade seed compost there is. Here is how to make it, which leaves to use, and how to turn it into a sowing mix rather than just another mulch.

• What it is cold, fungal breakdown of fallen leaves — a soil conditioner, not a fertiliser
• Collect late September to early December, from lawns, paths and drives — leave plenty in the borders for wildlife
• Ready 12 months as a coarse mulch; 18–24 months sieved as seed compost
• Kit black bin bags with holes punched in them, or a one-metre wire mesh cage — plus a watering can

Leaf mould is not compost

This is the distinction that makes everything else make sense. A compost heap is a bacterial process: it runs hot, needs a balance of nitrogen-rich green material and carbon-rich brown material, wants air and turning, and can be finished inside a season. Leaf mould is a fungal process: it runs cold, needs nothing but moisture and time, and is worked on by the same organisms that break down leaf litter on a woodland floor.

The reason leaves behave differently is their chemistry. They are high in lignin and very low in nitrogen — a carbon-to-nitrogen ratio somewhere around 40 or 80 to one, against roughly 30 to one for a balanced compost heap. Bacteria simply stall on that diet, which is why a barrowload of leaves tipped into your compost bin goes cold, mats into slimy layers and holds the whole heap up. Keep them separate; the details of the other process are in our guide to composting for beginners. The trade-off is that finished leaf mould contains very little plant food — a fraction of the nitrogen in good garden compost — so it improves structure, water-holding and root run rather than feeding anything. For feeding, see our guide to feeding and fertilising; for what leaf mould is unbeatable at, read on.

Which leaves to use

Anything deciduous will rot eventually, but the leaves differ enormously in speed and in the quality of what they leave behind. Oak, beech and hornbeam are the classic choices: slow, but they produce the finest, darkest leaf mould of all. Ash, cherry, birch, lime, hawthorn and field maple break down considerably faster and are perfectly good. Sycamore, horse chestnut, plane and poplar are big, tough and slow — shred them before bagging or you will still be looking at recognisable leaves in three years.

Keep evergreens out: holly, laurel and conifer clippings take years and are better shredded into a compost heap. Pine needles are worth bagging separately, since they make a pleasantly acidic mulch for blueberries and other lime-haters. Never collect leaves swept from road verges — they carry grit, road salt and traffic pollutants — and never strip a woodland floor, because that litter layer is somebody's habitat. Your own lawn, path and drive will give you more than enough, and clearing them is a job worth doing anyway, as our guide to autumn garden preparation explains.

How to make leaf mould, step by step

1. Collect after rain, and shred if you can Wet leaves are the whole point, so choose a still, damp day rather than a dry, windy one. The quickest method by far is to run the mower over a leafy lawn with the collector on: it shreds the leaves and mixes in a little grass, which roughly halves the time to usable leaf mould.
2. Choose bags or a cage For a small garden, fill black bin bags, punch a dozen holes in each with a fork and tie the neck loosely — they can be stacked behind a shed and forgotten. For volume, knock four stakes into the ground and staple chicken wire around them to make a cage about a metre square and a metre high. No lid and no base: rain and worms both need to get in.
3. Wet every layer as you fill This is the step that decides whether you have leaf mould in two years or a bag of dry brown leaves in five. Tip in a barrowload, water it with a can until it is as damp as a wrung-out cloth, then add the next. Leaves shed water once they are packed, so wetting the finished pile from the top does not work.
4. Firm it down and walk away Tread the cage down as you fill to squeeze out air pockets. Then do nothing at all — no turning, no activator, no added nitrogen, all of which push the pile towards composting instead. Check the moisture in spring and midsummer and water in a dry spell; that is the entire maintenance schedule.
5. Start a new pile every autumn Because the material takes one to two years, the only sensible system is a rolling one: this year's leaves, last year's leaves and the batch that is finally ready. Two cages side by side, or three neat rows of bags, and you will never be without it.

One year or two?

Leaf mould is useful at two quite different stages, and knowing which you have saves disappointment when you come to sow into it.

Using leaf mould for seed sowing

Here is the part that surprises people: the very thing that makes leaf mould a poor fertiliser makes it an excellent seed compost. A seed carries its own food supply and needs nothing from the medium except moisture, air and something to grip. Sow into rich compost and you get soft, leggy seedlings and, at worst, salt damage on the young roots. Low fertility is a feature, not a fault — the principle behind every commercial seed compost, and one our guide to germinating seeds explains in more detail.

The traditional mix is equal parts sieved leaf mould, sieved garden loam and sharp sand or fine grit; a simpler version is two parts leaf mould to one part sharp sand. Mix it a few days ahead, keep it just moist, and use it as you would any peat-free seed compost — the sowing technique itself is unchanged, as set out in our guide to starting seeds indoors. Two cautions. Leaf mould is not sterile, so sieve it fresh, sow into clean containers and give seedlings good airflow to avoid the problems in our guide to damping off and seedling mould. And for very fine or very expensive seed, use a bought seed compost and save the leaf mould for pricking out, where a few stray weed seedlings do no harm.

Outdoors it earns its keep just as well. A grit-topped pot of hardy perennial seed standing against a north wall all winter is happiest in a free-draining leaf-mould mix, exactly the method in our guide to growing perennials from seed. Raked over a seedbed it gives the fine, stone-free tilth that direct-sown root crops need — see how to grow radishes from seed for what that tilth is worth. And in containers or a raised bed, a couple of buckets stirred into the mix does more for water-holding than anything you can buy.

Common leaf mould mistakes

• Filling the bag with dry leaves — the commonest failure of all. Dry leaves sit unchanged for years. Water every layer.
• Adding grass clippings to speed it up — that turns it into a compost heap, and a slimy, anaerobic one at that.
• Sealing the bag airtight — the fungi need air. Punch holes and tie loosely.
• Expecting a fertiliser — leaf mould conditions soil; it does not feed plants. Use compost or a feed for that.
• Sowing into one-year-old leaf mould — too coarse and too twiggy. Sieve two-year-old material instead.
• Raking every last leaf out of the borders — leaves left on beds are free mulch and winter shelter for insects and hedgehogs.
• Collecting from road verges — salt, grit and pollutants come with them.

Frequently asked questions

How long does leaf mould really take?
Whole leaves of ash, birch or cherry take about twelve months to become a usable mulch and two years to sieve into seed compost. Oak and beech take longer but give a better product; sycamore and horse chestnut can take three years unless shredded. Mowing the leaves first is the single biggest accelerator.

Can I speed it up without ruining it?
Yes, in three ways: shred the leaves, keep the pile consistently damp, and make it big enough that the middle stays moist. What does not help is adding nitrogen, lime or a compost activator — those push the pile towards bacterial composting and you end up with something else entirely.

Does leaf mould contain any nutrients at all?
A little, but not enough to matter. Think of it as the peat-free equivalent of a soil improver: it adds humus, holds water, feeds soil life and improves structure on both clay and sand. Anything you grow in it for more than a few weeks will need feeding separately.

I have no space for a cage — can I still make it?
Easily. Four bin bags stacked behind a shed or under a bench will produce a barrowload of sieved leaf mould in two years, which is plenty for a season of seed sowing and pricking out. Punch the holes, wet the leaves, label the bag with the year, and forget it.

Leaf mould is the one garden project where doing less produces more, and the only cost is remembering to start it. Rake the leaves this autumn, wet them, bag them, and by the spring after next you will be sowing into something you made from what the trees threw away. While the pile matures, put the ground to work: green manures cover the empty beds, a mulch keeps the weeds down, and soil preparation now saves work in March. Sow perennial flowers, biennials, wildflowers, herbs and hardy vegetables this month — see what to sow in September — and if frost is on your mind, our guides to overwintering tender plants and extending the growing season cover the rest. Browse our organic seeds, our whole flower seed range and seed range, or read the rest of our growing guides.