What it is
Autumn leaves broken down by fungi, not bacteria. That distinction is the whole point.
Conventional compost runs hot and bacterial. High heat, fast turnover, hungry for nitrogen. Leaf mould runs cold and slow. Fungal hyphae weave the leaves into the soil over one to two years.
The finished product is dark, crumbly, sweet-smelling. It holds up to 500% of its weight in water. Garden soil holds about 60%.
Nutrients are low. That makes it a lousy fertiliser and an outstanding soil conditioner.
Why it works
Walk through any old woodland. Scrape the litter aside. You will find a dark spongy layer laced with white threads. That is leaf mould doing its job in the wild.
This is the layer native oaks live on. It is where mycorrhizal fungi feed and trade. Most gardens lack it entirely.
Make a pile. You are rebuilding that layer wherever you want it.
Build it
Collect leaves in autumn. Any deciduous species works. Birch, alder, and hazel break down inside a year. Oak, beech, and sycamore take 18 months to two years.
Skip evergreen leaves. Holly, laurel, and pine needles decompose too slowly and carry compounds that suppress other plants.
The cage. Four posts, chicken wire wrapped around them, roughly one metre square. Pack the leaves in. Wet them if they are dry. Walk away.
No turning. No activators. No nitrogen layer. The fungi will find the leaves.
The bag method. Stuff wet leaves into black bin bags. Poke a few holes for air. Tie loose. Stack behind a shed. The bags hold moisture and a little warmth, which speeds things along.
Shred to halve the time. Run a lawnmower over a layer of leaves on the lawn before bagging. More surface area, faster colonisation. Optional, never essential.
Tend it
At one year you have coarse, half-rotted material. Good for mulching trees and beds.
At two years you have the prize: fine, crumbly, fully decomposed mould. Use it as a seed-starting medium, a potting mix base, or a top-dressing.
Use it
Spread a 5 to 10 cm layer around trees, shrubs, and perennial beds in autumn or early spring. Unlike wood chips, leaf mould will not lock up surface nitrogen. Safe right against young transplants.
Work one-year material into new beds or wicking beds. On clay it opens structure. On sand it sponges water that would otherwise leach away.
In a food forest, this is the default mulch. Apply it around new plantings and under canopy. Use it on fruit tree guilds to push the soil toward fungal dominance, which is what fruit trees want.
Sieved two-year mould replaces peat in any potting recipe. Mix with garden compost and sharp sand for general use. Use it straight for woodland plants and ferns. Seedlings grown in leaf mould build stronger roots than peat-raised ones, almost certainly because mycorrhizal colonisation starts earlier.
Using it also keeps you out of the peat trade, which destroys wetlands that take millennia to form.
When it goes wrong
Pile dries out. Decomposition stalls. Soak it.
Bag stinks of ammonia. Too wet, no air. Open the bag, fork the contents, add holes.
Pile still looks like leaves after a year. Cold winter, tough species, or no shred. Wait another season. Patience is the only real input here.
Slugs move in. Fine. They are a sign of healthy biology, not a problem to fix.
See also
- Mycorrhizal Fungi the networks leaf mould feeds and extends
- Composting Methods bacterial compost as the complement to fungal mould
- Food Forest Design the system where leaf mould belongs by default
- Soil Cover leaf mould inside a broader cover strategy
- No-Dig the method that uses surface mulch best