Soil & compost
Leaf mould
Crumbly, dark material made from rotted-down autumn leaves, used as a soil improver, mulch or seed-compost ingredient.
Leaf mould is one of the simplest and most useful materials you can make in a UK garden, and it costs nothing but a little patience. It is made entirely from fallen autumn leaves, left to break down until they collapse into a dark, crumbly, soil-like material.
How it differs from compost
The key thing to understand is how leaves rot. Ordinary compost is broken down mostly by bacteria, which work fast and warm, generating heat as they go. Leaves are different: they are tough and waxy, and they are broken down mainly by fungi. Fungi work slowly and at cool temperatures, so a leaf-mould heap never heats up the way a compost heap does.
This slower, cooler, fungal process is why leaf mould is low in nutrients. It is not a feed for your plants — it is a soil conditioner. What it does brilliantly is improve soil structure: it opens up heavy clay, helps sandy soil hold on to moisture, and creates a lovely crumbly tilth for sowing into.
How long it takes
Leaf mould is a waiting game. Most leaves take one to two years to break down fully. After about a year you will have rough, partly rotted leaf mould — ideal for spreading as a mulch over beds or around shrubs. Leave it a second year and it becomes fine and crumbly enough to use as an ingredient in seed and potting compost.
Some leaves rot faster than others. Thin, soft leaves such as oak, beech, hornbeam and birch break down quickly and give the best result. Tougher, leathery leaves — sweet chestnut, horse chestnut, sycamore and plane — take longer, so it helps to shred them first by running a mower over them.
Avoid using evergreen leaves such as holly, laurel and conifer needles, which rot far too slowly to be worth it. Pine needles are an exception worth knowing: they rot down acidic, which makes them useful around blueberries and other acid-loving plants.
How to use it
Once it has rotted down, leaf mould is wonderfully versatile. Dig or rake well-rotted leaf mould into beds to improve the soil, spread it as a moisture-holding mulch, work it into a seedbed before sowing, or sieve it and mix it with garden compost and grit to make your own potting mix. Because it holds water so well, it is especially valued for raising seedlings and for top-dressing containers through a dry summer.
The only real cost is space and time — but for a free bin bag of autumn leaves, it is hard to beat.
In a UK garden
Bag up fallen leaves in October and November when they drop, and leave them to rot in a quiet corner for a year or two — perfect for gardens with deciduous trees.
Example
Stuff autumn leaves into a bin bag, punch a few holes, tie it loosely, and forget about it until it has crumbled into dark, soil-like material.