Autumn compost differs from summer compost in one crucial way: it's overwhelmingly carbon-rich. Leaves and woody stems are dry and low in nitrogen, so a heap built only from them barely decomposes — it will sit there for years looking much as it did on day one.
Browns and greens — the ratio everything depends on
Compost needs two things: carbon and nitrogen. Brown material (leaves, straw, twigs, cardboard) supplies carbon. Green material (grass clippings, kitchen scraps, fresh foliage) supplies nitrogen and moisture.
The ratio that works is roughly 2–3 parts brown to 1 part green by volume. In autumn the problem is always the same — far too much brown and not nearly enough green. The fix: don't dump collected leaves on in one go, but layer them with grass from the last cut, kitchen waste or vegetable trimmings.
What definitely doesn't belong on the heap
- Blighted foliage — especially from tomatoes, cucumbers and potatoes. In an ordinary garden heap that never reaches a high enough temperature, the spores overwinter perfectly happily.
- Weeds that have set seed — the seed survives, and in spring you spread it across the whole garden with the finished compost.
- Perennial weeds with running roots — couch grass, bindweed, ground elder. Give them a scrap of root and the heap becomes their nursery.
- Large quantities of walnut leaves — they contain juglone, which suppresses germination. A small proportion dilutes away; a heap made mostly of them does not.
- Meat, bones, fat, dairy — they attract rodents and smell.
- Ash from coal or briquettes — it carries heavy metals. Small amounts of wood ash matter less, but they do push the pH up.
How to build the heap
- Keep the base coarse — put shredded twigs or coarse stems at the bottom. The air gap that creates lets the heap breathe from below.
- Layer it, don't just tip it all on — alternate 10–15 cm of brown with about 5 cm of green.
- Shred the leaves first — the easiest way is to run the mower over them. Whole leaves mat into airless layers; shredded ones break down several times faster.
- Dampen each layer lightly — compost should be as moist as a wrung-out sponge, never sodden.
- Add a handful of soil or old compost — inoculating it with micro-organisms kick-starts decomposition.
Leaf mould — why leaves are best composted separately
Leaves aren't broken down by bacteria like the rest of the heap but by fungi, and it takes longer — one to two years. If you have leaves in quantity, it pays to bag them or pen them in a wire cage and set them aside. The resulting leaf mould isn't a fertiliser (it's low in nutrients), but it's one of the finest soil conditioners there is: it holds water and improves structure.
The heap through winter
Decomposition all but stops in frost, and that's fine — it restarts on its own in spring. What helps is covering the heap for winter (landscape fabric, an old tarpaulin, a layer of straw) so rain doesn't leach it and snow doesn't turn it into a block of ice. A covered heap holds its heat longer and starts up weeks earlier in spring.
There's no point turning compost in winter. Leave it alone and come back to it when the weather warms.
Frequently asked questions
Can I put walnut leaves on the compost?
In small amounts yes, in large amounts no. Walnut leaves contain juglone, a compound that suppresses germination and growth in other plants. As a fraction of the heap they break down and dilute away without consequence. If the compost would be mostly walnut, heap it separately and give it an extra year to mature.
Why isn't my compost breaking down?
Most often the heap is too dry, or it's all brown material. An autumn heap of pure leaves barely moves. Try soaking it through and layering in nitrogen-rich material — grass clippings, kitchen waste or a little manure.
Can I compost blighted tomato foliage?
No. Late blight and other fungal diseases survive in an ordinary garden heap that never gets above 60 °C, and next year they come back onto your beds with the finished compost. Put diseased foliage in the household waste instead.