Every autumn the trees hand you a few sacks of free soil improver. Most of it ends up in the brown bin. I did exactly that for years, until I worked out what fallen leaves actually do to soil.
The leaves are about to start dropping. So this is your nudge to sort out somewhere to put them before they are all over the lawn and you are sick of the sight of them.
Why fallen leaves are worth keeping
Left to rot on their own, leaves turn into leaf mould. It is dark, crumbly and smells like a woodland floor.
Leaf mould is made mainly by fungi working slowly in cool, damp conditions. Garden compost is driven more by heat-loving bacteria. In practice both involve a whole community of fungi, bacteria and small soil creatures, but that difference in emphasis is why one takes months and the other takes years.
It also explains what each one is for. Compost feeds your plants. Leaf mould fixes your soil. It is low in nutrients, so treat it as a soil conditioner rather than a fertiliser. What it does brilliantly is improve structure. Heavy clay opens up and drains better. Sandy soil holds on to moisture instead of letting it run straight through.
If your soil is claggy in winter and sets like concrete in summer, or if it dries out within a week of rain, leaf mould is the fix. Compost alone will not do it as well.
My autumn leaf routine
Collecting leaves is one of the few garden jobs my kids will actually volunteer for. We go out with a couple of sacks and they do more jumping in the pile than filling it. It takes twice as long and it is the best part of the job.
We do the lawn first. Anything that lands in the borders I leave alone, because that is free mulch already doing its job.
The sacks go behind the shed and I forget about them for a year. I do not turn them. The only thing I check is that they have not dried out.
I always keep some of the finished stuff back rather than using it all at once. Some gets spread as a mulch through the year. Some gets worked into the borders when I am planting something new. Soil that has had leaf mould added holds water better and digs like soil should.
Should you leave leaves on the lawn?
No. Leaves sitting on grass block light and air, hold damp against the sward and encourage disease.
Clear them roughly once a week through peak leaf fall. Grass can cope with a scattering, but a solid covering left for more than two or three weeks will thin or kill the turf underneath.
The easiest way to do it is with a rotary mower on a high cut. It shreds the leaves as it collects them and picks up a few grass clippings at the same time, which adds a bit of nitrogen and speeds up the rotting. Just do not go overboard on the grass, or the pile tips over into slimy half-compost instead of leaf mould.
Leaves on borders, and the ones you should bin
Leaves that fall on beds and borders can stay where they are. Worms pull them down, the soil gets fed and you save yourself a job.
Two exceptions.
The first is low, evergreen or alpine planting. A thick blanket of wet leaves will smother small plants, so pull leaves off those and put them in the pile.
The second matters more. Do not compost or make leaf mould from diseased leaves. Rose black spot, apple and pear scab, and box blight all overwinter on fallen foliage and will come straight back next season. Rake those up and put them in the council green waste collection, where commercial composting gets hot enough to kill them off. A cool leaf pile will not.
Worth knowing on slugs: leaf litter does give slugs and snails somewhere damp to hide. It also shelters ground beetles and centipedes, which eat them. In most borders it evens out. Near hostas and young seedlings, keep the mulch a bit thinner.
How to make leaf mould
There is not much to it.
In a bag. Fill a sturdy sack with damp leaves, punch a few holes for air, tie the top loosely and stick it somewhere out of the way. This is what I do. Reusable woven leaf sacks or hessian bags work better than single-use bin liners and you get years out of them.
In a cage. Four stakes and some chicken wire. Aim for about a metre wide and a metre tall, which is the minimum bulk needed to keep the middle damp and rotting evenly. Put it somewhere sheltered so the leaves do not blow away.
Then leave it. Most leaves take about two years to break down fully, though after around twelve months you will have a rough, part-rotted material that is already useful.
A few things that help:
- Shred the leaves first by running the mower over them. Smaller pieces rot faster and it genuinely makes a difference.
- Keep them damp. An open cage usually gets enough from the rain, but bagged leaves and piles under trees or eaves do dry out, so check them and water if needed.
- You do not need to turn a leaf pile. It will rot on its own. But turning it once or twice speeds things up and is the first thing to try if a pile has stalled.
One thing to expect. The pile shrinks dramatically, down to roughly a third of what you started with, and less once it is fully rotted. Make more than you think you need.
For the full method, including the two-bin rotation that gives you a supply every year, see my ultimate guide to leaf mould composting.
Which leaves work, and which need extra help
- The best. Oak, beech and hornbeam. Thin, and they rot down into lovely fine material. Birch is quick too.
- The slow ones. Horse chestnut, sweet chestnut and sycamore are thick and waxy. Left whole they can take two to three years. Shred them and they speed up a lot.
- Evergreens. Holly, cherry laurel and conifer clippings take forever in a cool leaf pile. Shred them and put them in the compost heap instead, where the heat gets through them. If you are shredding laurel, do it outdoors rather than in a closed shed. Also worth knowing that holly prickles survive long after the leaf has gone, which is an unpleasant surprise when you are handling the finished stuff.
- Pine needles. Keep these in their own bag. They rot slowly and make a fine mulch for rhododendrons, blueberries and heathers. I would not expect them to actually lower your soil pH though. That part is largely a garden myth, because soil pH is governed by what is underneath rather than what you scatter on top.
- Walnut. Walnut leaves contain juglone, which can hold back sensitive plants like tomatoes. It breaks down during composting, so you do not need to ban it. Shred the leaves and give them a good few months to rot before you use the results, and avoid spreading fresh walnut leaves straight onto a veg bed.
Skip anything swept up from a busy roadside. The Environment Agency has found compost made from street leaf sweepings carries much higher levels of polycyclic aromatic hydrocarbons than green waste compost, along with grit, salt, plastic and road grime. Leaves from your own garden or a quiet street are fine. Gutter sweepings from a main road are not, especially anywhere near food crops.
Leave some leaves for wildlife
This one matters. Leaf piles are a favourite spot for hedgehogs to build a winter nest.
Hedgehogs are in real trouble. The State of Britain's Hedgehogs report puts the decline in rural populations at somewhere between 30 and 75 per cent since 2000, with an estimated 879,000 left, and the species is now listed as Vulnerable to extinction in Britain. There is better news in towns and gardens, where numbers appear to be stabilising, which is exactly why what we do in our own gardens counts.
So before you move an old leaf pile, fork a compost heap or strim a corner, have a proper look first. And if you are building a bonfire, build it and light it on the same day, or move the whole pile to fresh ground just before lighting it. Lift it apart section by section with a broom handle rather than a fork, and light it from one side so anything inside has a way out.
It is not only hedgehogs. Frogs, toads and newts shelter under damp leaves, and a lot of beetles, spiders, moth pupae and queen bumblebees overwinter in leaf litter. Leaving one undisturbed pile in a quiet corner costs you nothing.
What to do with the finished leaf mould
After about a year it is dark and crumbly with bits of leaf still visible. Spread it around shrubs, trees and veg beds at about 5cm deep to hold moisture in and keep weeds down. Keep it clear of stems and away from tree trunks.
You can also work it into the top of a border as a soil improver, which is what I tend to do when planting. Plenty of gardeners now prefer to spread it on the surface and let the worms take it down instead, which protects soil structure and the fungal life that leaf mould is so good for. Both work. If your soil is in poor shape, a light forking in gets you there quicker.
After two years it is fine and soil-like. Sieve it and it makes a very good free seed sowing mix, either on its own or mixed with sharp sand. Its low nutrient content actually suits seedlings, because there is nothing to scorch them. One warning: leaf mould piles do get colonised by weeds, so use the maturest material you have and sieve it properly.
For a homemade potting mix, combine well-rotted leaf mould with equal parts sharp sand, garden compost and decent soil.
If you want to read more about mulching, I have covered that in my post on using compost as mulch.
The catch, and how to get compost faster
Leaf mould is free and it is easy. It is also slow, and it will not take your food waste.
If you want usable compost this season rather than in two years, and somewhere to put kitchen scraps as well as garden waste, you need something that runs hot. I use two at home.
The HOTBIN is the one I would point most people at. It is insulated, runs at 40 to 60°C and turns waste into compost in 30 to 90 days with no turning. Autumn leaves go in fine as long as you shred them and add them in small amounts alongside something easier to digest like grass or kitchen scraps, plus shredded paper and woodchip to keep the air moving. Use coarse woodchip rather than fine sawdust, which compacts and kills the airflow. It needs feeding regularly to stay hot. The 200L wants around 5kg of waste a week and the Mini around 2.5kg.
The Green Johanna takes cooked food, meat, fish and bones, which most bins will not. It reaches around 40°C on its own and up to 65°C with the insulating jacket fitted, giving compost in four to six months. If you want it working through a British winter, get the jacket. Mine has one. It is a faff to fit and it earns its keep from November onwards.
If you would rather not think about heat management at all, a Maze tumbler is the easy option. Two compartments, so you fill one while the other cooks, and a geared handle that makes turning a full barrel manageable. Around eight to twelve weeks in warm weather, longer over winter.
Run one of those alongside a couple of sacks of leaves and you have covered both jobs. Compost for feeding your soil, leaf mould for fixing it.
Common questions
Should I pick up leaves or leave them?
Clear them off the lawn. Leave them on borders, unless the plants underneath are small and evergreen, or the leaves came off a diseased plant.
Can I just put leaves in my compost bin?
A few, yes. They count as browns and balance out grass and kitchen waste. It is only bulk leaves that are a problem, because they mat together and rot slowly. That is when a separate leaf pile makes more sense.
How long does leaf mould take?
Around one to two years. Twelve months gets you a usable mulch. Two years gets you fine, sievable material.
Can I make it faster than that?
Shred the leaves, add a little grass or compost, keep it damp and turn it occasionally. Or run leaves through a hot composter mixed with other waste, which works in weeks rather than years.
What if I have far too many leaves?
Fill what bags you have, leave a pile for wildlife and put the rest in the council green waste collection. There is no prize for keeping every last one.
Now go and get the sacks out before the wind does it for you.
Further reading
I'm Adam, the founder of Compost Guy. I'm passionate about empowering people to embrace composting! Whether you're a seasoned composter or just starting your journey, I'm here to help.
