Making Leaf Mould

Fallen leaves turn into free mulchA layer of material spread over the soil around plants to suppress weeds, retain moisture, and improve soil structure as it breaks down. Full definition → and seed-sowing mix, but they rot badly in a compost heap and slow the rest of the heap down. Give them a pile of their own, keep it damp, and wait: about a year for a rough mulch, two for fine leaf mould.

Why leaves get a heap of their own

Ordinary composting is bacterial. Soft green material such as kitchen scraps and grass clippings supplies nitrogen, brown material such as cardboard and prunings supplies carbon, and the bacteria feeding on that mix make the heat. Get the ratio roughly right and a heap can give usable compost in four months to a year. If you have no room for a heap, a wormery handles kitchen scraps instead.

Leaves don't fit that recipe. They are nearly all carbon, low in nitrogen, and built around lignin, a tough fibre that breaks down slowly. A thick layer of leaves in a compost heap throws off the green-to-brown balance, mats down and shuts out air.

Left alone, leaves rot by a different route: a slow, cool process driven by fungi rather than bacteria. It needs a cooler, damper spot than a compost heap, which is why a dedicated pile in a shaded corner works better than mixing leaves into the bin. The same cool, damp conditions that hold back a hot compost heap do not stop it: it needs less heat, so it works in cooler climates and in piles left exposed to the weather, though a cool, wet Scottish year may mean the slower end of the timings below.

Three ways to make it

All three corral damp leaves somewhere they will stay damp and undisturbed for a year or more. The difference is space and tidiness: a wire cage is the least tidy option, and bags stacked behind a shed are slightly slower and out of sight.

A wire or chicken-wire cage

Make a square or round frame from chicken wire, supported at the corners or at intervals by stakes or bamboo canes, in a sheltered spot so the leaves don't blow away. Make the pile as large as you can: a big pile holds its moisture and rots faster. Moisten it if it dries out. Whole leaves in a pile take about two years. If you want to avoid wire, a heap contained by clematis stems or old runner bean stalks does the job.

Bin bags or sacks

Pack damp leaves into a bin liner, pierce it a few times with a fork or knife, tie the top loosely and stack the bags out of sight for up to two years. Thick reusable bags or hessian sacks are better still, and old builders' bags are ideal because they are portable. Make sure the leaves are damp before you tie the top. This is the low-effort, low-space option.

Mow them up first

A rotary mower run over a leaf-covered lawn collects and shreds the leaves in one pass, and picks up grass clippings that add nutrients the leaves lack. Set the blades high. Shredded leaves settle less densely, so more air gets in, and they usually give a usable, fine leaf mould in 6 to 12 months. Whole leaves give a coarser result. If you rake instead, rake in the direction the wind is blowing and pick the leaves up with leaf boards (two rectangles of plywood or plastic used like giant hands) or a lawn vacuum. Leaves are easier to collect when the weather is dry and still.

Where to get the leaves

Use leaves from your own garden or public places. Leaves from beside a busy main road may carry atmospheric pollution, so quieter streets and side roads are better, and street leaves often hide litter, so sort through them first. Leave woodland leaves where they are: they feed the woodland and shelter its wildlife. Leave some under your own hedges too, where they feed the soil and hedgehogs may be hibernating. Our guide to leaving autumn leaves for wildlife covers what else lives in them.

Which leaves work, and which don't

"Good leaf mould leaves" and "fast leaf mould leaves" are two different claims. RHS names oak, beech and hornbeam as the best leaves, and they need no shredding. But Garden Organic puts beech, birch, hornbeam, oak, sweet chestnut and magnolia in the higher-lignin, slower-rotting group, against ash, cherry, elm, linden, maple, poplar and willow as the quicker ones. Both hold: oak, beech and hornbeam make a fine result without help, but a pile made mostly of them sits toward the two-year end of the timeline.

Pine needles are worth their own pile because the finished leaf mould is acidic. That suits a mulch for rhododendrons, azaleas, camellias, heathers and blueberries, and does harm if it ends up in an ordinary bed by accident. Needles drop all year but mostly in spring, so gather them gradually. Garden Organic says leaves with black spots can go into the leaf mould pile too.

How long it takes, and what each stage is for

Leaf mould is useful at more than one point along the same slow decay.

After a year or so, leaves crumble easily in the hand but the leaf shapes are still visible. This young leaf mould suits:

• a summer mulch around herbaceous plants, shrubs, trees and vegetables, to hold moisture and suppress weeds;
• a winter cover for bare soil, which protects it from heavy rain washing nutrients out and feeds worms and soil life;
• digging in as a soil improver before sowing and planting, especially on heavy soil, where it helps drainage;
• an autumn top-dressingAdding fresh compost, feed or grit to the soil surface around an established plant, rather than digging it in. Full definition → for lawns.

After two years or more it is dark brown and crumbly, with no trace of the original leaves. It is low in nutrients and fine in texture, which is what seed germinationA seed sprouting: breaking dormancy and putting out its first root and shoot, given the right warmth and moisture. Full definition → wants. Use it well sieved on its own, or mixed with an equal part of sharp sand, for sowing seed. For a homemade potting compost, mix equal parts well-rotted leaf mould, sharp sand, loam (garden soil) and garden compost. Use washed, gritty horticultural sharp sand, not builders' sharp sand.

Weed seeds can colonise a leaf mould heap, so don't use it where weeds would be a particular problem. If a pile is slow to break down, turn it regularly to aerate it and speed up the process, and make sure it doesn't dry out in hot, dry weather. For more on spreading it, see how mulching works.

In Scotland

Birchwood is Scotland's most widespread and extensive type of woodland, so a lot of Scottish leaves are birch. Birch sits in the slower-rotting, higher-lignin group, so expect a birch pile to take toward two years unless you shred it. Pine needles make acidic leaf mould, and Scotland's soils tend to be more acidic than the rest of the UK, so check your soil pH before mulching ordinary beds with it.

Common questions

Sources