Building a Compost Heap from Tree Leaves and Clippings

By the time the last of the oak leaves have come down, most of us are looking at a garden that appears to have produced nothing but waste. A few hours with a rake and a tarpaulin changes that view completely. Leaves, hedge trimmings and soft prunings are the raw material for two of the finest soil amendments you can make at home, and neither costs a penny beyond a bit of space and a little patience. If you have ever paid for bags of leaf mould or soil conditioner, this is the year to stop.

Why keep garden waste on site?

Green waste carted to the recycling centre is processed in bulk, and while that is vastly better than sending it to landfill, the finished product rarely finds its way back to your own borders. Keeping it at home closes the loop on your plot. Leaf mould and compost improve soil structure, hold moisture through dry summers, feed the soil biology that feeds your plants, and suppress weeds when used as a mulch. They are not fertilisers in the concentrated sense, but they make whatever nutrients are already in your soil far more available.

There is a practical argument too. A mature sycamore or a row of hornbeam hedging can produce several cubic metres of material in a season. That is a lot of bags to load into the car.

Leaf mould and compost are not the same thing

It is worth understanding the distinction before you build anything, because the two processes want different conditions.

• Leaf mould is the product of fungal breakdown. Leaves are piled on their own, with no green material added, and the heap stays cool. It takes a year or two, but it demands almost no work.
• Compost is bacterial. It needs a balance of nitrogen-rich green material and carbon-rich brown material, plus air and moisture, to heat up and rot down quickly — often in three to six months.

Leaves are the classic brown: high in carbon, low in nitrogen, and slow to rot in a conventional heap. That is why they work far better in a dedicated leaf bin than thrown in with the kitchen peelings.

Building a simple leaf mould bin

Four sturdy stakes driven into the ground, wrapped in wire netting, gives you a bin roughly a metre square and a metre high. Site it somewhere shaded and out of the worst of the wind. A second bay alongside lets you turn the heap from one to the other after a year, which speeds things up considerably.

If space is tight, a few black bin bags with a handful of holes punched in the sides will do the job, tucked behind a shed. Damp the leaves as you fill — they should be moist but not sodden — and tread them down occasionally to reduce the volume. Left dry, they simply sit there.

Not all leaves behave the same way. Oak, beech and hornbeam break down steadily and give excellent results. Sycamore, lime and horse chestnut are quicker. Conifer needles, holly and thick evergreen leaves such as laurel and rhododendron are slow and acidic; keep them in a separate bin and expect to wait three years or more, or run them over with a mower first. Shredded material rots far faster than whole leaves, so a pass with the lawnmower is never wasted effort.

Expect a rough, coarse mulch after twelve months and fine, crumbly leaf mould after two years. Sieve the finished product if you want it for seed sowing.

Composting hedge clippings and green waste

Fresh, soft hedge trimmings — privet, box, beech, hornbeam, yew — are greens, and they are a valuable source of nitrogen. Layer them with your leaves, shredded cardboard, torn paper or chopped woody prunings. Aim for roughly two to three parts browns to one part greens by volume; this is a guide, not a rule, and you will quickly develop a feel for it.

If the heap is too wet and slimy, add more browns. If it is dry and nothing seems to be happening, add greens and water it. The old test still works: squeeze a handful and it should feel like a wrung-out sponge. Turn the heap every few weeks to reintroduce air, and you should see it heat up noticeably in the middle.

A few things to leave out:

• Perennial weed roots — couch, bindweed, ground elder and the like will happily survive a cool heap.
• Diseased material — leaves carrying black spot, rust or honey fungus are best kept out.
• Large volumes of grass clippings in one go. Spread them thinly or mix them through, or they will mat down and go anaerobic.
• Anything treated with a persistent herbicide. Clippings from a lawn recently treated in this way can damage tomatoes and other broadleaf plants for years.

Thick woody prunings should be shredded or chopped before they go in; otherwise they will still be there in five years. Leylandii is fine in a hot heap but slow in a cool one, so mix it well with other material rather than dumping it in a single layer.

Using what you make

Leaf mould is a superb mulch for woodland plants, ferns, rhododendrons and anything that enjoys cool, moist roots. Forked in, it lightens heavy clay and helps sandy soils hold water. Mixed roughly equally with garden soil and a little sharp sand or grit, it makes a perfectly good seed and potting compost for the year ahead.

Your finished compost is best used as a soil improver, spread on beds in autumn or early spring and let the worms work it in. It also makes an excellent top dressing for lawns, sieved and scattered thinly. Around trees and shrubs, mulch generously but keep the material clear of the trunk — a ring of compost touching bark invites trouble. Spread it out to a depth of five to eight centimetres and let the rain do the rest.