Why Leaf Mould Is Worth the Wait

Every autumn, the garden hands you a free gift and then asks you to tidy it away. Instead of treating fallen leaves as a chore, treat them as the raw material for the finest soil conditioner you will never have to pay for. Leaf mould is not compost. It is made by fungi rather than the hot bacterial process of a compost heap, and the end result is a dark, crumbly, sweet-smelling material that holds water beautifully and improves almost any soil it touches.

It takes patience rather than skill, and it costs nothing but a few bin bags and a corner of ground you were not using anyway. For anyone growing in pots, courtyards, balconies or a small raised-bed plot, that is a serious advantage: a free, light, peat-free component for your growing mix that you can make in bulk without needing space for a compost bay.

Which Leaves to Keep (and Which to Set Aside)

Most leaves will rot down eventually, but they do not all behave the same way. Thin, soft leaves break down quickest and can be ready in a year:

• Ash, birch, cherry, lime, willow and poplar
• Sycamore and field maple
• Fruit tree leaves from apples, pears and plums

Tougher, waxy leaves take longer — often two years, sometimes three. Oak, beech, horse chestnut, sweet chestnut and holly all fall into this group, so if you have a lot of them be prepared to wait, or shred them to hurry things along. Conifer needles and evergreen leaves such as laurel and rhododendron break down very slowly and produce an acidic mould; keep them in a separate bag to use around blueberries, rhododendrons and camellias. Skip leaves from busy roadside verges, which carry grime, and any that are clearly diseased.

The Bin Bag Method, Step by Step

This is the simplest system going, and it works in the smallest of gardens. You need black bin bags, a fork or sharp knife, and a spare half hour.

• Gather dry-ish leaves with a rake and a trug, or by hand. Soaking wet leaves are heavy and slow to handle.
• Punch eight to ten holes around the sides and base of each bag with a garden fork. Air is essential — sealed leaves turn into a slimy, sour sludge rather than mould.
• Fill each bag about two-thirds full. Leaves compact as they break down, and the material needs room to breathe.
• Add a handful of garden soil or a scoop of last year's leaf mould. It introduces the fungi that do the work.
• Moisten if the leaves are dry. Aim for the dampness of a wrung-out sponge — a sprinkling of water, or an hour left open in the rain, is plenty.
• Tie the top loosely, or simply fold it over. Never seal it airtight.
• Label the bag with the month, the year and the type of leaves. You will not remember next spring.

Where to Tuck Your Bags

Leaf mould is unfussy, but it does best somewhere sheltered and out of sight. Behind the shed, behind a water butt, along a north-facing wall or under a hedge are all ideal. Black bags in a sunny spot warm up and rot faster, so if you are impatient, choose the brightest corner you can spare. Check the bags every few months. If the contents feel bone dry, add water. If they smell sour or look slimy, open the bag, turn the contents and mix in a few dry leaves or a torn-up cardboard tube to restore airflow.

Troubleshooting and Speeding Things Up

If your leaves are still recognisable after two years, the usual culprits are dryness or a lack of air. Both are easy to fix. Running a lawnmower over the leaves before bagging them halves the waiting time, because small pieces rot far faster than whole ones. Shredded leaves also pack down more, so use more bags. A thin scattering of grass clippings through the mix adds nitrogen and speeds things up, but keep it thin or the bag will go anaerobic and smell. Turning the bags once or twice a year helps. Cold winters slow everything down, which is fine — leaf mould is forgiving and largely foolproof, and a bag forgotten for three years produces something even finer.

Using Leaf Mould in a Small Garden

Spread it as a mulch in a five-centimetre layer around perennials, shrubs and fruit bushes, where it will suppress weeds and feed the soil life beneath. Fork it into raised beds to open up heavy clay and to help sandy soils hold water. For containers, it is invaluable: mix equal parts sieved leaf mould, garden compost and loam, with a handful of horticultural grit for drainage, and you have a decent peat-free growing medium for mature plants. Sieved finely and mixed half and half with sharp sand, it makes a serviceable seed mix for larger seeds, and on its own it is a lovely top dressing for pots and seedlings. Start a couple of bags this autumn and you will have a free supply for years to come.