Why Leaf Mould Earns Its Place in the Garden

Every autumn, Britain hands us a free harvest. The leaves that tumble onto the lawn, the path and the flowerbeds contain everything a tree has drawn up from the soil over the year, and when they rot down they become one of the finest soil conditioners you can get hold of. Leaf mould is dark, crumbly and sweet-smelling. It holds water like a sponge, improves the structure of both sandy and heavy clay soils, and it costs nothing but a little patience.

The catch is the patience. Leaves are mostly carbon and break down slowly, and the fungi doing the work are in no hurry at all. Expect to wait between twelve months and two years. Fill a few bags this autumn and you will be spreading the results on your borders the autumn after.

Which Leaves Are Worth Collecting

Not all leaves behave in the same way, and knowing which is which saves you disappointment later.

• Break down quickest (a year or less): ash, sycamore, horse chestnut, lime, poplar and willow.
• Slower but excellent (up to two years): oak, beech, hornbeam and sweet chestnut. Beech and oak make particularly fine, long-lasting leaf mould.
• Best kept separate: thick evergreen leaves such as laurel, holly, rhododendron and magnolia, which can take three years or more. Conifer needles are slow and acidic, so pile them on their own and use them around blueberries and rhododendrons.

Skip leaves from very busy roadsides, which carry traffic grime, and leave any that are clearly diseased out of the pile. If your horse chestnuts are riddled with leaf miner damage, the leaves will still rot down perfectly well, but only if the bags are left long enough for the grubs to be thoroughly dealt with.

The Bin Bag Method: Simple and Surprisingly Effective

You do not need a compost heap, a shredder or any special equipment for this. Old compost bags and black bin bags do the job beautifully, and the method could hardly be simpler.

• Collect the leaves on a damp day, or water dry ones before bagging. Moisture is essential — the fungi need it.
• Fill each bag loosely, to about two-thirds full. Do not cram them tight.
• Punch six to eight holes in each bag with a garden fork, on both sides, so air can move and excess water can drain.
• Add a handful of garden soil or a few grass clippings to each bag to introduce the right microbes.
• Tie the top or fold it over, then stack the bags in a corner behind the shed, out of the fiercest sun and weighted down so they do not blow about.

That really is it. Check them the following autumn. If the contents feel dry, add a splash of water. Turning a bag out into a bucket once or twice a year will speed things along, but it is not essential.

A Wire Cage for Bigger Batches

If you have a large garden and a lot of leaves, a cage is more practical than dozens of bags. Make a square roughly a metre by a metre from chicken wire wrapped around four stakes, and pile the leaves in. Leave the top open so rain can get in, and the sides will let air through as the heap settles.

Two tricks speed up the process. First, mix in thin layers of grass clippings or a light dusting of a nitrogen-rich fertiliser — thin is the word, because thick layers of clippings go slimy and smelly. Second, tread the heap down as it settles and keep it damp. A heap that dries out will sit there for years without changing.

Knowing When It Is Ready, and How to Use It

Leaf mould is ready when it is dark brown, crumbly and smells pleasantly of woodland soil, with only the odd skeleton leaf still recognisable. You will usually find two grades in the same bag: a coarse, partly broken material and a finer, sieved product underneath.

• Coarse leaf mould is ideal as a mulch around shrubs, fruit bushes and perennials, spread 5cm thick over damp soil in autumn.
• Sieved leaf mould makes a superb autumn lawn dressing, scattered thinly and brushed into the grass.
• The finest, sieved leaf mould can be mixed with loam and grit to make a seed sowing compost, or used on its own for potting on established plants.
• Dig it in to vegetable beds to improve water retention and soil structure over several seasons.

Common Mistakes Worth Avoiding

• Do not tip large quantities of leaves onto your normal compost heap — they slow everything else down.
• Do not use an airtight bin. The process needs air as well as moisture, or the leaves simply go sludgy and sour.
• Do not let the stack dry out completely in a hot summer.
• Do not expect a finished product by spring. Leave it alone, and let nature take its time.

Start a couple of bags this weekend and forget about them. A year from now you will have a free, peat-free, garden-made soil conditioner that money genuinely cannot buy.