Why leaf mould deserves a corner of your garden

Every autumn, Britain's trees hand us a free gift, and most of us rake it into the green bin without a second thought. Fallen leaves are the raw material for leaf mould, a dark, crumbly soil conditioner that costs nothing but patience. Unlike compost, which is made largely by bacteria and needs a good mix of green and brown material, leaf mould is the work of fungi. They break the leaves down slowly and coolly, which is why the whole process takes a year or two rather than a few months.

The reward is a material that improves soil structure, holds moisture, and feeds the soil life beneath your feet. It is low in nutrients compared with garden compost, so it will not overwhelm young plants, but it conditions soil beautifully — ideal for sowing, mulching and reviving tired beds. If you have a few trees within reach, you already have everything you need.

Which leaves are worth collecting

Not all leaves behave the same way. Thin, soft leaves rot down quickly and give you usable mould in about twelve months. Thick, leathery or waxy leaves take far longer and are best kept separate or left where they fall.

• Fast breakdown (about a year): sycamore, lime, cherry, plum, ash, horse chestnut, birch and willow.
• Slower breakdown (up to two years): oak, beech and hornbeam. These make superb mould, so they are well worth the wait, especially if you shred them.
• Very slow or best avoided: holly, laurel, rhododendron and most evergreen leaves. Conifer needles can be collected separately and used as a mulch for acid-loving plants such as rhododendrons and blueberries, but they will not rot down into a general-purpose conditioner in any reasonable time.

If your lawn has a mix of everything, do not worry. Gather them all and accept that the resulting mould will simply take a little longer.

The bin bag method, step by step

You do not need a fancy leaf bin. Heavy-duty bin bags or old compost bags are perfect, and they let you tuck the whole operation behind a shed where it is out of sight.

• Collect the leaves while they are dry. A dry day makes raking far easier, and leaves sweep up quickly from lawns and paths.
• Shred them if you can. Run the lawnmower over a pile, or give bags a good shake and a stamp. Smaller pieces rot considerably faster.
• Damp them down. This is the step most people skip. Sprinkle water over the leaves as you fill the bag so they are moist but not sodden — think wrung-out sponge, not swamp.
• Fill loosely, then tie the top. Do not cram them in tightly. Leave a little air in the bag and tie it loosely rather than sealing it airtight.
• Punch a few drainage holes in the bottom and sides with a fork or secateurs so excess water can escape and worms can find their way in.
• Label and store. Write the month and year on the bag with a marker, then stack the bags in a shady corner. Two or three bags is plenty for most gardens.

Keeping it damp and letting time do the work

Once bagged, leaf mould is a wonderfully low-maintenance crop. Check the bags every few months, squeezing a handful of leaves to see how moist they are. If they feel dry and crisp, add water and retie. If they are dripping wet, open the top for a few days and let them breathe.

Store the bags somewhere cool and shaded. A hot, sunny spot will dry them out and slow the fungal work you are relying on. Behind a shed, under a hedge or against a north-facing wall is ideal. Open wire rings or a simple cage of stakes and chicken wire work just as well, though they dry out faster and attract more wildlife.

After a year, open a bag and take a look. If the leaves have turned into a crumbly, dark brown material with an earthy, woodland smell and only a few visible leaf skeletons, it is ready. If not, dampen it, tie it back up and check again in six months. Oak and beech can easily take two years, and there is no harm in leaving them longer.

How to use your finished leaf mould

Leaf mould is remarkably versatile, and because it is low in nutrients it can be used generously without burning plants.

• As a mulch: spread a 5–8cm layer around shrubs, perennials and trees in spring or autumn to lock in moisture and suppress weeds.
• As a lawn dressing: sieve it and scatter thinly over the lawn in autumn, then brush it in to improve the soil beneath the turf.
• In potting mixes: mix roughly equal parts leaf mould, garden compost and sharp sand or grit for a fine seed-sowing compost.
• For woodland plants: spread it around ferns, hellebores, snowdrops and other shade lovers that relish a rich, leafy floor.
• To improve heavy clay or sandy soil: dig it into beds in autumn and let the worms work it in for you.

A riddle is handy for producing a finer grade for seed sowing, while the coarser material left behind is perfect for mulching.

A few things that go wrong

If the bags smell sour or slimy, they have gone anaerobic — too wet and too tightly packed. Empty them, mix in some dry leaves or shredded cardboard, and start again with looser bags. If the leaves are simply not breaking down, they are probably too dry or too whole; add water and shred them. Squirrels and foxes sometimes tear bags open looking for grubs, so tuck them behind something solid or use a wire ring instead. And if you have far more leaves than bags, do not despair: a loose heap in a quiet corner will rot down eventually, and it will be full of worms and hedgehogs in the meantime.

Start a bag this autumn and you will have a free supply of soil conditioner next year, and even better stuff the year after. It is one of the few jobs in the garden that rewards you for doing almost nothing at all.