Leaf mold: what to do with fallen leaves in your yard?

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Every fall, it's the same scene: leaves pile up on the lawn, along the walkways, and at the foot of the hedges. Many gardeners bag them up for the recycling center or, worse, burn them, which is prohibited. That's a shame, because fallen leaves are a free and plentiful resource.

You can of course add them to your compost bin. But you can also turn them into a product in its own right that gardeners love: leaf mold. Here's how to make it, which leaves to choose, and how to use it.

Leaf mold and compost: two different products

In a compost bin, bacteria do most of the work, breaking down a mix of green and brown materials. A pile of leaves on their own behaves differently. Decomposition is carried out mainly by fungi, slowly and without heating up. That's why it takes more patience.

The result is different too. Leaf mold is low in nutrients, so it isn't a fertilizer. It is, however, an excellent soil amendment with a fine, crumbly texture that improves soil structure and helps it retain water. That's exactly what you want for starting seeds, mulching, or loosening heavy soil.

Ideal leaves and leaves to shred

Not all leaves break down at the same speed. Here are the general trends:

• Oak, beech, and hornbeam make the best leaf mold and break down without any special help.
• Sycamore maple, walnut, horse chestnut, and chestnut are much slower: shred them first, or put them in the compost bin instead.
• Holly, aucuba, and cherry laurel, with their tough evergreen foliage, are best shredded and put in the compost bin.
• Conifer needles do eventually break down, but they take two to three years. Kept separate, they produce an acidic leaf mold that's useful for acid-loving plants.

You'll sometimes read that oak leaves are “forbidden” in compost. That's a myth: in fact, they're among the best. Thick, slow leaves, such as walnut, simply need to be shredded.

Two precautions: collect healthy leaves whenever possible, and leave out those from roses affected by black spot (marsonia). Also avoid leaves gathered along major roads, which may be polluted, and pick out any trash if you collect them from the street.

The bag method or the cage method

The bag: simple and discreet

This is the ideal method for small yards. Fill a large trash bag with leaves, moisten them if they're dry, then punch a few holes in the bag with a garden fork or a knife to let air in. Close it loosely and store the bag in a shady corner, out of sight. Then all you have to do is wait.

The cage: for large volumes

If you have a lot of leaves, build a square or round wire mesh enclosure, held up by four stakes, in a spot sheltered from the wind. The bigger the pile, the faster it breaks down. Fill it up over the course of the fall and water it if it dries out, especially during hot summers. A slatted compost bin such as the Compostys wooden compost bin can also double as a leaf bin. In the illustration: the bag method (1) and the wire mesh cage method (2).

Willy's tip

Rake up your leaves in damp weather, right after a rain: they pack down better, and you won't need to water them before bagging them.

The lawn mower: a shredder in disguise

There's no need to invest in a shredder for leaves. Simply run the lawn mower, with the bagger attached, over the leaf-covered lawn. It chops and collects them at the same time, which noticeably speeds up decomposition. Bonus: a little grass clippings get mixed in with the leaves and add nitrogen that enriches the leaf mold.

On a walkway or a patio, rake the leaves into a pile, spread them in a thin layer over the lawn, then run the mower over them.

How long does it take?

You'll need to be patient: most leaves take around two years to break down completely. Expect a first product you can use as a soil amendment after about a year, then one to two more years to get fine, dark leaf mold rich in humus.

To speed things up:

• shred the leaves;
• keep them moist, never soggy;
• make the pile as large as possible;
• add nitrogen, for example by alternating layers of about 12 in (30 cm) of leaves with a little roasted horn meal or chicken manure, or some nettle tea.

These nitrogen boosters are natural activators. We look at how useful they really are in our guide Compost activators: useful or not?

Uses for leaf mold

Depending on its age, leaf mold has several uses:

• Starting seeds: well-rotted leaf mold, more than two years old, can be used as is for sowing seeds.
• Potting: mixed in equal parts with sand, compost, and good soil, it makes an excellent growing medium. We go into these mixes in detail in our guide Making your own potting mix.
• Mulching: younger or coarser leaf mold can be spread as a mulch at least 2 in (5 cm) thick, from late fall to late winter.
• Soil amendment: worked into the soil, it loosens heavy soils and helps sandy soils hold water.
• Lawn: once sifted, it can be spread in a thin layer in the fall.

A pile of leaves may contain weed seeds, so avoid using it where sprouting weeds would be a real problem.

Did you know?

In the forest, nobody rakes up the leaves: they protect the soil from drying out, feed a host of organisms, and slowly turn into humus. Leaf mold simply recreates this natural cycle in your garden.

What about a regular compost bin?

Fallen leaves are also an excellent carbon-rich brown material for your compost bin. The problem is that they all show up at once, in the fall, while your food scraps come in all year round. The solution: fill a few bags with dry leaves and store them next to the compost bin. That way you'll have a supply of “browns” to balance your food scraps until spring. We explain this balance in our guide Green and brown materials.

For a worm composter, however, we recommend brown cardboard as the dry material instead: leaves collected from the yard can bring in slugs, insect eggs, or unwanted seeds. We explain this in our guide on what you can feed your worms.

Key takeaways

Oak, beech, and hornbeam: in a bag or a cage for leaf mold. Tough or evergreen leaves: shredded, in the compost bin. And in every case, keep a few bags of dry leaves on hand for the year.

In summary

Fallen leaves aren't waste but a resource. In a bag with holes or a wire mesh cage, kept moist and ideally shredded with the lawn mower, they turn into fine leaf mold in about two years, ideal for starting seeds, mulching, and improving the soil. Slow or tough leaves are better off in the compost bin, where they'll serve as brown material all year long. A little patience is all it takes to give back to the soil what the trees gave it in the fall.

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