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Making Leaf Mold

Create a soil-enriching mulch with shredded leaves

Electric leaf shredder

The Electric Leaf Shredder reduces fall leaves to buckets of shreds.

In Britain, leaf mold is the connoisseur’s choice for mulching perennial gardens. It’s easy to see why this cocoa brown, sweet-smelling, moisture-retentive mulch is so popular. The curious thing about leaf mold is that so few gardeners in the U.S. even know what it is. That’s probably because here in the States there’s only one way to get leaf mold—you need to make it yourself!

Leaf mold is nothing more than partially decomposed leaves that are somewhere along the continuum between shredded leaves and humus. If you wonder what it looks like, next time you’re in the woods, just kneel down and push away a small area of dry leaves. Underneath, you’ll reveal a layer of leaf mold—a crumbly brown material with a pleasant, earthy scent.

The Benefits of Leaf Mold

Leaf mold has several great attributes. The first is that it can hold up to 500 percent of its own weight in water. Besides helping retain moisture in the soil by reducing evaporation, leaf mold also absorbs rainwater to reduce runoff, and in hot weather, it helps cool roots and foliage.

Most leaves are slightly acidic when they fall, with a pH below 6. However, as the leaves break down into leaf mold, the pH goes up into more a neutral range. Leaf mold will not correct pH problems, but will have a moderating effect.

Over time, yearly applications of leaf mold mulch can significantly improve the quality of your soil. The result will be better water-holding capacity, a more friable texture, and an increase in beneficial soil life. Though leaves are not high in nitrogen, phosphorus and potassium, tree roots mine calcium, magnesium and many other trace minerals from the soil and your garden will also benefit from these nutrients.

From Leaf to Leaf Mold

Unlike making regular compost, making leaf mold is a “cold” composting process. The decomposition is done primarily by fungi, rather than bacteria, and it is considerably slower. The rate of decomposition is largely determined by four key factors.

Shredded leaves

A bag of shredded leaves, ready for composting.

First is the type of leaves in your pile. Some leaves, such as oak and holly, are higher in lignin (cellulose) than others, and therefore take much longer to break down. Combining different types of leaves, like a mixed salad, is a good way to balance lignin content and also improve the quality of the finished product.

Moisture is another factor to keep in mind. Remember that fungi are doing the work, and they need a moist environment. An unattended pile of dry leaves could take three years or more to break down. Keep the pile covered and moist (not wet), and you may have ready-to-use leaf mold in a year.

Another consideration is nitrogen. Freshly fallen leaves have a carbon-to-nitrogen ratio in the range of 30 to 1, which is ideal for quick decomposition. Old leaves, including those that have been on the ground for just a few weeks, will have already lost most of their nitrogen content. If you can gather fresh leaves and get the process underway, there will still be a good amount of nitrogen to speed up the initial decomposition.

Whole or Shredded?

The easiest way to make leaf mold is to just rake your leaves into a big pile and let the pile sit there for two or three years. If you aren’t quite that patient or you don’t have enough room for three giant leaf piles, you’ll need to shred them. Leaves break down much more quickly if they’re shredded, largely because it increases the amount of surface area, which makes it easier for fungi to do their work. Shredding the leaves also prevents them from packing together into stacks that repel moisture and seal out air. It also makes it easier to fit a large quantity of leaves into a relatively small space.

The simplest way to shred leaves is to run over them with the lawn mower a few times and then rake them up. You can also rake the leaves and run them through a leaf shredder. Or use a hand-held leaf vacuum with a shredding capability.

Assembling Your Leaf Mold Pile

A leaf pile needs to be fairly substantial in size, in order to retain enough moisture and heat to get finished leaf mold within 12 months. Six feet square and five feet high seems to be an ideal size. It takes about 25 trash bags full of leaves to make a pile this large.

A second option is to pile the leaves into a wire or wood enclosure. Again, 4 or 5 feet square is ideal. Wet the pile thoroughly and cover it with a tarp. Check the moisture level several times during the year. It should be like a well wrung sponge. If you live in a dry climate, you might want to line the enclosure with cardboard or plastic to help retain moisture. If you have a minute when you’re checking the moisture content, use a fork to stir the leaves and incorporate a little fresh oxygen.

Another easy, yet very effective way to make leaf mold, is to pack the leaves into black trash bags. If the leaves are fresh and shredded, just moisten them, close up the bag, and poke a few holes in the sides of the bag. If the leaves are whole or dry, moisten them well and add a shovelful of garden soil, compost, or manure. Then just stash the bags out of the way for a year or two.

How to Use Leaf Mold

Leaf mold is ready to use when it’s soft and crumbly. Distribute it around your perennials, vegetable plants (shrubs, too if you have a lot of it), no more than about 3 inches thick. Because leaf mold retains so much moisture, be sure to keep it several inches back from the crown or base of the plant. This will help prevent pest and disease problems.

You can also incorporate leaf mold right into the soil. Unlike raw leaves, it will not steal nitrogen from the plants around it, so it’s safe to use in vegetable gardens and around annual flowers. You can also add it to new garden beds, use it instead of peat moss to lighten the soil in containers, use it to enhance the soil in a shade garden, or to improve any soil that’s too sandy or too heavy.

Someday, gardeners here in the States may catch on to the value of leaf mold. Leaves are certainly an abundant natural resource in most parts of the country. For now, they’re still free for the taking, so don’t delay. Grab a rake and start making your own super-premium, extra-fancy leaf mold mulch.

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