How to compost leaves: from fallen leaves to garden soil

Fall leaves are free, plentiful, and one of the best things you can add to garden soil. Here’s how to turn them into compost or leaf mold, and what to do with it.

Fallen leaves are one of the easiest things to compost. They’re free, there are lots of them at once, and with a little help they turn into dark, crumbly material that improves almost any garden soil. This guide covers the two main ways to do it, a compost pile and the even simpler leaf mold, plus what to do with the finished product. If you’re still deciding whether to rake at all, see should you rake your leaves?

Why compost leaves

Compost adds organic matter to soil. It improves the soil’s structure, helps it hold water and nutrients, and reduces erosion (U.S. EPA). Leaf compost is fairly low in nutrients, so think of it as a soil builder rather than a fertilizer (New Jersey Department of Environmental Protection).

In compost terms, dry leaves are a “brown”: rich in carbon. Grass clippings and vegetable scraps are “greens,” rich in nitrogen (U.S. EPA). The microbes that do the work break things down fastest at roughly 30 parts carbon to 1 part nitrogen by weight. Leaves on their own run about 40 to 80 parts carbon to 1, and grass clippings about 12 to 25 (Texas A&M AgriLife Extension). That’s why a pile of leaves alone breaks down slowly, and why mixing in some greens speeds it up. It varies by tree, too: oak leaves are among the slowest, partly because of their tannins, while maple leaves are closer to the ideal (Cornell Waste Management Institute).

Shred them first

Smaller pieces break down faster (Iowa State University Extension), and shredding keeps leaves from matting into soggy layers that shut out air. It also shrinks the pile dramatically, to as little as a tenth of its original volume (Alabama Cooperative Extension).

You don’t need a special machine. A mulching mower works best, but a regular mower is fine: mow slowly over a shallow layer of fairly dry leaves, then go over them again in another direction for finer pieces (Penn State Extension). Aim for a coarse shred rather than dust. Very fine leaf bits can pack down and cut off air, and some gardeners do fine with whole leaves as long as the pile is fluffed and turned (Colorado State University Extension).

Whole leaves

Light, airy, and slow to break down

Shredded

Smaller pieces break down faster

Finished

Dark, crumbly, and earthy-smelling

Build a compost pile

Size matters: about 3 feet wide, deep, and tall (a cubic yard) is the usual minimum for a pile to hold its heat (Texas A&M AgriLife Extension). Piles much wider than about 6 feet can slow down because the middle doesn’t get enough air (Colorado State University Extension). A bin is optional. Wire, wood, or a simple loose heap all work (Penn State Extension).

Build in layers. One common method is 6 to 8 inches of leaves, then a thin layer of soil or finished compost, repeated until the pile is 3 to 5 feet high (Iowa State University Extension). Mix in greens as you go. Recommended proportions of leaves to grass clippings vary widely between sources, so start with roughly equal amounts and adjust: add more leaves if the pile smells, and more greens if it stays cold (Colorado State University Extension; Clemson Extension).

Leave out meat, fish, bones, dairy, fats and oils, pet waste, diseased or pest-infested plants, weeds that have gone to seed, and plants treated with weed killer (U.S. EPA).

Keep it damp and let it breathe

The pile should feel about as damp as a wrung-out sponge. Squeeze a handful and only a few drops should come out (Texas A&M AgriLife Extension). Dry leaves shed water at first, so wet them as you add them and check the pile often (Colorado State University Extension). A pile that’s too dry just sits there. One that smells bad is too wet or short of air, so add more leaves and turn it (U.S. EPA; Clemson Extension).

An active pile heats up, typically to somewhere around 130 to 160°F in the middle (NC State Extension), and a leaf pile can reach about 150°F within a few weeks. Hotter isn’t always better: decomposition is fastest between about 90 and 140°F, most of the microbes doing the work can’t survive much above 140°F, and above 160°F a pile can sterilize itself (Cornell Waste Management Institute). Turning a pile that’s running very hot lets heat escape. Turning it about once a month in warm weather brings fresh air and the cooler outside material into the center. In cold weather, turn it less often, since each turn lets heat escape (New Jersey Department of Environmental Protection).

How long it takes

It depends on how much attention you give it. A well-managed pile of shredded leaves mixed with greens, kept damp and turned, can be ready in about 2 to 4 months. An average pile takes 6 to 8 months, and a pile of whole leaves left alone can take more than a year (University of Georgia Extension), sometimes two or three (Colorado State University Extension).

Compost is finished when it’s dark, loose, and crumbly, smells like fresh soil, no longer heats up, and has shrunk to about a third of its starting volume (U.S. EPA).

The easy way: leaf mold

If turning a pile sounds like too much work, make leaf mold. It’s leaves alone, broken down slowly by fungi rather than the heat-loving bacteria of a hot compost pile (University of Georgia Extension). Pile damp leaves in a corner, in a wire or wood bin at least 3 feet tall, or in a yard bag with a few slits cut for air, and wait (Penn State Extension). Water the leaves if they dry out.

Left alone, leaves become leaf mold in about two years. Shredding, wetting, and turning can cut that to 6 to 12 months (Penn State Extension). The result is dark, crumbly, and earthy-smelling. It’s low in nutrients, but it’s excellent for improving soil structure and helping soil hold water, and it makes a good mulch or potting-mix ingredient (Penn State Extension).

Leaves that need a little care

Black walnut. Walnut leaves contain juglone, a natural compound that can harm sensitive plants such as tomatoes. Juglone breaks down with air, water, and microbes, and thoroughly composted walnut leaves are generally considered safe (University of Wisconsin Extension). A hot pile breaks it down best (Oregon State University Extension). Don’t use uncomposted walnut leaves as mulch, and if you’re unsure, keep walnut compost away from tomatoes and other sensitive plants.

Oak leaves. They’re slow to break down, but they won’t make your soil acidic. Finished compost, including compost made with oak leaves, is close to neutral or slightly alkaline (University of Illinois Extension), and research has found that mulching oak and maple leaves into lawns didn’t change soil pH (Penn State Extension).

Pine needles. Fresh needles are acidic, but the soil buffers them, so they acidify soil no more than other mulches do (NC State Extension).

Diseased leaves. Ordinary backyard piles usually don’t get hot enough to kill plant diseases. If leaves had a disease such as apple scab, either compost them in a hot pile that stays around 140°F or more, turned so the outer leaves reach the center, or bag them for disposal. Since ordinary backyard piles don’t reliably get that hot throughout, keeping heavily diseased leaves out of home compost is the safer choice (University of Georgia Extension; University of Wisconsin–Madison Extension). The same goes for weed seeds, which can survive a pile that doesn’t heat up (University of Illinois Extension). For more on leaf diseases, see what’s damaging these leaves?

Using finished compost and leaf mold

For garden beds, mix 1 to 2 inches of finished compost into the top 6 to 8 inches of soil. Use it more generously in heavy clay and new beds (University of Georgia Extension; Virginia Cooperative Extension). You can also spread it on top as mulch, top-dress a lawn with a thin layer of up to about a quarter inch (Virginia Cooperative Extension), or blend it into potting mix, where it shouldn’t make up more than about a quarter to a third of the volume (University of Georgia Extension).

Or skip the pile: mow them into the lawn

The least work of all is leaving leaves where they fall and running a mower over them. Purdue research found that shredding leaves into turf didn’t hurt the grass (Purdue University), and mulched oak and maple leaves don’t add thatch or encourage disease (Purdue Turfgrass Science). Mow often enough that the leaves stay manageable, and stop adding more once about half the grass is hidden; any extra can go to the compost pile (University of Minnesota Extension).

Save a few bags for later

Fall brings more leaves than a pile can use at once. Bag the extra and keep it nearby (Alabama Cooperative Extension). Next spring and summer, when you have grass clippings and kitchen scraps but no browns, a bag of saved leaves gives you browns to mix in. The EPA suggests burying food scraps under several inches of leaves or other browns (U.S. EPA).

A few safety notes

Wear safety glasses when shredding leaves with a mower or shredder (Cornell Waste Management Institute). Compost and potting mix can occasionally carry bacteria that cause a type of pneumonia, so health researchers advise keeping dust down, keeping your face away when you open or turn compost, and washing your hands before touching your face (Centers for Disease Control and Prevention).

If you can’t compost at home, many towns collect leaves or run drop-off composting sites. Across the U.S., about 63 percent of yard trimmings were composted in 2018 (U.S. EPA).