Using Autumn’s Bounty: Leaf Mold, Mulch and Compost
By Bill Kohlhaase, Planet Natural
Leaves, easily turned into protective mulch, soil-enhancing leaf mold or rich compost, are the fall season’s gift to the composter. After the last tomatoes are picked, the standing greens harvested, the squash brought in and the carrots pulled, nature provides a bounty that assures the next year’s crops will have the best soil possible. Let your non-gardening neighbors curse autumn’s raking tasks. Composters rejoice in the piles of mineral-rich organic material that trees graciously shed just for them.
Okay, okay, maybe that’s a little too much hyperbole. Still, it’s hard not to get poetic about leaves. Sure, raking can be hard work even for composters who know the value in each and every leaf. But leaves have long been a treasure for the gardeners: easily available, rich in nutrients, an effective mulch in winter and summer and, once decomposed, extremely beneficial to the soil.
But making leaf compost isn’t as easy as piling up a bunch of leaves and spreading them in the garden the following spring. Leaves, by themselves, do not make the rich soil amendment that all composters strive to achieve (but they will make leaf mold, a valuable soil addition; more below). Many of us started composting with leaves alone and it took a few seasons worth of experience to learn just what to add and how to maintain our heaps to turn our leaves into rich hummus. But leaves, in their abundance, can be the primary ingredient in successful compost. And they’re use is one of the most rewarding green practices a gardener can employ.
It’s difficult to estimate the amount of leaves that go into U.S. landfills and, of course, the estimates vary by season and location (weight versus volume is also a factor; leaves are the largest component of yard waste by volume, grass the largest component by weight). The EPA says 13 per cent of municipal waste volume nation-wide is from lawns, parks and other growing spaces. By weight, it is over half. Eight million tons of leaves went into landfills in 2005. It’s estimated that amount is somewhat less today thanks to the use of composting.
Of all green waste, the amount of leaves included can range from 5 to 50 percent depending on the season. The McGraw-Hill Recycling Handbook, Second Edition states that overall leaves make up 25 percent of all yard wastes in the U.S. The Pennsylvania Department of Environmental Protection estimates that grass, leaves, and other wastes from lawns and backyard gardens account for an estimated 18 percent of the annual municipal waste stream. In the fall, leaves can account for as much as 60-80 percent of that waste. In New Jersey, five to 30 percent of municipal solid waste is believed to be leaves. In the fall, this figures jumps to almost half. Because of its dry climate and short-growing season, the state of Wyoming estimates that its percentage of green waste is far lower than the national average.
These figures are in constant flux as individuals and communities apply composting methods to their green waste. But the fact remains that leaves are a tremendous and largely unnecessary burden on our landfill systems. And as a valuable resource to the gardener, the shame is wasting them at all. Stu Campbell, the author of Let It Rot! writes, “throwing them away is one of the worst kinds of conspicuous waste I know.”
What’s wasted? Pound for pound, the leaves of most tress contain twice the mineral content of manure. Because they’re a form of organic roughage, they can dramatically improve drainage and aeration of the soil. And they provide the perfect nutrition for beneficial microbes. In short, they make soil come alive.
Leaves are rich in the trace elements your soil needs. Trees are an effective mineral extractor, putting down deep and intricate root systems that funnel calcium, magnesium, potassium and phosphorus from the soil into their trunks and out to its leaves. 50 to 80 percent of all the nutrients trees extract from the ground end up in the leaves. Gathered at their peak and composted correctly, leaves will transfer this nutrition to your soil.
But all leaves are not created equal. The leaves of the eastern hemlock have twice as much nitrogen as the leaves of the red maple. White ash leaves are loaded with calcium, hemlock not so much. White ash leaves have a pH of 6.8, sugar maple leaves have a pH of 4.30. Some leaves aren’t suitable at all for composting, or should be used very sparingly. The leaves of black walnut trees and eucalyptus trees contain a natural herbicide that may keep your garden seeds from germinating.
To avoid wasting all these valuable nutrients and roughage, it’s important to know how to use leaves effectively. Leaves are at their nutrient best shortly after they’ve fallen from the tree. Soon thereafter, their nutrient value begins to disappear. Leaves left on lawns or in piles over winter lose much of their mineral value to leaching. Leaves composted without shredding and not mixed with a green source of nitrogen may sit for years before decomposing. Without a source of nitrogen, leaves will not become compost but instead become leaf mold, a valuable soil addition in terms of drainage and water-holding capability, but not as valuable as mineral-rich compost.
Leaf Compost, Leaf Mold, Leaf Mulch
What you intend to make with your leaves will determine the process you use. Many gardeners, especially those with abundant access to leaves, will have use for all three leaf products: compost, mold and mulch. Some will be looking only to make compost to enrich their soil. Gardeners with soil drainage problems will want to make leaf mold to improve the crumb and friability of their soil. Those with perennial plantings and extensive shrubbery will want leaf mulch to protect their plants and improve the soil’s water holding capabilities. Making the decision easier is the fact that any of the products can be used more or less effectively for any of these uses. But for the best utilization of leaves’ nutrition, you’ll want to make compost.
The Rake’s Progress
Let’s start at the beginning. Leaves should be gathered as soon as they start falling from your trees. At this point, they contain the most nitrogen and their cells are still pliable and friendly to decomposition. Not only do leaves give up nitrogen as they sit around, the cells walls harden, becoming resistant to break down. As the lignin between cell walls dehydrates, it not only resists decomposition but its ability to transmit nutrients through the soil (cation exchange) is decreased. Using freshly fallen leaves to make mold or compost not only preserves the leaves’ mineral content, it increases the function that transmits that nutrition from soil to plants. Lignin also provides nutrition for the bacteria that will facilitate the decomposition process. The more viable the lignin, the faster you’ll have compost.
Yes, gathering leaves is a chore, one that extends a month or two through the fall season. But as The Complete Gardening Compost Guide by Barbara Pleasant and Deborah L. Martin points out, its also good exercise. Plan to spread your raking out over the season and you can give up your gym membership for the entire fall. Pleasant lists ”12 Rules of Raking“ to make the job easier and more effective. While most of these rules come from common sense, the last, “Keep in mind that leaf season will last for several weeks, so you have plenty of time to let yourself enjoy the weather and the work,” is one the more ambitious among us might need to be reminded of.
Leaves break down slowly. A pile of unshredded leaves without added nitrogen sources may sit for years before it will be completely decompose. Early-season raking of clipped grass and leaves help solve this problem by supplying an already mixed source of leaves and grass. As the season moves on, only leaves will be available. To make quality compost, leaf shredding is essential. This can be done by commercial shredders, which are notoriously expensive, noisy and fragile. Or shredding can be done with your home lawn mower. Don’t be content to run over your leaves once. Maximum shredding is important for quick breakdown. It’s easier if you employ help to pile up the leaves again once you’ve passed over them with the mower. Several passes will give you a fine, quick-to-decompose product. This is true if you’re making compost or leaf mold. In a pinch, a Weed Whacker or other line trimmer can be used to reduce leaves to a more compostable size.
Unshredded leaves left to mold will pack tightly in layers, delaying the molding process sometime for as much as two or three years. Even in a compost tumbler, unshredded leaves will sit through the season while all other green materials around it decompose.
Piling On Leaf Mulch
Now’s the time to decide what to do with your leaves. If using them as mulch, they can be applied directly under trees shrubs and plantings to protect the soil and provide insulation from the cold. Don’t be afraid to pile it on. Loft is important; the higher the pile and the more air trapped inside it, the better the insulating properties. Several inches is a good start. The leaves will compress and layer as the season progresses. In extremely cold climates, a foot of leaf mulch is not too much. Remember that leaves generally increase the acidity of soil. It’s a good idea to test soils in the spring and add lime or other alkaline substances if you pH is not to your plants’ liking. If using whole leaves or those not finely shredded, you’ll want to pull them back in the spring to allow the soil to warm. Unshredded leaves can also make a sort of canopy over soils, allowing moisture to run-off and not get to the ground. Finely shredded leaves tend to work themselves into the soil and encourage moisture absorption. Also, shredded leaves will not inhibit the spring soil warming process as much.
Studies have found that mulching leaves directly into turf, lawns and gardens has many benefits and a few drawbacks (see When Leaves Turn Into Litter). Generally, mulching directly into turf increases aeration and friability of soils, allowing grasses to spread and thicken. It will also lower nitrogen to carbon ratios of soils if done to extremes. Large amount of shredded leaves left on turf results in leaf litter being apparent the next spring and a chance that new grass growth will be discouraged by the cover.
If you have an abundance of leaves, it’s a good idea to store some in contained heaps to use later during the growing season as mulch. Yes, they’ll lose some of their nutritional benefit through leaching and off-gassing. But come spring, they’ll help conserve moisture in the soil during the growing season and will slowly become integrated into your garden. The decomposition that occurs during the storage process is beneficial. You’re making leaf mold.
Mold Does Mulch One Better
Leaf mold is a step past leaf mulch. It’s made in much the same way as compost, but with little or no nitrogen added to the leaves. Leaves left in contact with the earth and its wealth of beneficial microbes will slowly turn to leaf mold. The speed at which this happens depends mainly on the size of the leaves, shredded or not. Just leaving leaves where they fall will eventually result in leaf mold, not a bad thing in wooded areas, but not a good thing on your lawn (see “leaves on turf above). Some gardeners with whom patience is a virtue, see little reason to “artificially” make leaf mold. Those of us without that patience are glad to encourage the natural process.
Making leaf mold is similar to making compost. Piling leaves in heaps or in bins and cages is about all that’s necessary. Keep the piles uniformly moist. Turning them on occasion is helpful but not necessary. Matting, a problem with leaf-only piles, is minimized by frequent turning. Keeping the pile under a plastic tarp will help conserve heat and moisture. Be sure that the pile has access to air. Even piled in cages, leaves can take three years to reach optimum condition. But if you shred finely, turn the pile and keep it uniformly moist, you’ll have usable product in six to 12 months. Leaf mold can also be made in plastic bags by filling lawn bags with shredded leaves, dampening and poking a few holes to let in air.
Making leaf mold (or compost for that matter) in raised beds can greatly increase the volume of your soil. Filling a raised bed with shredded leaves in the fall and turning them into the soil as soon as possible is one of the most beneficial things you can do for your contained soil. Covering the bed with plastic over the winter will speed the assimilation process.
Leaf mold absorbs five times its weight in water. Turned in to hard and clay soils, it will help make them more friable and root-friendly while maintaining good moisture levels. And any leaf mold not used in your garden makes a great addition to your compost heap.
Making leaf compost isn’t different than making other compost. Bins, cages, piles and tumblers will all give satisfactory results though at different speeds. Because leaves are mostly carbon (60 parts carbon to one part nitrogen) more attention must be paid to the carbon-nitrogen balance. Not only will the right ratio of leaves to green material or manure yield a more nutritious product, it will also give you compost more quickly.
Chopping and mixing leaves with other brown and green ingredients will speed decomposition by four times. Five parts leaves to one part manure will get your compost pile up and hot. Using only grass clippings requires five part leaves to two or three parts clippings. Kitchen waste including coffee grounds and those last trimmings from your garden will also increase the nitrogen content of your pile. But don’t over do it. Too much nitrogen will help make your heap smell or turn anaerobic. Being sure your pile gets enough oxygen will help prevent this problem. To avoid matting, frequent turning of leaf piles is a must. Turning distributes moisture among water-repellent leaves, making for more uniform decomposition.
Maintaining correct carbon-to-nitrogen ratios is not always easy. Measuring green and brown materials in buckets, bushels or wheel barrow loads, not an exact science, will give close proximity. Because manure has more weight per volume, less of it than what appears to be 20 percent by volume will give a correct balance. While the traditional layering method isn’t necessary to make compost, it does help you eyeball well-balanced green and brown ratios.
Because leaves are often available in such large quantities, it is impractical to expect your compost tumbler to consume all of them. If you have a bounteous supply of leaves, you’ll want to use bins, cages or heaps to begin the compost process. Leaves from the heaps can always be added to your tumbler when a new batch is being started. Again, because of their availability, it’s tempting to construct very large piles. But large piles are harder to turn and contain. Two or three manageable piles, all with sufficient nitrogen source added, are much more effective and more easily worked. The classic “three bin” method of composting is a great way to keep large amounts of leaves organized and progressing through the decay cycle.
Some gardeners have developed shortcuts that help them utilize fall’s bounty more efficiently. One method is to rake leaves directly over the remains of your vegetable garden at the end of the season, then rototill the entire plot to break up the leaves and greens and mix them with the soil. The plot can then be covered with plastic if the size of your garden makes it feasible. Adding a little manure or fertilizer will help the carbon to nitrogen balance. A second rototilling a week or so later further breaks down the leaves, integrates them with the soil and aerates it all. Recover for the winter. Spring rototilling should reveal that the leaves have become part of your soil.
One last caution when using your finished leaf compost. Some leaves will yield a more acidic product, especially if pine needles have been included (though it takes large amounts of needles to effectively change the pH). Measuring the pH of your soil after adding compost is a good idea. Supplement to bring soil pH in line with your plants’ needs. Or just add a bit of lime to compost high in pine needles and acidic leaves (oak, maple) before using it.