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The Dirt on Mulch—It's Great for the Garden and Can Even Help Old-Growth Forests
Plants & Gardens News | Volume 22, Number 2 | Summer 2007
by Janet Marinelli
When it comes to mulching, like almost everything I do in my garden, I try to think like a forest. Forests are the most biologically productive and diverse terrestrial habitats, so I figure they're good role models. Prairies and other plant communities have natural layers of leaf litter too, but forests, especially deciduous forests, are nature's ultimate mulching machines.
Pine needles from evergreens are a great source of mulch for acid-loving plants.
Each autumn, deciduous trees relinquish their summer vestments in a rain of blazing foliage. Leaves contain a good deal of the forest's biomass, and also many of the trace minerals essential for growth that tree roots pluck from the soil. As the fallen leaves slowly decay and release their nutrients, they are transformed into food for the plant community. They also provide food and habitat for earthworms and other soil organisms, which are eaten in turn by birds and other small animals. The blanket of organic matter insulates the soil, protecting the plants from extremes of temperature. It prevents soil from eroding away with storm water or wind and conserves precious soil moisture that otherwise would evaporate readily into the atmosphere.
Mulching can have all these advantages in the garden—and more. However, not all mulches are equally beneficial, and some can have environmental effects that are downright destructive. For example, recent reports have documented how the growing demand by gardeners for cypress mulch has led to increased logging in endangered cypress swamps, which provide critical flood control, particularly in Louisiana and Florida. Fortunately, many environment-friendly mulches are available, and the "greenest" and most inexpensive mulch often can be found right in your own backyard.
A Garden's Guardian
Mulch's ability to conserve water in the garden has long been recognized—no small matter when you consider that 30 percent of the water in the East goes to irrigate gardens; in the West, this figure is 60 percent. A blanket of mulch keeps the soil around your plants' roots from frying in summer and in winter helps prevent the soil from alternately freezing and thawing, which leads to soil heaving and root damage. By cushioning the impact of pounding rainfall, mulch also hampers soil compaction. While rain runs right off the surface of rock-hard soil, loose soil allows water to penetrate and plants' roots to breathe. Over time, organic mulches—those made of plant material—decompose and add organic matter to your soil, improving even more its ability to retain water and nutrients. As a result, your plants are more vigorous and less vulnerable to pests and diseases.
Put down your rake and let autumn leaves fall where they may! As fallen leaves decay and release their nutrients, they are transformed into food for the plant community.
Mulch not only promotes a more bountiful crop of flowers and vegetables but also eliminates the need for hours of backbreaking work. It suppresses weeds. And since it keeps the soil loose, there's no need for repeated cultivation with hoe or scuffle.
Marvelous Mulches
The best mulches are both light and open enough to permit water and air to penetrate to the soil and substantial enough to inhibit the growth of weeds. Mulches are divided into two basic categories: organic and inorganic. Inorganic mulches, including crushed stone, gravel, or recycled synthetic rubber chips, are not made of plant material. In most situations organic mulches are preferable because they eventually break down and enrich the soil.
A mind-boggling array of bagged mulches is available. But you can save yourself some serious money, and also make your garden function more ecologically, by mulching fallen leaves from your own property. In the typical home garden, leaves and lawn clippings are bagged up and trucked off to the local landfill, accounting for more than 12 percent of the 236 million tons of municipal solid waste generated in the U.S. in 2003, according to the EPA. To compensate for the nutrients lost when leaves are discarded, we buy fertilizers. By contrast, forests are "closed-loop" natural systems. Leaves fall to the ground, decompose, and become plant food.
If you have some large trees on your property, you can use the fallen leaves, nature's favorite mulching material, in your landscape. Whole leaves tend to mat together and block the movement of water into the soil, so before applying them to your planting beds, shred them with a leaf shredder or simply pass over them with your mower. Pine needles from evergreens on your property are another great source of mulch.
Wood chips are another natural mulch.
If you need more mulch than you can produce from your garden, avoid commercial products made from species of conservation concern, such as cypress. To be sure you're buying a mulch that is a recycled byproduct of timber operations and not from whole trees felled solely for the manufacture of mulch, look for products expressly labeled "bark mulch," not just wood or hardwood mulch. Bark mulches must contain at least 85 percent bark.
Be aware, too, that hardwood mulch can contain recycled construction and demolition debris treated with chromated copper arsenate (CCA), which means it is contaminated with arsenic, a human carcinogen. While CCA is no longer utilized to treat most residential wood products, it was used for decades in the manufacture of pressure-treated wood to resist rot. To make sure you are not purchasing hardwood mulch contaminated with arsenic, look for products certified by the Mulch & Soil Council, a trade group. Certified mulch is tested chemically to ensure that it contains no CCA-treated wood. Among the other advantages of buying certified mulch is that you can be sure that the product label accurately represents what's in the bag—that, for example, a bag labeled bark mulch is indeed bark.
A variety of organic byproducts available locally also make excellent mulches—and recycle a material that might otherwise be disposed of as waste. Midwestern gardeners have long reaped the benefits of mulch made from ground-up corncobs, an agricultural byproduct readily available in the region. Pecan shells and cottonseed and peanut hulls in the South, cranberry vines from Cape Cod and Wisconsin bogs, and spent hops from the local brewery are among the many good candidates for mulching. So is a thin layer of coffee grounds from your own coffee machine or the neighborhood café.
Look also for mulches made from invasive trees in your area—for example, melaleuca mulch, which is now commercially available in Florida. First brought to that state from Australia about a hundred years ago, melaleuca trees (Melaleuca species) proceeded to colonize an estimated 20 percent of all natural lands south of Lake Okeechobee, choking out native species. Such mulches have the biggest environmental benefits of all. They not only have the horticultural advantages of other organic materials but also encourage the removal and recycling of invasive trees that are destroying native habitats.
BBG's former director of Publishing, Janet Marinelli has pioneered a people—and wildlife—friendly approach to gardening in several books, including Stalking the Wild Amaranth: Gardening in the Age of Extinction. Her latest book, Plant, profiles 2,000 imperiled plants around the world.