Audubon Living
The Dirt on Mulch
Don’t kill a forest to grow a plant. Choose mulch alternatives that are more accessible, easier on the environment, and often as cost-effective as they can be (free).
By Janet Marinelli
If you’re looking for mulch, consider skipping the garden center and getting creative. Crushed pecan shells (shown here and on following pages), coffee grounds, cranberry vines, ground corncobs, cottonseed hulls, peanut hulls, and spent brewery hops are some good options.
A few years ago Jacqui Sulek was on a mission to find an environment-friendly mulch for her Fort Lauderdale, Florida, garden, something less destructive than the cypress mulch so popular at local nurseries and superstores. She had heard the growing demand for cypress mulch was causing increased logging in critically endangered cypress swamps, particularly in Louisiana and Florida—including the historic habitat of the recently “rediscovered” ivory-billed woodpecker. She had her heart set on mulch made from invasive nonnative melaleuca trees being removed from the Everglades.
At the time melaleuca mulch wasn’t widely available, but with a little persistence she managed to track down a local commercial supplier. Sulek, who was recently hired as the chapter coordinator for Audubon of Florida, drove over, loaded 10 bags of the stuff into her station wagon, and put it to good use in her backyard butterfly garden. “I felt like I was doing a good thing,” says Sulek—and with good reason. She was conserving water and other resources in her garden by adding mulch, and also hastening the removal of one of the most invasive plant pests plaguing south Florida.
Jacqui Sulek is one of a growing number of gardeners looking for alternatives to mulches with deleterious environmental effects. Fortunately, there are a variety of nontraditional, and often local, sources of mulch for beds and borders—everything from coffee grounds to corncobs to pecan shells. What’s more, the “greenest” and most inexpensive mulch can often be found right in your own backyard.
Successful gardeners have long recognized mulching’s benefits. It can go a long way toward conserving water in the typical suburban landscape—no small matter when you consider that 30 percent of the domestic water used in the East goes to irrigate home gardens; in the West this figure is 60 percent. And like the protective layer of leaf litter in forests, prairies, and other native plant communities, a blanket of mulch helps keep plants healthy. Mulch has great insulation value. It keeps the soil around plant roots from frying in summer, and in winter helps prevent the ground from alternately freezing and thawing, which leads to soil heaving and root damage. By cushioning the impact of pounding rainfall, a layer of mulch also hampers soil compaction. While rain runs right off the surface of rock-hard soil, loose soil allows water to penetrate and plant roots to breathe. Over time, organic mulches, those made of plant material—as opposed to those made from crushed stone, gravel, or recycled rubber—decompose and enrich garden soil, improving even more its ability to retain water and nutrients. As a result, plants are more vigorous and less vulnerable to pests and diseases.
Mulched soil is health food for garden fauna as well as flora. It becomes fodder for earthworms and other soil organisms. The small animals that find refuge in the rich, damp environment provided by mulch in turn help sustain larger ground-feeding animals—like the flocks of grackles and robins that use their beaks to flip over the leaf mulch in my Shelter Island, New York, woodland garden all summer long. The birds are probing for pill bugs, worms, and other delectable morsels. Mulch pays large dividends to a garden’s human inhabitants, too. It suppresses weeds, eliminating the need for hours of backbreaking work. I’d much rather while away my summer days watching swallowtails sip nectar from my flowers than pulling weeds. And since a layer of mulch keeps the soil loose, there’s no need for repeated cultivation with hoe or scuffle.
Almost any kind of material applied to the surface of the soil can serve as mulch. But the best mulches are both light and permeable 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: inorganic and organic. Inorganic mulches, including crushed stone, gravel, or recycled rubber chips, are not made of plant material. (These, however, make a great porous paving material for patios and pathways.) In cultivated areas of the garden, organic mulches are preferable, because they eventually break down and fortify the soil.
A mind-boggling array of bagged organic mulches are available at local garden centers, as well as at Home Depot, Lowe’s, and other superstores. But you can save yourself some serious money, and also make your garden function more ecologically, by mulching fallen leaves and clippings from your own property. In the typical American neighborhood, leaves and lawn clippings are bagged up and shipped off to the local landfill, accounting for more than 12 percent of the 236 million tons of municipal solid waste generated each year in the United States. To compensate for these lost nutrients, we’re forced to buy fertilizers synthesized from fossil fuels that pollute waterways and poison wildlife. Forests or prairies, by contrast, are “closed-loop” natural systems; leaves and branches fall to the ground, decompose, and become food for the plant community and the many creatures it supports.
If you’re lucky enough, as I am, to have woodlands or some large trees on your property, you can use the fallen leaves—nature’s favorite mulching material—in your landscape. You don’t even have to rake—simply bag them up with your lawn mower. Whole leaves tend to mat together and block the movement of water into the soil, so before applying leaves to your planting beds, put them through a shredder, or simply pass over them with the mower. Needles from any pine trees on your property are aromatic, attractive, and easy to handle.
Almost anyone with a yard has ready access to grass clippings. The best way to recycle these is to leave them on your lawn. Just follow the “one-third” rule: Trim your lawn often enough so that you’re not shearing off more than one-third the length of the grass blades each time you mow. After you do, the grass clippings dehydrate and disappear, forming a water-conserving mulch on the lawn.
If you need more mulch than you can produce from your garden, a little ingenuity can go a long way, as Ellen Rathbone learned. When the local power company went through town cutting down the trees under power lines and putting them through a chipper, the Newcomb, New York, gardener stopped by and asked if she could have some. Two loads were promptly delivered to her property. “Now everyone in town wants to know what I am going to do with this mountain of mulch,” says Rathbone, an environmental educator for the Adirondack Park Agency. Because she gardens a lot, it won’t go to waste. “It may not be the most creative mulch,” she says, “but it is certainly the most cost-effective—it’s 100 percent free.”
Another good idea is to make friends with farmers. They offer an opportunity to recycle organic material that might otherwise be tossed. And such local sources of mulch avoid the pollution created by long-distance shipping. Ann Shahid, also an environmental educator, who lives in Ridgeville, South Carolina, near the heart of the state’s pecan-growing country, points out that until recently pecan shells were considered waste. Then landscapers began to appreciate the fact that the pecans, with their uniform size and shape, make attractive and somewhat formal-looking mulch when crushed, and because their shells are to some extent rot resistant, they don’t break down as quickly as other kinds of mulches. At the same time growers began to realize the shells have value to gardeners. The only drawback is that locating a supply can entail a bit of detective work. You might need to find and approach a pecan grower yourself, says Shahid. But it’s worth the trouble. “I think it’s a wonderful idea to use something that would otherwise be thrown away,” she says.
No matter where you live, you should never be short on mulch. Midwest gardeners can reap the benefits of mulch made from ground-up corncobs. Cottonseed hulls and peanut shells in the Deep South, cranberry vines from Cape Cod and Wisconsin bogs, or spent hops from a local brewery are among the many other good local candidates for mulching. And don’t forget about those coffee grounds from your percolator or neighborhood café; they can also provide nutritious cover for plants.
A number of conventional commercial mulch products are sustainably produced. But avoid mulches made from species of conservation concern, like cypress. Other materials to shun are salt hay and peat moss, both taken from ecologically valuable wetlands. Salt hay is made of saltmarsh grasses from coastal areas that should be left alone. Peat moss, which is extracted from Canadian bogs and sold as a soil conditioner, doesn’t even make good mulch; little if any rain can percolate through it and reach the soil.
When buying bagged tree-based mulches, you can maximize the chances that you’re buying something that is a recycled byproduct of timber operations—and not from whole trees felled solely for the manufacture of mulch—simply by looking 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 containing recycled construction and demolition debris may included wood treated with chromated copper arsenate (CCA), which contains arsenic, a human carcinogen. CCA is no longer used to treat most residential wood products, but it was 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 with labels stating that they have been 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 the product label accurately represents what’s in the bag—that, for example, a bag labeled “bark mulch” has the required percentage of bark.
Targeting mulches made from invasive trees in your area, as Jacqui Sulek did, may require a little sleuthing, but it can produce the biggest environmental payoffs of all. Just make sure such mulches have been partially composted to kill any remaining seeds. Mulches produced from invasive pests have all the horticultural advantages of other organic materials, and purchasing them supports efforts to remove nonnative trees that are degrading native habitats, whether Norway maple in the Northeast or Russian olive in the West. The melaleuca mulch helped work wonders on Sulek’s block in downtown Fort Lauderdale by nurturing the Simpson’s stopper and the other native plants she and her neighbors had nestled into the ground to entice butterflies. They knew their work had paid off when some rare species appeared among the waves of flitting wings. The plants “attracted hairstreaks,” she says, “including the amethyst hairstreak, which hadn’t been seen in the area for 10 years.”
Janet Marinelli is the author of Plant: The Ultimate Visual Reference to Plants and Flowers of the World (DK Publishing).
Lay It On Thick
Mulching is one of the simplest garden practices. As a rule of thumb, most mulches should be applied about three inches deep for best results, although this varies to some extent, depending on the type of mulch. The finer the material, the thinner the layer should be; a mulch of coffee grounds, for example, should be no more than an inch thick.
Optimum mulch depth also depends on the type of soil. Sandy soils, which lose moisture rapidly, benefit from a thicker mulch than clay soils, which tend to retain water. To avoid diseases and other problems, never pile mulch against tree trunks or plant stems; instead, pull it back an inch or two. Before applying mulch, remove any existing weeds, and water if the soil is dry.
Timing is everything. To be most effective, apply mulch around heat-loving vegetables like peppers and tomatoes after the soil has warmed—in mid or late spring in most areas. Cabbages, greens, and other cool-weather crops can be mulched in early spring. Mulches used primarily to protect shrubs and flowers from severe winter cold should be laid down in early winter, when the soil has cooled but not frozen hard; recycled holiday evergreens are a great material for this purpose. Mulch can be applied anytime in perennial beds and around trees and shrubs.
There are a few circumstances in which it’s best to leave the soil uncovered. Don’t mulch a low-lying area that’s apt to become waterlogged, unless you plan to grow plants adapted to these conditions. Likewise, don’t mulch seedlings planted in very moist soils, because excessive wetness is an invitation for damping-off, an often fatal fungal disease. Once seedlings are well established, it’s safe to mulch.
Because organic mulches eventually break down and become part of the soil, mulch must be renewed, usually once a year.—J.M.
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