How Green Is Your Garden?
With proper planning, native plants and some appropriate actions, homeowners can create carbon-neutral landscapes
DOUG KENT is a bit of an iconoclast. "Gardeners think green is good," says the environmental landscape designer as he ushers a visitor through one of his garden projects in Manhattan Beach, California. Because growing plants is the essence of gardening, and the plants pull carbon dioxide (CO2) from the atmosphere during photosynthesis, we figure our landscapes must be beneficial in an age of climate change fueled heavily by this greenhouse gas. But the reality is that most gardens are sources of the substance. The amount of energy used to construct and maintain a typical garden can result in a surprisingly large level of CO2 emissions, says Kent, who maintains it is possible to create landscapes that are carbon-neutral or, even better, that function as backyard "carbon sinks."
The Manhattan Beach garden is a case in point. Kent designed it to be carbon-neutral, though you might not know it by looking at it. Like many suburban yards, it has several shade trees, including an impressive California sycamore that casts a leafy canopy over much of the backyard. California flannelbush (Fremontodendron californicum), golden monkeyflower (Mimulus guttatus) and fuchsia-flowering gooseberry (Ribes speciosum) are just a few of the wildflowers growing on the property that attract butterflies and hummingbirds.
The landscape's mostly native plants require very little water, and no chemical pesticides or fertilizers—only homemade compost. What the garden does not have is a lawn, which can be a significant source of greenhouse gas emissions depending on the products used to maintain it. Gardeners unwittingly contribute to the problem of global climate change in a variety of ways. For starters, we consume energy directly, by deploying the entire panoply of power equipment deemed essential for a proper modern landscape. These tools, particularly gasoline-powered equipment, collectively can generate large amounts of CO2 emissions.
The energy involved in pumping and distributing the water we use for irrigation can be another major source of CO2. Generally, the more arid the area, the higher the water's "embodied energy," the technical term for this indirect form of energy consumption. A study commissioned by the city of Irvine, California, found that the energy used to deliver water was second only to the fuel consumed by the service vehicles of municipal landscapers—and far more water is lavished on private gardens than public landscapes.
The fertilizers and pesticides with which gardens are routinely coddled account for still more energy consumption and carbon emissions. "Most gardeners are surprised to learn that often the biggest contributor to greenhouse gas emissions from home gardening and lawn care is associated with use of nitrogen fertilizers," says David Wolfe, a plant and soil ecologist at Cornell University. The manufacture of synthetic fertilizer is extremely energy intensive. Manures and other organic sources are better alternatives because the CO2 emissions associated with their manufacture are mostly eliminated. But using either synthetic or organic fertilizers releases nitrous oxide, which in Wolfe's words "has 300 times more global warming potential per molecule than carbon dioxide." Chemical pesticides also have high embodied energy, and they are toxic to boot.
Garden maintenance is just part of the problem. Garden construction also has an impact on the local environment. In some cases, it can be so energy intensive that it may take many years to offset the carbon emissions. Paving surfaces represents an especially large amount of embodied energy. Throw in processes involved in producing other accoutrements of a well-appointed landscape, and the emissions can pile up.
Having a garden that goes easy on the atmosphere requires changing some routine practices, but ultimately it is both better for the environment and less trouble to maintain. Kent divides the process of going carbon-neutral into four steps. The first is to determine the largest sources of energy consumption, and therefore greenhouse gas emissions, in your garden. (You can use the seven-page audit at the end of his book, Gardening in a New Era, to calculate energy consumption down to the nearest Btu, or British thermal unit.) This, in turn, will reveal the most promising ways to conserve energy in your landscape. Cutting your garden's energy consumption is the logical next step in reducing its carbon footprint.
Conservation alone won't make your garden carbon-neutral, however. To achieve that goal, you have to accumulate biomass—a fancy way of saying that you need to grow a lot of plants. To capture as much carbon as possible, maximize the planting area on your property and minimize the pavement. Then you must store the resulting biomass for as long as possible.
"This tree represents 1,000 pounds of biomass—that's 900 pounds of carbon dioxide," says Kent, pointing to a tree in the front yard of the Manhattan Beach garden. "Most people are aware of the fact that plants pull CO2 from the atmosphere to create their living tissue," he writes in his book. But few gardeners are aware that 90 percent of a plant's dry weight, its biomass, comes from this atmospheric CO2. That means the more plants actively growing and capturing carbon dioxide in a garden, the better. And according to Kent, storing the carbon captured by your plants means preventing it from decomposing, because all the CO2 they consume during photosynthesis is released back to the atmosphere when they die and begin to decay and rot.
Taking the steps outlined in "Six Ways to Save Energy and Reduce Your Yard's Carbon Footprint" (below) will go a long way toward reducing your garden's contribution to global climate change. If you want to go even further, Kent advocates the following additional measures:
Think of your yard as an urban or suburban woodlot and, if space permits, add new seedlings to your yard to augment mature trees that have finished their phase of greatest growth and therefore may be no longer capturing more carbon daily than they release as CO2 during respiration. This will help ensure that the garden is in a constant state of growth.
Repudiating current environmental practice, Kent also recommends composting only as much green waste as necessary to renew your soil. Only about 5 percent of a plant's biomass comes from the soil in the form of trace elements, and the biggest byproduct of decomposition is CO2, which is released back into the atmosphere.
For these reasons, Kent believes that while 100 percent of the green waste generated in brand-new gardens should be composted and returned to the soil, a mature landscape requires only 10 percent as a rule. He advocates sending excess trimmings to state-of-the-art landfills.
Cornell University's Wolfe disagrees. "The fundamental rule of thumb should be to recycle everything from the garden back to the garden, except of course what you are harvesting," he says. "The age of the garden makes no difference." Compost is essential not just for recycling trace elements but also for replenishing the organic matter in the soil. Wolfe adds that you can save yourself some work by letting the soil microbes "compost" the material in place.
In fact, one of the best things about carbon-neutral gardens is that they don't require the constant weeding, watering and feeding that conventional landscapes do. Instead of working, you can be out relaxing in your atmosphere-friendly yard like the owners of the Manhattan Beach garden.
New York journalist Janet Marinelli received NWF's magazine writing award for 2008. You can visit her blog on natural gardening and sustainable living at www.janetmarinelli.com.
Six Ways to Save Energy and Reduce Your Yard's Carbon Footprint
Want to save energy and reduce your backyard's carbon footprint too? Here are six steps you can take:
1. Reduce the size of your lawn. Better yet, consider eliminating it entirely. Families with young children require only a small area of lawn where the kids can play. Everyone else can manage without turf by creating patios for living space, enlarging planting beds or installing a rock garden.
Tip: Consider replacing your lawn with a native wildflower meadow. This will provide habitat for wildlife and requires no watering after its young plants are established. Since introducing plants to your property that are not indigenous to your region can contribute to ecological problems, ask your local native plant society which species are appropriate to cultivate.
2. Use hand tools instead of power equipment. When you reduce the size of your lawn, for example, you'll only need a push mower.
3. Choose materials with low-embodied energy. Brick and concrete have large carbon footprints compared to gravel and especially wood. Used brick and other recycled materials are good choices, too.
4. Emphasize woody plants that capture more carbon than fleshy herbaceous species. Create a flower meadow or vegetable patch, but plant most of your property with low-maintenance native trees and shrubs, preferably those that also provide food and nesting and resting places for birds and other wildlife. Again, choose species native to your region.
5. Plant trees and shrubs where they will block winter winds and provide shade in summer. This will reduce the amount of energy required to heat and cool your home and thus reduce your carbon footprint even further. The particular landscape strategy depends on your climate.
Tip: For more details, see "Landscaping for Energy Efficiency," a booklet produced for the U.S. Department of Energy and available online at www.eere.energy.gov.
6. Minimize, or better yet eliminate, the use of fertilizers and pesticides on your property. Use compost and mulch produced from garden trimmings to enrich your soil instead, and use native plants that are naturally pest resistant.
NWF Priority: Creating Low-impact Backyard Habitats
As part of its efforts to combat climate change, NWF produced a special report, The Gardener's Guide to Global Warming, which includes a range of steps you can take around your property to help reduce your carbon footprint. For other actions you can take inside and outside your home, see Cool It!® Tips for Going Green. Through its Certified Wildlife Habitat™ program, NWF also offers the information you need to make your yard more inviting to animals throughout the year. Once you create the haven, you can have your property certified as an official NWF wildlife habitat.