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The Climate-Friendly Gardener

A Guide to Combating Global Warming from the Ground Up

Union of Concerned Scientists

 

 

 

 

Seventy percent of American households engage in some level of gardening or lawn care every year.1   Some do it for beautiful flowers, lush grass, or fresh fruits and vegetables; some for the peace and quiet or the connection to nature. But there is another reason to grow plants in your yard: certain gardening practices can help combat global warming. This guide will show you how. First, we explain the science linking soil, plants, and climate change; then we provide practical tips for a more climate-friendly garden, and links to resources that will help you adapt these tips to your own needs.

 

 

 

Carbon is constantly cycling from the air into plants and soil, and back into the air. Global warming is largely a result of an imbalance in this carbon cycle, due to the release of vast quantities of ancient carbon that have been burned as fossil fuel. Adapted from www.farmingfutures.org.uk/x510.xml

The Relationship between Gardens and Climate

 

Scientists agree that global warming is well under way, the result of a buildup in the atmosphere of carbon dioxide (CO2) and other heat-trapping gases generated by human activities including the burning of fossil fuels. When too much global warming pollution is released into the air, it acts like a blanket, trapping heat in our atmosphere and altering weather patterns around the world. This climate disruption is likely to have wide-ranging consequences not only for our own health and well-being, but that of other living things as well.

 

Many gardeners already see evidence of global warming in their own backyards. Summers are getting hotter, and generally milder winters mean plant hardiness zones are shifting.2 These changes can make planning your garden—which plants to choose, how soon to put new seedlings in the ground, and when to harvest vegetables—more of a challenge. While a longer growing season will benefit some gardeners, climate disruption is also projected to increase the frequency and severity of extreme weather (for example, droughts, floods) and to benefit some pests and weeds, which will further challenge the ability of gardeners (and farmers) to grow plants successfully.

 

Gardeners can do more than merely adapt to global warming, however—they can make choices in their gardens that don’t add to the problem. That’s because each patch of soil (and the plants that grow in it) takes in and gives off various types and amounts of heat-trapping gases, depending on how it is managed.

 

 

 

 

 

The Secret Is in the Soil

 

All living organisms are based on carbon. Carbon atoms can bond with as many as four other kinds of atoms, resulting in carbon chains and other compounds such as proteins, fats, and carbohydrates that are essential to life on Earth. Carbon continually moves through living organisms, the oceans, the atmosphere, soil, and rocks in a phenomenon known as the carbon cycle.

 

Gardeners participate in this complex cycle. Plants capture CO2 from the air and convert it to carbohydrates (starches and sugars) along with other carbon compounds that become the tissues of the plant. When these carbon-rich plant tissues are consumed by animals, or when the plant dies and microorganisms in the soil cause it to decompose, CO2 is formed again, and its return to the atmosphere completes the cycle.

 

Global warming is largely a result of the carbon cycle being out of balance. When we burn oil, coal, and natural gas we release vast quantities of ancient carbon that had been stored underground for millions of years. One way to re-balance the carbon cycle is to "lock up" some of Earth’s carbon atoms again.

 

In the garden, the key to locking up carbon is soil. Putting carbon-rich organic matter such as plant parts or manure into the soil may store (or "sequester") the carbon there for a period of time. Although some of this carbon is re-released quickly into the atmosphere as CO2, some remains bound to minerals or in organic forms that break down into CO2 slowly in the soil, helping to reduce the buildup of carbon in the atmosphere. Gardeners can help their soil store more carbon through a number of practices discussed in this guide.

 

Building carbon-rich soil has other benefits besides combating global warming. Soils rich in organic matter drain well, prevent water pollution, support many beneficial microbes and insects, and sustain plant growth with little or no synthetic fertilizer (which is derived from fossil fuels). A study of Seattle, where single-family yards and gardens account for an estimated 25 percent of the city’s land area, compared a "climate-friendly" lawn and garden care approach with one relying on fossil-fuel-intensive inputs and practices. The study concluded that each household converting to climate-friendly practices produces environmental and public health benefits—not including carbon storage benefits—worth nearly $75 a year.

 


How to Limit Chemicals in Your Garden

Use people power. Weed, prune, and rake leaves by hand whenever possible. And if you have a lawn, consider using a push or electric mower rather than a gasoline-powered one; every gallon of gasoline you burn puts 20 pounds of CO2 into the atmosphere.6 The average lawn mower also emits as much smog-forming pollution in one hour as an average car traveling almost 200 miles.7

Choose non-synthetic products. With many more products from which to choose, the number of U.S. households using non-synthetic fertilizers and insect- or weed-control products increased from an estimated 5 million households in 2004 to 12 million in 2008.8

One organic fertilizer option, composted manure, has a smaller carbon footprint than industrial fertilizers.9 And compared with chemical pesticides, organic options such as beer bait for slugs, insecticidal soaps, neem oil, and powders and sprays derived from the naturally occurring Bt bacterial toxin are more climate-friendly and safer for pets, kids, and wildlife.

Rotate annual crops. If you have a large vegetable garden, don’t plant the same crops in the same spot year after year. Moving them around your garden in a four-year rotation (for example, leafy vegetables, fruiting crops (like tomatoes) root vegetables, legumes)10 can help prevent some crop-specific pests and diseases from establishing a foothold, making them easier to manage without chemicals. Additionally, by rotating legumes with non-legumes, you’ll need less fertilizer because legumes leave nitrogen in the soil.

Know your fertilizer needs. Many gardeners simply guess at what their soil needs when applying compost or other fertilizers. As a result, many either under- or over-apply fertilizer. To get a clearer picture of your soil’s fertility, have it professionally analyzed about every three years by a university extension service or private laboratory. Standard soil tests are inexpensive and measure phosphorus, potassium, calcium, and magnesium levels, soil pH, and other indicators. You can also have your soil’s organic matter content analyzed. You’ll need to take several samples from around your garden in spring or fall; contact your state’s Cooperative Extension office    for specific sampling and mailing instructions.11,12

Avoid peat. Sphagnum peat moss, a component of many potting and seed-starting mixes, has long been controversial because its mining (primarily in Canada and Scandinavia) destroys peat bogs important to wildlife and water quality. Peat bogs also store considerable amounts of carbon that is released when the peat is mined and used.13 Look for peat-free mixes or use your own compost.


Why You Should Be Growing Your Own Food

Growing food is another way to make your garden climate-friendly. Most supermarket produce travels more than 1,500 miles (often in a refrigerated truck) to get to your dinner plate.25 Eliminating some of that transportation and refrigeration by growing your own produce not only conserves fossil fuels and prevents carbon emissions, but also saves money. One vegetable gardening expert notes that a typical 20-foot by 30-foot garden produces about 338 pounds of produce—worth more than $600—per growing season.26

Incorporating fruit trees and berry bushes into your yard instead of purely ornamental plants increases the amount of food you can grow. And homegrown produce is fresher and better-tasting because it’s picked ripe and usually eaten within hours of harvest.


How to Capture Carbon with Trees

Because of trees’ cooling effect and other benefits, some U.S. cities and towns have adopted tree-planting goals, and the U.S. Environmental Protection Agency has suggested tree planting as a way for cities and states to help meet their clean air targets.34,35 Individual gardeners and homeowners can contribute to these efforts by following this advice:

 

Choose trees for the long run. Not all trees are equally effective at storing carbon. Trees that grow larger will store more carbon over their lifetimes than smaller trees, and faster-growing trees accumulate carbon faster. Opt for native, long-lived, low-maintenance, moderate- to fast-growing species that are large at maturity, such as sweetgum (Liquidambar styraciflua), tulip poplar (Liriodendron tulipifera), or white oak (Quercus alba).

 

Ensure trees stay healthy. Living trees and intact wood can store carbon for a long time, but dead trees and rotting wood release much of their accumulated carbon back into the atmosphere. Therefore, start with longer-lived trees, which hold their carbon longer,36 and native species, which are well adapted to local conditions. Think twice about those that would be planted at the southern end of their current hardiness zone, as they may not adapt well to the warmer temperatures resulting from climate change. Finally, don’t neglect trees after they are planted; although regular watering and pruning can produce CO2 (see the section on lawns for more about water use), it’s important to help young trees become established to increase their likelihood of a long life. Minimize fossil fuels in tree maintenance. Tree-care equipment powered by fossil fuels (chainsaws, leaf blowers, trucks used by tree-care crews) release heat-trapping gases and other pollutants, offsetting the climate benefits provided by trees. Opt for low-carbon methods such as hand pruning where feasible and safe.

 

Location, location, location. Plant trees in strategic energy-saving locations, such as on the south side of your home in warm climates, or where evergreens will provide a break from prevailing winter winds in colder climates.

 

More information about tree care and the benefits of planting trees is available on the Arbor Day Foundation website http://www.arborday.org

 


How to Use Cover Crops in Your Garden

Farmers have long planted cover crops to promote soil fertility and control erosion. Now, many state and local cooperative extension services recommend cover crops to home gardeners as well.27,28,29 Planting the seeds and turning the cover crops under before planting your flowers or vegetables takes extra time and effort, but you may find the effort pays off in the form of a more productive garden (not to mention the added climate benefits). A few tips:

• Choose cover crops that will germinate and become established quickly in your local climate, will provide enough shade to prevent weed growth, and will be easy to work into the soil in the spring. Combining a legume with a grass or cereal plant (vetch with rye or oats, for example, or Austrian peas or garden peas with winter wheat or rye) is a good strategy in many parts of the country.30

• Winter cover crops should be sown in late summer or early fall, after vegetables have been harvested or summer flowers have faded but temperatures are still warm enough for the seeds to germinate and become established. In some cases, sowing shortly before harvest (between rows of growing plants) does not interfere with the harvest, and gives the cover crop more time to become established.

• You can also grow cover crops at other times of year in any unused portion of your garden (as part of a crop rotation system that will help reinvigorate the soil and retain soil nitrogen).

• About three weeks to a month before you plant your garden for the next season, turn the cover crop under if the soil is not too wet.31 Or cut it to the ground and leave it as mulch.

When planted in the fall and turned under in spring, cover crops such as Austrian winter peas, or field peas, suppress weeds, prevent erosion, improve soil fertility, and store carbon.


The Best Recipe for Compost

If you don’t have municipal composting in your area, find out whether any local regulations affect your ability to have a compost pile on private property. Then follow these basic guidelines, while taking steps to avoid attracting pests:

Balance carbon and nitrogen. The most efficient composting process provides microorganisms with the correct ratio of carbon- and nitrogen-containing materials (carbon serves as the microbes’ energy source and nitrogen supplies the building blocks for proteins). A carbon-to-nitrogen ratio ranging between 25:1 and 30:1 is optimal. Remember that carbon-rich materials tend to be dry and brown (leaves, straw, wood chips), while nitrogen-rich materials are usually wet and green (fresh grass clippings, food waste).

Let it breathe. Turning or mixing your compost regularly exposes the decomposing materials to oxygen, which makes the process more efficient and minimizes the production of methane (a potent heat-trapping gas). It also helps the compost heat up to a temperature that kills weed seeds and disease-causing organisms.

Close the loop. Compost is finished when it looks dark and crumbly and smells pleasantly earthy. Use it in your garden as a substitute for synthetic fertilizers and peat-based potting or seed-starting mixes; master gardeners recommend anywhere from one-quarter of an inch to three inches of compost per year.49 It also makes excellent mulch.

To read more about the science of composting and tips for choosing a system appropriate for your yard, visit the University of Illinois Extension website (http://web.extension.illinois.edu/homecompost/intro.html).

Turning garden and kitchen waste into compost reduces heat-trapping methane emissions from landfills, improves your garden’s soil, and helps it store carbon.

 

The Climate-Friendly Gardener

Careful attention to the world’s soils is one piece of solving the climate problem. Agriculture and forestry, which dominate managed soils worldwide, clearly have the largest roles to play. However, as cities expand in the United States and elsewhere, there is a growing opportunity for urban and suburban areas to play a part. Recent studies suggest that urban green spaces (lawns, gardens, parks, golf courses, and trees planted along streets) have the potential to capture CO2 and "store" that carbon over time (see the box to the left).3,4 Practices that maximize carbon storage without generating too much global warming pollution in the process (such as crop rotation, cover crops, tree planting, and "low-input" lawn and garden maintenance) can help slow the pace of climate change. As a gardener, you can lead the way in your own yard.

 

Five Steps to a Climate- Friendly Garden

A garden can be thought of as climate-friendly if it stores (or prevents the release of) more heat-trapping gases than it generates. While the movement of carbon and other global warming pollutants in the garden is highly complex and challenging to measure, researchers in agriculture, climate and soil sciences, forestry, and urban ecology have identified a number of practices likely to move your garden in the right direction:

 

Step 1. Minimize Carbon-Emitting Inputs

The kinds of tools and chemicals you use in the garden—what scientists call "inputs"—can affect the amount of heat-trapping gases your garden absorbs or emits. Gasoline-powered tools such as lawn mowers and leaf blowers are obvious sources of CO2. But common garden chemicals can also contribute to global warming.

 

Synthetic fertilizers—especially nitrogen-based ones—require a lot of energy to manufacture, which generates a significant amount of CO2.14 Even organic nitrogen-based fertilizers (compost, animal manure) can create additional heat-trapping gases, especially if the timing and amount of fertilizer are not precise.15 On U.S. farms, crops absorb less than half of the nitrogen fertilizer applied. Some of the excess runs off into rivers and other bodies of water, some seeps into groundwater, and some is converted into nitrous oxide—a heat-trapping gas 320 times more potent than CO2.16, 17 Similarly, studies have suggested that fertilizer overuse on home gardens and lawns—especially well-watered lawns—can be a source of nitrous oxide.18, 19

 

Pesticides (which include insecticides, herbicides, and fungicides) also contribute to global warming. Like synthetic fertilizers, pesticides require a lot of energy to manufacture, package, and transport. A recent review of farm operations indicates that the production of herbicides (weed killers, the fastest-growing class of pesticides used in the garden20) generates more carbon emissions than other kinds of pesticides. And glyphosate—the active ingredient in the widely used herbicide Roundup—is among the most carbon-intensive.21

 

Step 2. Don’t Leave Garden Soil Naked

Whether you grow summer vegetables, annual flowers, or perennial borders, there are periods of time—perhaps as much as half the year, depending on your local climate—when plants are not actively growing. During these times, bare soil is vulnerable not only to erosion and weeds but carbon loss as well.

 

The use of cover crops—grasses, cereal grains, or legumes that can be grown when other plants cannot—is a mainstay of organic farming systems because it helps develop healthy and productive soil, reduce the need for energy-intensive chemical fertilizers and pesticides, and store large amounts of carbon. Cover crops are not meant to be harvested but to stabilize, build, and add nutrients to soil that would otherwise remain bare.

 

Cover crops have been described as a "winter blanket" for soils because they are often planted in fall.22They suppress weeds, buffer the soil from rain and wind (reducing erosion and water runoff), and increase the soil’s water-holding properties (improving the ability of crops to withstand drought). But most importantly for the climate-friendly gardener, when cover crops are turned under in the spring, their organic matter improves the soil, which will store carbon for years while providing nutrients for subsequent plantings and a variety of beneficial organisms.

 

Peas, beans, clovers, and other legumes grown as cover crops may supply most or all of the nitrogen needed for the next season’s plants to thrive, substantially reducing gardeners’ need for synthetic nitrogen fertilizers. Legumes do this by partnering with common soil bacteria to "fix" nitrogen (that is, to convert it from the abundant but unusable form found in the atmosphere into forms that plants can use). During their growing season, legumes incorporate fixed nitrogen into proteins and other large molecules in their tissues;23when this plant matter is returned to the soil, it is broken down by microbes, releasing the fixed nitrogen for use by the next crop.

 

Non-legume cover crops such as rye and winter wheat are also useful. When planted at the correct time, they can capture excess nitrogen from fertilizers or decaying plant matter; several studies showed these cover crops reduced nitrogen losses into groundwater by about 70 percent.24 By holding nitrogen in their tissues, they also prevent soil microbes from converting it into heat-trapping nitrous oxide. And non-legume cover crops typically grow larger and faster compared with legumes, absorbing more CO2 from the atmosphere and returning more organic matter to the soil.

Step 3. Plant Trees and Shrubs

If you have space in your yard, planting and maintaining one or more trees or large shrubs is an excellent way to remove more heat-trapping CO2 from the atmosphere over a long period of time. All plants absorb CO2 through their leaves, storing the carbon in their tissues. But trees and shrubs, because they are large, woody, and long-lived, can store larger quantities of carbon than other plants, for longer periods of time.

 

A recent multi-city study estimated that, as a whole, the urban trees of the contiguous United States accumulate nearly 23 million tons of carbon in their tissues per year.32  That’s more than all of the homes, cars, and industries in Los Angeles County emit each year, or about as much as all of the homes in Illinois or Pennsylvania, or all of the power plants in Massachusetts.33

 

While all trees store carbon, urban and suburban trees can also prevent CO2 emissions because they help reduce energy use in nearby homes and buildings. Well-placed trees can shade buildings from the summer sun or buffer them from cold winter winds, reducing the need for air conditioning and heating. And through evapo-transpiration (the movement of water from the soil, through plant leaves, and into the air), urban and suburban trees can lower an entire neighborhood’s temperature during the summer, further reducing the need for air conditioning.

 

Most supermarket produce travels more than 1,500 miles to get to your dinner plate. Growing food in your garden eliminates some of that transportation and its heat-trapping emissions.

 

Thus, one study found that in addition to storing between 10 and 24 pounds of carbon annually, a single shade tree in Los Angeles helps residents and businesses save enough energy to avoid the release of nearly 40 pounds of carbon from power plants each year.37 Another study from California estimated that urban trees offset Sacramento County’s CO2 emissions by nearly 2percent.38

 

Energy savings also translate into cost savings. A recent study of three U.S. cities, for example, found that planting an average of four shade trees per house would reduce annual citywide energy expenditures by $1.5 million in Salt Lake City, UT, more than $6 million in Baton Rouge, LA, and nearly $13 million in Sacramento, CA.39

 

Finally, in addition to their energy and climate benefits, urban trees in the contiguous United States remove an estimated 711,000 tons of toxic pollutants (such as sulfur dioxide and ground-level ozone, or smog) from the air each year.40

 

Step 4. Expand Recycling to the Garden

According to the U.S. Environmental Protection Agency, yard trimmings and food waste together make up about one-quarter of the country’s municipal solid waste.41 When these organic wastes are disposed of in landfills, much of the carbon is stored under ground, but the waste that breaks down releases methane—a heat-trapping gas 23times more potent than CO2. Worldwide, methane produced in landfills and other solid waste disposal sites represent about 3 to 4 percent of all human-caused heat-trapping gases.42

 

Gardeners can help reduce the global warming pollution associated with waste disposal by turning leaves, grass, woody garden clippings, dead garden plants, and kitchen waste into mulch or compost, then using it in the garden. "Recycling" these wastes will not only reduce methane emissions from landfills but also improve your garden’s soil and help it store carbon.

Compost, which can be any mixture of decaying organic materials (for example, leaves, animal manure, food scraps), is created by a natural process in which bacteria, fungi, and other organisms break down wastes into a nutrient-rich soil amendment. While composting does produce global warming gases, studies indicate that the best practices for creating and using compost have a smaller climate impact than landfills.43,44,45

 

That’s because efficient composting takes place aerobically (that is, in the presence of oxygen), which minimizes the formation of methane. By contrast, landfills lack oxygen circulation, so organic materials are broken down primarily by bacteria that thrive in the absence of oxygen and produce methane. One study suggests composting is also better than incinerators for reducing heat-trapping emissions from organic waste disposal.46

 

Some localities, including the cities of San Francisco and Seattle and Minnesota’s Western Lake Superior Sanitary District, collect yard waste, kitchen scraps, or both for centralized composting, which encourages residents to participate.47 Transporting wastes to centralized facilities produces CO2 that could be avoided by composting at home, but these emissions are small relative to those averted in the process. And because conditions at centralized facilities are optimal, they may come closer to eliminating methane emissions than the average home composting pile or bin.48

 

Step 5. Think Long and Hard about Your Lawn

About 80 percent of all U.S. households have access to a private lawn,50 and the total area of the contiguous United States covered with turf grass (including home lawns, parks, golf courses, and athletic fields) is estimated at more than 40 million acres—three times the area devoted to our irrigated corn crop.51 Can lawns be made climate-friendly?

 

A growing body of research indicates that soils covered in turf grasses can capture and store significant amounts of carbon. On the other hand, a small number of newer studies have shown that lawns have the potential to generate heat-trapping nitrous oxide. Some have found these emissions to be significant; 52,53,54 others have not.55 Although the science is unsettled, it appears that practices designed to maximize lawn growth and health with minimal inputs of fertilizer and water may achieve the best balance of carbon storage and nitrous oxide emissions.

 

A lawn is made up of many thousands of individual grass plants, each of which absorbs CO2 from the air and converts it into carbohydrates through photosyn-thesis. Grass undergoes a seasonal cycle of rapid growth periods (spring and fall for cool-season grasses, summer for warm-season grasses) and flowering (which may not occur if flowering stalks are frequently mowed). Throughout this cycle, lawn grasses continuously shed root material,56 depositing carbon in the soil. Grass clippings left on the lawn to decompose after mowing are another source of carbon that can be stored in the soil below.

 

Unlike gardens, which are often tilled and replanted each year, lawns are typically left undisturbed, enabling the soil to store carbon for long periods of time.

 

Rates of carbon storage vary depending on the climate, soil type, rainfall, and how the grasses are managed. A study of 15 golf courses documented rapid rates of carbon storage during the first 25 to 30 years of high-input turf management,63 and other studies have demonstrated that home lawns can also capture carbon.64,65,66 Well-watered and fertilized lawns store the most carbon—especially those with an ample supply of nitrogen.67 However, nitrous oxide emissions from lawns have been linked to the application of nitrogen fertilizer and generous watering.68,69, 70

 

Today the Backyard, Tomorrow the Nation

Gardening practices alone won’t solve global warming. As with switching to more efficient compact fluorescent light bulbs or reusable shopping bags, cultivating a climate-friendly garden or lawn is just a small piece of the puzzle. But together, the 81 million U.S. households that own a small piece of the outdoors can play an important part in the fight against climate change by storing carbon in their soil and trees, and reducing heat-trapping emissions from pesticides, fertilizers, and garden waste and equipment.

 

Moreover, home gardeners can point the way to climate-friendly practices on a much larger scale: our nation’s farms. With their 900 million acres of intensively managed soil, U.S. farms and ranches have an opportunity to play a truly significant role in curbing global warming. Many have already adopted practices such as cover cropping, crop rotation, and low-input agriculture, and the Union of Concerned Scientists is working to ensure that future farm policies encourage many more to do so.

 

The federal food and farm bill passed by Congress every five years represents the best vehicle to make climate-friendly practices the norm rather than the exception in American agriculture. For that to happen, climate-savvy gardeners and others will need to weigh in on the debate.

 

By adopting the same climate-friendly practices described here, farms can play a significant role in curbing global warming. Many have already moved in this direction, but new farm policies are needed to encourage many more to follow suit.

 

 


 

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