Climate-friendly gardening refers to gardening practices that aim to reduce the release of greenhouse gases like carbon dioxide, methane and nitrous oxide in order to aid the reduction of global warming. These practices can include soil management, rain capture, cultivating native plants, and utilizing biochar, nitrogen-fixing plants and compost in order to eliminate the use of chemical fertilizers. Avoiding actions like burning garden waste, excessive digging and the utilization of gas-powered tools may also help to lower greenhouse gas emissions. Techniques such as planting trees and ground cover, utilizing mulch and limiting soil disturbances can aid in soil health and the sequestering of carbon in the soil.
Land use and greenhouse gases
The burning of fossil fuels is the main source of the excess greenhouse gases causing climate change, but there are other sources to consider as well. A special report from the Intergovernmental Panel on Climate Change (IPCC) estimated that, in the last 150 years, fossil fuels and cement production have been responsible for only about two-thirds of climate change while the other third has been caused by human land use.
The three main greenhouse gases produced by unsustainable land use are carbon dioxide, methane, and nitrous oxide. Black carbon, or soot, can also be a product of unsustainable land use, and, despite not being a gas, it can behave like greenhouse gases and contribute to climate change.
Carbon dioxide
Carbon dioxide, , is a natural part of the carbon cycle, but human land uses often creates excess amounts, especially from habitat destruction and the cultivation of soil. When woodlands, wetlands, and other natural habitats are turned into pasture, arable fields, buildings and roads, the carbon held in the soil and vegetation becomes extra carbon dioxide and methane to extract more heat in the atmosphere.
Gardeners may cause extra carbon dioxide to be added to the atmosphere in several ways:
Using peat or potting compost containing peat; Buying garden furniture or other wooden products made from woodland which has been destroyed rather than taken as a renewable crop from sustainably managed woodland; Digging the soil and leaving it bare so that the carbon in soil organic matter is oxidised; Using power tools which burn fossil fuel or electricity generated by burning fossil fuel; Using patio heaters; Heating greenhouses by burning fossil fuel or electricity generated by burning fossil fuel; Burning garden prunings and weeds on a bonfire, though pyrolysis of wood turns 35% of its carbon (which would otherwise decompose to ) into biochar, which remains stable in the soil for thousands of years; Buying tools, pesticides, synthetic nitrogen fertilizers (over 2 kilograms of carbon dioxide equivalent is produced in the manufacture of each kilogram of ammonium nitrate), and other materials which have been manufactured using fossil fuel; Heating and treating swimming pools by burning fossil fuel or electricity generated by burning fossil fuel; Watering their gardens with tapwater, which has been treated and pumped by burning fossil fuel, with a greenhouse gas impact of about 1 kg CO2e/m3 water. Buying garden products which have been transported by vehicles powered by fossil fuel.
Methane Methane, CH4, is a natural part of the carbon cycle, but human land uses often add more, especially from anaerobic soil, artificial wetlands such as rice fields, and from the guts of farm animals, especially ruminants such as cattle and sheep.
Gardeners may cause extra methane to be added to the atmosphere in several ways:
Compacting soil so that it becomes anaerobic, for example by treading on soil when it is wet; Allowing compost heaps to become compacted and anaerobic; Creating homemade liquid feed by putting the leaves of plants such as comfrey under water, with the unintended consequence that the plants may release methane as they decay; Killing pernicious weeds by covering them with water, with the unintended consequence that the plants may release methane as they decay; Allowing ponds to become anaerobic, for example, by adding unsuitable fish species which stir up sediment that then blocks light from and kills submerged oxygenating plants.
Nitrous oxide
Nitrous oxide, N2O, is a natural part of the nitrogen cycle, but human land uses often add more.
Gardeners may cause extra nitrous oxide to be added to the atmosphere by:
Using synthetic nitrogen fertilizer, for example "weed and feed" on lawns, especially if it is applied when plants are not actively growing, the soil is compacted, or when other factors are limiting so that the plants cannot make use of the nitrogen; Compacting the soil, such as by working in the garden when the soil is wet, which will increase the conversion of nitrates to nitrous oxide by soil bacteria; Burning garden waste on bonfires.
Black carbon
Black carbon is not a gas, but it acts like a greenhouse gas because it can be suspended in the atmosphere and absorb heat.
Gardeners may cause extra black carbon to be added to the atmosphere by burning garden prunings and weeds on bonfires, especially if the waste is wet and becomes black carbon in the form of soot. Gardeners are also responsible for extra black carbon produced when they buy garden products which have been transported by vehicles powered by fossil fuel especially the diesel used in most lorries.
Gardening to reduce greenhouse gas emissions and absorb carbon dioxide
There are many ways in which climate-friendly gardeners may reduce their contribution to climate change and help their gardens absorb carbon dioxide from the atmosphere.
Climate-friendly gardeners can find good ideas in many other sustainable approaches: Agroforestry; Forest gardening; Orchards; Organic gardening; Permaculture; Rain garden; Vegan organic gardening; Water-wise gardening; Wildlife garden; Biochar.
Protecting and enhancing carbon stores
Protecting carbon stores in land beyond gardens
Climate-friendly gardening includes actions which protect carbon stores beyond gardens. The biggest carbon stores in land are in soil; the two habitat types with the biggest carbon stores per hectare are woods and wetlands; and woods absorb more carbon dioxide per hectare per year than most other habitats. Climate-friendly gardeners therefore aim to ensure that nothing they do will harm these habitats.
According to Morison and Morecroft (eds.)'s Plant Growth and Climate Change, the net primary productivity (the net amount of carbon absorbed each year) of various habitats is:
Tropical forests: 12.5 tonnes of carbon per hectare per year; Temperate forests: 7.7 tonnes of carbon per hectare per year; Temperate grasslands: 3.7 tonnes of carbon per hectare per year; Croplands: 3.1 tonnes of carbon per hectare per year.
The Intergovernmental Panel on Climate Change's Special Report Land use, land-use change, and forestry lists the carbon contained in different global habitats as:
Wetlands: 643 tonnes carbon per hectare in soil + 43 tonnes carbon per hectare in vegetation = total 686 tonnes carbon per hectare; Tropical forests: 123 tonnes carbon per hectare in soil + 120 tonnes carbon per hectare in vegetation = total 243 tonnes carbon per hectare; Temperate forests: 96 tonnes carbon per hectare in soil + 57 tonnes carbon per hectare in vegetation = total 153 tonnes carbon per hectare; Temperate grasslands: 164 tonnes carbon per hectare in soil + 7 tonnes carbon per hectare in vegetation = total 171 tonnes carbon per hectare; Croplands: 80 tonnes carbon per hectare in soil + 2 tonnes carbon per hectare in vegetation = total 82 tonnes carbon per hectare.
The figures quoted above are global averages. More recent research in 2009 has found that the habitat with the world's highest known total carbon density - 1,867 tonnes of carbon per hectare - is temperate moist forest of Eucalyptus regnans in the Central Highlands of south-east Australia; and, in general, that temperate forests contain more carbon than either boreal forests or tropical forests.
Carbon stores in Britain According to Milne and Brown's 1997 paper "Carbon in the vegetation and soils of Great Britain", Britain's vegetation and soil are estimated to contain 9952 million tonnes of carbon, of which almost all is in the soil, and most in Scottish peatland soil:
Soils in Scotland: 6948 million tonnes carbon; Soils in England and Wales: 2890 million tonnes carbon; Vegetation in British woods and plantations (which cover only 11% of Britain's land area): 91 million tonnes carbon; Other vegetation: 23 million tonnes carbon.
A 2005 report suggested that British woodland soil may contain as much as 250 tonnes of carbon per hectare.
Many studies of soil carbon only study the carbon in the top 30 centimetres, but soil is often much deeper than that, especially below woodland. One 2009 study of the United Kingdom's carbon stores by Keith Dyson and others gives figures for soil carbon down to 100 cm below the habitats, including "Forestland", "Cropland" and "Grassland", covered by the Kyoto Protocol reporting requirements.
Forestland soils: average figures in tonnes carbon per hectare are 160 (England), 428 (Scotland), 203 (Wales), and 366 (Northern Ireland). Grassland soils: average figures in tonnes carbon per hectare are 148 (England), 386 (Scotland), 171 (Wales), and 304 (Northern Ireland). Cropland soils: average figures in tonnes carbon per hectare are 110 (England), 159 (Scotland), 108 (Wales), and 222 (Northern Ireland).
Protecting carbon stores in wetland
Climate-friendly gardeners choose peat-free composts because some of the planet's biggest carbon stores are in soil, and especially in the peatland soil of wetlands.
The Intergovernmental Panel on Climate Change's Special Report Land Use, Land-Use Change and Forestry gives a figure of 2011 gigatonnes of carbon for global carbon stocks in the top 1 metre of soils, much more than the carbon stores in the vegetation or the atmosphere.
Climate-friendly gardeners also avoid using tapwater not only because of the greenhouse gases emitted when fossil fuels are burnt to treat and pump water, but because if water is taken from wetlands, the carbon stores are more likely to be oxidized to carbon dioxide.
A climate-friendly garden therefore does not contain large irrigated lawns, but instead includes water-butts to collect rainwater, water-thrifty plants which survive on rainwater and do not need watering after they are established, trees, shrubs and hedges to shelter gardens from the drying effects of sun and wind, and groundcover plants and organic mulch to protect the soil and keep it moist.p. 242p. 80–82
Climate-friendly gardeners will ensure that any paved surfaces in their gardens (which are kept to a minimum to increase carbon stores) are permeable, and may also make rain gardens, sunken areas into which rainwater from buildings and paving is directed, so that the rain can then be fed back into groundwater rather than going into storm drains. The plants in rain gardens must be able to grow in both dry and wet soils.
Protecting carbon stores in woodland Wetlands may store the most carbon in their soils, but woods store more carbon in their living biomass than any other type of vegetation, and their soils store the most carbon after wetlands. Climate-friendly gardeners therefore ensure that any wooden products they buy, such as garden furniture, have been made of wood from sustainably managed woodland.
Protecting and increasing carbon stores in gardens
After rocks containing carbonate compounds, soil is known to be the biggest store of carbon on land. Carbon is found in soil organic matter, including living organisms (plant roots, fungi, animals, protists, bacteria), dead organisms, and humus.