Original publication by Isabelle Gerretsen for bbc.com on 13 June 2022
From patches of wilderness to decomposing plants, turning your garden into a carbon sink isn’t just about adding lots of trees.
During World War Two, the UK ministry of agriculture encouraged gardeners to “Dig for Victory” and grow their own vegetables to help feed the country. Allotments sprung up in private gardens and public parks – even the lawns outside the Tower of London were transformed into vegetable patches.
Almost 100 years later, the “Dig for Victory” slogan has been repurposed by the UK’s Royal Horticultural Society (RHS). The gardening charity aimed to mobilise the biggest gardening army since World War Two to fight the biggest threat of the 21st Century: climate change. The tools at their disposal? Planting trees, using rainwater instead of sprinklers, and making compost.
If every one of the UK’s 30 million gardeners planted one medium-sized tree and let it grow to maturity, they would store the same amount of carbon as is produced by driving 284 billion miles (457 billion km), 11 million times around the planet, research by the RHS shows. If every gardener produced 190kg of compost each year, they would save the amount of carbon produced by heating half a million homes for a year.
As governments and companies race to slash their emissions, there is increasing interest in the ability of natural landscapes, such as forests, wetlands and mangroves, to protect against the risks posed by climate change. Horticulturalists say the humble garden can also serve as a powerful tool in this fight.
“Gardens are becoming shop windows for the wider environment, demonstrating the dangers of pests and threats of climate change and showing what can be done to tackle it,” says Simon Toomer, curator of living collections at Kew Gardens in the UK.
To cope with climate change, gardens must become more resilient to hotter and drier conditions in the summer and more rainfall in the winter, the RHS warns.
The ideal low-carbon garden has a wildness to it. It is packed with plants and teeming with life. The gardener in this sustainable haven is equally mindful of nurturing life below the ground as she is of tending to her flower displays and shrubs. She recycles every grass clipping, fallen leaf and broken twig within the garden and avoids toxic chemicals to boost plant growth, relying instead on home-made compost and living mulch to create a thriving habitat.
“In the past everyone wanted a pristine lawn, but now there’s a big movement in gardening for more natural landscapes which is really quite exciting,” says Justin Moat, senior research leader on Kew Gardens’ Nature Unlocked programme, which explores nature-based solutions to climate change and food security.
21ST CENTURY GARDENING
From working with contaminated city soil to reconsidering weeds,
pests and even lawns,
gardening is changing as we adapt it to the realities of modern life. This series takes a look at the future of gardens in the 21st Century – and explores how it can be updated to fit with modern sensibilities and challenges, such as environmental awareness and pollution.
“We need to put up with scruffy lawns,” says Moat. This may be wishful thinking, as BBC Future revealed recently: we appear addicted to manicured lawns (read more about their strange appeal and the people who think we should get rid of them).
In the UK, gardeners were recently encouraged to let nature take its course during “No Mow May”. Environmentalists say if left alone, lawns could become thriving wildlife hotspots. Given that an estimated 23% of urban land is covered by lawns, there is great potential for them to help fight the global biodiversity crisis.
Leaving the lawn mower in the shed also benefits the climate. One of the most important things gardeners can do in the short-term is reduce their energy consumption, from lawn mowers and sprinklers, says Toomer.
Operating a petrol lawn mower for one hour releases as much smog-forming pollution as driving for 160km (100 miles), says the California Air Resources Board (CARB).
Sally Nex, a professional gardener and author of the book How to Garden the Low Carbon Way, switched her petrol mower for a battery-powered one years ago after learning how many toxic fumes it spews out.
“There’s no regulation on the maximum emissions for petrol powered tools – it’s really shocking,” says
Other gardening tools are just as polluting as mowers. Using a petrol-powered leaf blower produces the same amount of emissions as a 1,770km (1,100 mile) car journey – the distance from Los Angeles to Denver – according to CARB.
Moat says the Nature Unlocked programme has highlighted the “phenomenal” power soil has to transform our gardens into biodiverse havens that can help mitigate climate change.
“So much more is happening underground than above it,” he says. “We need healthy soil for our food production and we need it to trap carbon.”
Replenishing and restoring the world’s soils – both in farming and natural landscapes – could help remove up to 5.5 billion tonnes of CO2e every year, according to a 2020 study. That is equivalent to the annual greenhouse emissions of the US, the world’s second largest polluter, in 2020.
Healthy soil offsets emissions by soaking up carbon from dead plant matter. To lock in as much carbon as possible, soil needs a good balance of water, pockets of air, living organisms, such as fungi, and nutrients. Gardeners maintain this balance by constantly adding organic material to their soil.
“I compare it to a carbon checking and savings account,” says Andrea Basche, assistant professor at the department of agronomy and horticulture at University of Nebraska. “You need a constant input of decaying plant matter and roots into the soil checking account to feed all the living organisms.”
Gardeners shouldn’t press the soil down too much or use heavy equipment when it’s wet as this will cause it to become compacted, closing vital air pockets and preventing water from draining, says Mark Gush, head of environmental horticulture at the RHS.
If left bare and exposed to the elements, soil will degrade and its carbon stocks will deplete. Covering the bare soil with plants, such as clover, and mulches – loose coverings of biodegradeable materials – is therefore key to prevent CO2 from seeping into the atmosphere, Gush says.
A recent study by Penn State University found that cover crops were more effective at protecting corn and soybeans from pests than applying pesticides.
Mulching has transformed Nex’s garden. “When I stopped digging and started mulching, I realised my topsoil was getting deeper and deeper,” she says. “The soil is black and teeming with life – it’s very rewarding.”
Mulching also suppresses weeds, helps soil retain moisture and protects plant roots from extreme temperatures.
Fallen leaves and broken twigs don’t need to be removed from flower beds but can be treated as “living mulches”, which are contributing vital nutrients to the soil. “Essentially leave any organic matter to feed into the soil,” says Toam.
Living mulches can also reduce gardeners’ reliance on nitrogen fertilisers, many of which have a high carbon footprint. Basche says farmers in Nebraska are having to use less fertiliser on their crops after growing a cover crop and using living mulches for several years. Legumes, such as beans and peas, act as a green manure by adding valuable nitrogen – vital for plant growth – to the soil when they decompose. Introducing a legume crop for one year at a cereal farm in Scotland could reduce the amount of nitrogen fertiliser needed over the entire five-year cycle by almost 50%, according to a 2021 study.
An easy way to enrich your soil is by adding homemade compost. Healthy compost should contain a 50:50 mix of materials that are rich in nitrogen, such as grass clippings and vegetable peels, and carbon, such as woody stems and paper towels.
Composting also allows you to discard any leftover food in a sustainable way. When dumped into landfill without oxygen, food waste rots and releases methane, a highly potent greenhouse gas which, although shorter-lived in the atmosphere, has a global warming impact 84 times higher than carbon dioxide (CO2) over a 20-year period.
But on a compost heap, exposed to oxygen, organic waste is converted into stable soil carbon, while retaining the water and nutrients of the original matter. Food which is composted releases just 14% the greenhouse gases of food that is thrown away.
“I dispose of all my garden waste, vegetables and peelings in the garden. Every time I harvest vegetables or prune roses, I’m removing carbon from the garden, so it’s important to return that carbon to the soil,” says Nex.
Compost heaps must be turned regularly – the RHS recommends once a month – to add air to the biomass and keep it moist. Garden compost can take up to two years to reach maturity, when it turns a dark brown colour, has a crumbly texture and smells like damp woodland.
If you plan on buying compost, avoid one containing peat, says Gush. Peatlands cover just 3% of the planet’s surface, but store twice as much carbon as all the world’s forests. They lock in carbon over thousands of years, with 1cm of peat forming roughly every 10 years.
“Peat bogs are very important sinks, they have accumulated carbon over millennia,” says Gush. “As soon as they are drained and the peat is exposed to the air, carbon is unlocked and released back into the atmosphere.”
The UK government said last year it plans to ban the sale of peat compost to gardeners by 2024, but critics warn that the two-year delay will add more than 1.5 million tonnes of CO2 to the atmosphere – the equivalent of the annual emissions of 214,000 UK residents.
WHAT IS CO2 EQUIVALENT?
CO2 equivalent is the metric measure used to compare the emissions from various greenhouse gases on the basis of their capacity to warm the atmosphere – their global warming potential.
While some gardeners might desire a uniform look for their flower beds and lawns, growing a wide range of plants is beneficial if you are looking to transform your garden into a miniature carbon sink.
Plant diversity has been shown to increase productivity and the amount of carbon stored in the soil. “Increased plant diversity boosts carbon sequestration by optimising use of available space in a garden, both above-ground and below-ground,” says Gush.
It’s important to grow layer plants in your garden and grow crops with roots that will reach different depths so that they can penetrate all parts of the soil and spread nutrients around. “This facilitates maximum carbon drawdown,” says Gush.
For those on a mission to transform their gardens into a carbon sink, growing long-lived trees seems like the most obvious option. To make your garden climate-resilient, the RHS recommends planting a mix of drought-tolerant trees, such as snow gum and holm oak, and ones that can withstand waterlogging, such as red maple and golden willow.
But trees are far from the only plants that can help offset your garden’s carbon footprint. Native grasses have extensive root systems – reaching more than 2ft into the ground – and act as reservoirs for carbon, which transfers into the soil when the roots die and decompose.
Woody shrubs, such as spindle and sweet briar and herbs like rosemary and thyme, can help boost your garden’s carbon stocks, Nex recommends in her book.
If you’re set on sprucing up your garden with colourful crops, it’s best to steer clear of annual flowers which need to be dug up every year – releasing locked-in carbon in the process – and opt for hardy perennials instead, such as peonies and sunflowers, says Nex.
Planting hedges is another worthwhile investment. A well-grown hedge, rich in biomass, helps suck carbon out of the atmosphere and into plants and soil. One study found that hedgerows store similar amounts of carbon to woodland. Hedges also harbour rich biodiversity and are teeming with wildlife. A British ecologist who monitored an old hedgerow near his home in Devon counted a remarkable 2,070 species, ranging from pollinators to lizards and mammals, visiting or residing there.
Ponds may also play an important role in gardens’ fight against climate change. One study of small, lowland ponds in north-east England found that they stored much higher rates of carbon (79 to 247g per square metre per year) compared to surrounding woodland or grassland (2-5g).
However, not all ponds act as carbon sinks. A US study found that man-made ponds collecting stormwater run-off in Florida emit more carbon than they store in their mucky sediment.
“That finding means some ponds are doing us an ecosystem ‘disservice,'” Mary Lusk, the study’s co-author and assistant professor of water and soil sciences at the University of Florida, said when the study was published. “Our results suggest that when they’re new, [the ponds] emit large proportions of carbon from the landscape.”
Ponds can also emit large amounts of potent methane into the atmosphere. One study by the University of Exeter concluded that ponds smaller than one square metre are responsible for releasing around 40% of all methane emissions from inland waters.
However, not all environmental benefits are about carbon – and ponds come with many other advantages, such as boosting biodiversity. In fact, some charities say that adding a pond to your garden is one of the best things you can do for wildlife (more on this later in the series).
“If you are disturbing the sludge at the bottom of the pond, your pond will release more methane than it will absorb carbon,” says Nex. To keep the noxious gas contained, Nex recommends removing dead foliage from your pond surface as rotting debris will give off methane and netting it in the autumn.
Gardeners who adopt low-carbon practices will be rewarded with thriving biodiversity and borders brimming with lush plants.
“My plants now grow so much better. It’s very flattering to me as I’m not doing very much!” says Nex. “It has really improved the appearance of my garden – it’s quite breath-taking actually.”