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.”
If you liked this story, sign up for the weekly bbc.com features newsletter, called “The Essential List” – a handpicked selection of stories from BBC Future, Culture, Worklife, Travel and Reel delivered to your inbox every Friday.