Text by Brian Barth for the January/February 2019 issue of Horticulture.
When people think of solutions to climate change, solar panels, electric vehicles and shuttered coal plants are often the images that come to mind. But these all have to do with reducing emissions. The other side of the climate-change equation involves removing the greenhouse gases that are already in the atmosphere—what is commonly called carbon sequestration. This is where your garden comes in.
Many folks know that forests help to sequester carbon, but it’s hard to imagine that a mere few trees in an urban yard will do much good. Yet if you add up all the trees in all the yards on the planet, they amount to an enormous forest.
The assumption has long been that wild landscapes — undisturbed, fully functioning ecosystems — are the real stars of carbon sequestration. But a study published last spring by a University of Wisconsin PhD candidate found that soils in the urban yards of Madison actually stored more carbon than nearby forests and fields. The precise reasons for that remain unclear at this point, as scientists continue to refine their knowledge of how carbon cycles through, and is stored in, different ecosystems, and how that’s impacted by different land-management strategies.
But there is a robust body of research that points to best practices for climate-friendly cultivation. “Carbon farmers” increasingly employ these techniques on rural acreage, as they aspire to raise crops and livestock while helping to avert planetary disaster. You can also put these practices to use in your garden, as carbon-farming expert Eric Toensmeier has. He lives not on a farm but on a tenth-of-an-acre lot in Holyoke, Mass., and he claims that his garden sequesters enough carbon to offset the average annual emissions attributed to one adult American.
Based on the available science to date, here is what Toensmeier and other climate-friendly gardening gurus believe to be most helpful in the fight against climate change:
To start with, it’s worth noting that your yard, like all landscapes, both stores and emits carbon as a part of natural processes. Thus, the goal is to store more and emit less. One common gardening activity that causes the soil to release carbon is tillage — that’s right, every time you break open the earth, whether with a rototiller or a digging fork, greenhouse gases are going up into the atmosphere.
focus on perennials
Eliminating tillage, of course, means eliminating most annual flowers and vegetables that fail to thrive if they don’t have a fluffy bed of soil in which to reside. This is why carbon farmers focus on perennials, of which there are plenty of beautiful options to choose and even some edible ones. Fruiting trees, shrubs and vines provide a fun way to produce food at home, and there are quite a few perennial herbs, such as sage and thyme, and vegetables, like artichokes and asparagus, to consider.
To an extent, the more biomass you can produce on your property, the more carbon you will sequester. A single enormous tree with nothing growing beneath it does not store as much carbon as a jungle-like mix of shrubs, vines and herbaceous plants, plus trees large and small, packed into the same area of land. Toensmeier boasts 300 species in his tiny plot. This may sound unruly—and it can be—but there is an art to pruning and taming such plantings into a harmonious composition. This is how Mother Nature plants the planet, after all, which we inevitably admire in natural settings. Climate-friendly gardening is a matter of reproducing that organized chaos in your yard.
boost organic matter
In many ways, what’s good for the planet is also good for the garden. It’s not just plants that store carbon; soil actually has enormous sequestration ability, especially the dark, crumbly, miraculously fertile part called humus, which is 60 percent carbon. Humus isn’t the sort of thing you can just go to the garden center and buy in a bag, however. It is the end result of the decomposition of organic matter.
A healthy microbial population in the soil and a steady supply of carbon-rich root exudates are prerequisites for humus production. If you’re planting densely and avoiding tillage, you should be in good shape on those two accounts. But the main ingredient is organic matter. This comes in a variety of forms: mulch (wood chips, straw, etc.), compost (you can spread it on top of the soil like mulch, rather than tilling it in) and yard waste (better to let the leaves decompose on the ground in fall than to rake the yard bare).
if you mow, go gas-free
Dense jungles are revered for their carbon sequestration abilities. But wild grasslands actually sequester copious quantities as well. This is in part a product of grasses’ extraordinary photosynthetic capabilities, which produce huge quantities of carbon exudates below ground. Thus the carbon sequestration in this case is concentrated in roots and humus, rather than aboveground vegetation.
Of course, wild grasslands are quite a different beast than ordinary lawns, which contain very little biomass and tend to deplete humus and organic matter content, especially when fertilized solely with synthetic products. A better bet if you want an open area of grass is to establish a native bunchgrass meadow, the larger the species the better. The unmown hummocks can be truly gorgeous, especially when sprinkled with wildflowers.
But even some ordinary lawn grasses can sequester quite a bit of carbon. In fact, what gardeners know as warm-season grasses (such as kikuyu, buffalo, zoysia, centipede and St. Augustine) are monstrous carbon producers. That’s because unlike cool-season grasses, these are “C4 species” — grasses that use a different form of photosynthesis that results in far higher carbon sequestration. Only about 1 percent of plant species on earth utilize C4 photosynthesis, but these account for roughly 30 percent of terrestrial carbon fixation globally—and many of them just happen to be lawn grasses.
Don’t negate the carbon sequestration of your lawn by maintaining it with a smog-spewing mower. Keep the lawn small enough to be maintained with a push mower, or at the very least use an electric mower. (It probably goes without saying, but all types of gas-powered equipment are a faux pas for climate-conscious gardeners. Befriend the rake instead of a blower.)
Image credit: Photos by R A Kearton/Moment/Getty Images