I hope we agree that gardening is good for you, whatever you decide to grow. Indeed, a fair case can be made for the value of just sipping a gin and tonic while watching the weeds grow. But, I would also submit, the very highest level of satisfaction comes from growing at least some of your own food. Quite apart from the fresh air and exercise involved in the actual cultivation, and the health benefits of eating your own fresh produce, the environmentally aware gardener will be chuffed by reducing a few food miles, and thus doing their bit to save the planet.
If the latter consideration is important to you, though, what should you grow? Specifically, what will lower your food-related carbon footprint the most?
This is no joke: the growing, transportation, packaging, retailing and cooking of food, together with the clearing of land to grow food in the first place, accounts for as much as 30 per cent of Britain's carbon footprint. A recent paper in the journal "Landscape and Urban Planning" considered this question, specifically in the context of urban farms, but the principles apply equally to the private gardener.
Tomatoes grown in a polytunnel have much higher yield
A key consideration, not surprisingly, is yield – all things being equal, you're better off growing high-yielding crops than low-yielding ones. So courgettes, for example, which are both high-yielding and also almost entirely imported, are a good choice. But yield isn't everything: potatoes are almost all grown in Britain and their carbon footprint is small, so little is gained (in CO2 terms) by growing your own. Sometimes yield outweighs other concerns. Although it takes a lot of energy (and therefore CO2 emissions) to manufacture a greenhouse or polytunnel, the higher yield of greenhouse-grown tomatoes more than compensates for that. Tomatoes are also mostly imported, and even those grown in Britain come from heated greenhouses that use a lot of energy. Strawberries, on the other hand, are not worth growing in a polytunnel – at least not to save CO2 emissions – because yields are much lower than tomatoes.
Sometimes the CO2 emissions from transport are of primary importance. Exhibit A here is green beans, which are imported year-round in large quantities by Britain from outside Europe, mostly from Kenya. Only in mid to late summer do home-grown beans make a dent in our imports. Because the beans are imported by air, their carbon footprint is huge, making it very worthwhile to grow your own. Here, however, the arguments start to get more complicated. First, "CO2-saving" depends on the assumption that your own produce directly substitutes for (imported) produce from the shops.
If you care about your carbon footprint, though, maybe you wouldn't consider buying beans airfreighted from Africa in the first place? In which case, your CO2 saving is more notional, and depends on the carbon footprint of what you would eat (if you weren't growing your own). Plus, there's always the difficulty of the welfare of the two million Kenyans who depend on us to buy their fruit, flowers and vegetables.
Beans growing in Kenya. Working out the carbon footprint of food is not simple
If we grow all the right things, how well are we doing, relatively speaking? That all depends on what you choose to compare it with. Planting trees soaks up CO2, but growing your own fruit and veg does much better, in fact about 10 times better, than planting trees on the same land. On the other hand, most of the emissions of CO2 (and other greenhouse gases) from food come from the production and consumption of meat and dairy products.
If you gave up eating meat, or significantly reduced your consumption, your carbon footprint would be greatly improved, almost irrespective of where your fruit and veg came from.
Ken Thompson is a plant biologist with a keen interest in the science of gardening. He writes and lectures extensively and has written four gardening books, including 'Compost' and 'No Nettles Required'. His latest book is 'Do We Need Pandas? The Uncomfortable Truth About Biodiversity'.