Time to consider vegetable gardens in urban climate policies!

More and more people around the world are starting to grow vegetables in their gardens and backyards. If the social and health benefits of gardening are well documented, its environmental benefits are somewhat less studied. For instance, to date, very few data were available to assess the climate impact of home vegetable gardens. This led a team of researchers and students from the University of California at Santa Barbara to ask the following question: what if households in Santa Barbara county started growing some of the vegetables they need in home gardens, that replaced part of their lawns? The result of their work was recently published in the journal Landscape and Urban Planning. Their article shows that it is time for urban policy makers to start tapping into the potential that vegetable gardens offer to tackle climate change.

So, what would happen if households used a part of their lawns to grow food?

Vegetable Gardens Help Mitigate Climate Change…

Researchers have mapped the possible consequences of such a change through a life-cycle assessment. Growing half of the vegetables they normally eat in a home garden (instead of buying them) could mean that households would:

• Reduce the size of their lawns, and therefore the amount of fertilizers and water to maintain them, and the energy to cut them.
• Only purchase half of their vegetables in the conventional food system.
• Make compost from food and yard waste to use in their vegetable garden, and reduce the amount going to landfills when it produces greenhouse gases.

Results show that such a change would lead to a decrease of 2,1 kg of greenhouse gas per kilogram of vegetable per year.

If households grew half of the vegetables they consume in their gardens, which would require an area of about 19 m² (only 3% of the average lawn area), they would decrease their food system greenhouse gas emissions by 4%, but their total household emissions by only 0.6%.

However, if half of the households in Santa Barbara County grew half of their vegetables, they would contribute 3.3% of Santa Barbara County’s greenhouse gas mitigation target for 2020, and if half the households in California did this, they would contribute 7.8% of the State of California’s target for 2020.

… provided Households Do not Use Fertilizers and Know how to Compost!

However, researchers found that this result is highly dependent on two factors:

• First, households’ gardening skills. Indeed, the researchers assumed an “ideal” situation, that is, one in which households produce good yields, do not use imported fertilizers and do not waste food grown in their gardens. These characteristics all decrease the greenhouse gas emissions per kg of vegetable.
• Second, households’ composting skills. Indeed, not everyone knows how to make a compost so it does not produce excess methane and nitrous oxide, very powerful greenhouse gases. Because it can be difficult for households to find the time for learning good composting, and actually doing it, it may be better not to compost organic waste at home. David Cleveland, the researcher from the University of California at Santa Barbara who carried out the study with his students as part of a research seminar, says that this was a major surprise to them. Ensuring that composting is done in a good way is therefore a major policy challenge, to the point that it may be better for households to send their organic waste to landfill facilities with gas to energy generation, or to larger scale, more efficient professional composting facilities that would give them compost back.

When the Climate Policy Officer Meets the Food Policy Officer

The policy implications of such research are clear: cities should start integrating measures into their Climate Plan to incentivize households to grow food in their gardens and make quality compost. Such actions could include a subsidy to cover start-up costs, education programs for new gardeners or even tax reduction for households who grow vegetables. They could even develop good quality composting facilities for households who do not feel like learning how to make compost.

More importantly, this research provides evidence for city officials who are willing to encourage households to grow food, but are reluctant to do so because the environmental benefits cannot be quantified. The methodology developed in the paper could be used both in food and climate policies. That’s (another!) good reason for food policy officers to go and talk to their climate colleagues! And vice versa…

Source: Cleveland, D.A., Phares, N., Nightingale, K.D., Weatherby, R.L., Radis, W., Ballard, J., Campagna, M., Kurtz, D., Livingstone, K., Riechers, G., Wilkins, K. (2017), The potential for urban household vegetable gardens to reduce greenhouse gas emissions, Landscape and Urban Planning, 157, pp. 365-374

NB: the author would like to thank David Cleveland for his inputs and comments.

Image: Pixabay