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Community Gardens Have Six Times the Carbon Footprint of Agriculture

Tomatoes growing on the vine.
Credit: Markus Spiske/Unsplash
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Fruits and vegetables grown in urban gardens have a surprisingly large carbon footprint, according to new research.

Published in Nature Cities, the study assessed the associated greenhouse gas emissions of 73 urban farms and gardens in 5 countries.

On average, these hobby gardens had a carbon cost six times as high as conventional agriculture.

In a few exceptional instances, however, city-grown crops, including tomatoes, were more carbon efficient than their industrial counterparts.

Urban sins

Whether tucked behind residential housing in cities or proudly placed in the hearts of villages, urban agriculture areas (allotments, community gardens) are key resources for countless diets around the world.

Yet, while these hand-grown, earthy brassicas and strawberries may appear sustainable, there was hitherto little research to prove that assumption.

Deciding to fill this knowledge gap, the research team from the University of Michigan (UM) set out to compare the carbon footprints of urban gardens across Europe and the United States.

They began by recruiting community gardeners in 73 sites across France, Germany, Poland, the UK and the US. These citizen scientists kept daily diary entries to record inputs and harvests from their food-growing sites throughout the 2019 season.

Inputs fell into three main categories: infrastructure (such as the raised beds in which food is grown, or pathways between plots), supplies (including compost, fertilizer, weed-blocking fabric and gasoline for machinery) and irrigation water.

The researchers calculated the climate-altering greenhouse gas emissions associated with all three elements over the lifetime of the farm. The emissions, expressed in kilograms of carbon dioxide equivalents (kg CO2e) per serving of food, were then compared to foods raised by conventional methods.

On average, food produced through urban agriculture emitted 0.42 kg CO2e per serving, six times higher than the 0.07 kg CO2e per serving of conventionally grown produce.

“Most of the climate impacts at urban farms are driven by the materials used to construct them—the infrastructure,” said study co-lead author Benjamin Goldstein, assistant professor at UM's School for Environment and Sustainability.

“These farms typically only operate for a few years or a decade, so the greenhouse gases used to produce those materials are not used effectively. Conventional agriculture, on the other hand, is very efficient and hard to compete with.”

“This dataset reveals that urban agriculture has higher carbon emissions per serving of fruit or vegetable than conventional agriculture – with a few exceptions,” Goldstein added.

These exceptions included tomatoes grown in the soil of open-air urban plots, which, on average, had lower carbon costs than tomatoes grown in conventional greenhouses.

“The exceptions revealed by our study suggest that urban agriculture practitioners can reduce their climate impacts by cultivating crops that are typically greenhouse-grown or air-freighted, in addition to making changes in site design and management," said study co-lead author Jason Hawes, a doctoral student at the UM’s School for Environment and Sustainability.

Ultimately, Hawes and his colleagues identified three best practices crucial to making low-tech urban agriculture more carbon-competitive with conventional agriculture:

  • Extend infrastructure lifetimes: extend the lifetime of urban agriculture materials and structures such as raised beds, composting infrastructure and sheds. A raised bed used for 5 years will have approximately 4 times the environmental impact, per serving of food, as a raised bed used for 20 years.
  • Use urban wastes as UA inputs: conserve carbon by engaging in “urban symbiosis,” which includes giving a second life to used materials, such as construction debris and demolition waste, that are unsuitable for new construction but potentially useful for urban agriculture.
  • Generate high levels of social benefits.: in a survey conducted for the study, the volunteer farmers and gardeners overwhelmingly reported improved mental health, diet and social networks. While these benefits of community gardening don’t reduce its carbon footprint, “growing spaces which maximize social benefits can outcompete conventional agriculture when UA [urban agriculture] benefits are considered holistically,” according to the study authors. 

“Urban agriculture offers a variety of social, nutritional and place-based environmental benefits, which make it an appealing feature of future sustainable cities,” said Hawes. “This work shines light on ways to ensure that urban agriculture benefits the climate, as well as the people and places it serves.”

Reference: Hawes JK, Goldstein BP, Newell JP, et al. . Comparing the carbon footprints of urban and conventional agriculture. Nat Cities. 2024. doi: 10.1038/s44284-023-00023-3

This article is a rework of a press release issued by the University of Michigan. Material has been edited for length and content.