Fruit and vegetables grown on urban farms, which are often touted as a healthier and more sustainable, are often more carbon-intensive than conventionally grown food, according to research published Monday in Nature Cities, which highlights the crops and methods the growing number of urban farmers around the world can use to help make the practice more environmentally friendly.
On average, fruit and vegetables grown on urban farms have carbon footprints six times greater than produce grown using conventional agriculture, according to a study of 73 urban farms and gardens led by researchers at the University of Michigan.
Each serving of homegrown or urban-farmed fruits and vegetables generates an average of around 500 grams of carbon dioxide (around a pound) compared to around 70g to 80g (0.15lb-0.18lb) per serving of conventionally produced produce, the researchers found.
The scientists claim their research is the first large- scale study of the increasingly popular practice that actually accounts for how most crops are grown in urban settings, with most previous research focusing on high-tech, energy-intensive methods like rooftop greenhouses and vertical farms as opposed to low-tech farms growing crops in soil on open air plots.
However, some crops are still competitive with conventional agriculture, the scientists said, notably tomatoes—which are conventionally grown in energy-intensive greenhouses—and asparagus, which have a relatively short shelf life and are often shipped by air freight.
An analysis of the 17 urban agriculture sites that outperformed conventional agriculture identified several ways urban farmers could be more carbon-competitive, the researchers said, such as extending the lifetime of infrastructure like raised beds and sheds (large-scale commercial agriculture often uses equipment for decades, lowering carbon costs).
Urban farmers could also reuse waste like construction debris—which they could “upcycle” for buildings and infrastructure—or use rainwater to lower their carbon footprint, the researchers said.
“Most of the climate impacts at urban farms are driven by the materials used to construct them—the infrastructure,” said Benjamin Goldstein, who co-led the study and is an assistant professor at Michigan’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.”
Goldstein told The Hill crop choice goes a long way to determining how carbon-friendly an urban farm is. He said the findings that the most carbon-competitive crops included things like tomatoes and asparagus can be used to “extrapolate to similar crops.” Other greenhouse-grown crops like tomatoes like cucumbers, lettuce, peppers, spinach and strawberries could also make carbon-competitive crops for urban farmers, he said. Other air-freighted crops like green beans and berries could also make good choices, he said, pointing to asparagus as an example.
While the researchers say their findings suggest steps need to be taken to ensure urban agriculture “does not undermine urban decarbonization efforts,” they stress there are many benefits to the practice beyond its unrealized potential to cut carbon emissions. They say a survey of urban farmers overwhelmingly reported improved mental health, diet and social networks from the practice. Other experts point to enhancements urban agriculture can bring to local ecosystems by boosting local biodiversity, creating more resilient food systems, improving air quality, keeping cities cooler by countering the “urban heat island” effect and helping control water runoff from storms.
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