For gardeners looking to do their part to slow the heating of the planet, all vegetables aren’t created equal.
Every serving of homegrown or urban-farmed fruits and vegetables contributes nearly a pound of Earth-warming carbon dioxide to the global climate, according to findings published on Monday in Nature Cities.
That’s roughly six times the levels of carbon pollution released by the same amount of produce if it’s grown in sprawling large-scale conventional farms — with their heavy use of pesticides, fossil-fuel-powered tractors and energy-dense artificial fertilizers.
But while most urban farms and home gardens used more carbon per serving than conventional farms — 57 percent and 75 percent, respectively — a minority of those city-based operations were more efficient than agro-industrial operations.
And those more efficient farms help show the path of how to green the rest, coauthor Benjamin Goldstein of the University of Michigan told The Hill.
“Urban farming can be climate beneficial for cities if you grow the right things in the right ways,” he said.
The scientists looked at real-world urban agriculture of all sizes — individual home plots, shared community gardens and commercial urban farms — at 73 sites across the U.S., France, Germany, Poland and the United Kingdom.
In Nantes, France, Goldstein recalled one farm that ran a particularly efficient operation, in climatic terms. It drew together sprawling outdoor fields with raised beds, a hydroponic greenhouse and outbuildings for packing and production.
Usually, these would have been built from new materials, recently created — their entire energy-intensive supply chains behind them.
But in this case, the farm’s structures were crafted from reclaimed materials like shipping crates and wood pallets.
“They had found ways to collect a lot of — I don’t want to say garbage, because it refers to something that doesn’t have value, but residual waste,” Goldstein said.
This humble example represents a sea change in the climatic benefits of an urban farm, the researchers found.
That’s because the ecological cost of urban farming largely comes down to the reliance on brand-new materials for inputs and infrastructure — problems that particularly plague the smallest farms and gardens.
That means that those materials must “pay back” the planet-warming energy used to create them within just a few years before being thrown away, whether or not they have use left in them.
“A raised bed used for five years will have approximately four times the environmental impact, per serving of food, as a raised bed used for 20 years,” they wrote.
By contrast, infrastructure for commercial agriculture may be in use for decades, reducing the total carbon cost of the food that is grown there.
To increase the benefits of locally growing these high-value plants, the authors urged urban farmers and city planners to try to squeeze as much life out of their materials — and “residual waste streams” — as possible.
They also suggested reusing urban waste streams — from construction materials to build raised beds, city compost to infuse garden soils with nutrients and recaptured greywater for irrigation — to further cut the climate costs of growing locally.
The more climate-friendly sites that they studied cut emissions by nearly half by “upcycling” waste wood and stone for building gardens.
If that practice were universally applied across urban farms, community gardens and personal gardens, “all three forms of urban agriculture would be carbon-competitive with conventional agriculture,” they wrote.
“That is, there is no statistically significant difference.”
In general, they emphasized, urban agriculture was best for the climate “when it serves as a hub for symbiosis of building materials, organic waste and rainwater.”
Above all, the authors encouraged urban growers to spend their carbon wisely.
In large measure, that means picking the right crops.
The study revealed that city gardeners and farmers can reduce their climate impacts by cultivating crops that are typically greenhouse-grown or air-freighted" said study coauthor Jason Hawes, a doctoral student at the University of Michigan’s School for Environment and Sustainability.
From those, Goldstein said, “you can extrapolate to similar crops,” which are either grown in a greenhouse powered by electricity from fossil fuels or flown long distances.
In the first category, along with tomatoes, other fruits and vegetables that straddle the line between greenhouse-grown and yard-adapted are cucumbers, lettuce, peppers spinach and strawberries.
Several commonly air-freighted crops can grow well in home plots, like green beans and berries.
When farmers grow these high-value crops in structures made of repurposed materials, generate compost locally or trap rainwater for irrigation — urban agriculture can beat conventional, the authors found.
But even when it doesn’t, Goldstein said, it’s often still worth doing — because urban farming also provides benefits well beyond the food.
When the team asked urban farmers across the Global North what they got out of urban farming, “we found a whole host of social and economic,” Goldstein said.
The hard-to-quantify public health benefits of urban agriculture can include the friendships that build up around a community garden; the improved mental health in neighborhoods where gardening is common and the spillover effects on diet that come with spending a day tending to vegetables — whatever the yields might be.
It’s important to “focus not just on growing food but on social benefits, because if you can mix those in, all that carbon generated by farming — directly or indirectly through its materials — is not just going towards food but towards job training, community building, healthier communities,” Goldstein said.
“It’s a way to spread out those carbon impacts. Not just the food, but to other benefits.
Increasing those benefits doesn’t make the carbon costs of urban agriculture any lower, the authors acknowledged.
But they’re an important part of the story, Goldstein said. “There are lots of reasons to do urban ag even if not a silver bullet for the climate crisis,” he said.
City-based farming can be “low carbon and carbon competitive, and it can also have lots of other benefits,” he said.
“But it has to be done in the right way.”
