JENNIFER BOUSSELOT HAS had one hell of a summer harvest. On a 576-square-foot plot of land, she’s pulled up over 200 pounds of produce—cucumbers, peppers, tomatoes, and basil, among other goodies—and the growing season isn’t anywhere close to done. Despite that resounding success, Bousselot is no farmer; she’s a horticulturist at Colorado State University, and that plot of land is actually up in the sky. The garden, atop a building near the Denver Coliseum, was purpose-built for Bousselot’s brand of research in an up-and-coming scientific field: rooftop farming.
As more people pour into metropolises—urban populations are projected to double in the next three decades, according to the World Bank—scientists like Bousselot are investigating how designers and planners can ruralize cities, greening roofs, and empty lots. The concept is known as “rurbanization,” and it could have all kinds of knock-on benefits for ballooning populations, from beautifying blocks to producing food more locally. It dispenses with the “city versus country” binary and instead blends the two in deliberate, meaningful ways. “You don’t have to set this up as a dichotomy between urban and rural, really,” says Bousselot. “What we should probably focus on is resilience overall.”
“The rurbanization idea is: OK, if we mix this up a bit, maybe we can create benefits on both sides,” adds Jessica Davies, principal investigator of Lancaster University’s Rurban Revolution project, a scientific investigation of the concept. “So if we bring some of what we grow nearer to where we live, can we enhance our connection with food? Can we make food more accessible? Can we improve local ecosystems?”
Recent research has begun to provide data on how well urban agriculture actually works if you’re planning to, you know, eat. A review paper published last month by researchers working on the Rurban Revolution project surveyed previous studies and determined that on average, urban agricultural yields (including both outdoor and indoor growing operations) were on par or higher than those of typical farms. But certain crops, like lettuces, tubers, and cucumbers, had yields up to four times higher when grown in cities. A separate team of scientists in Australia looked at 13 urban community farms for a year and found their yields to be twice that of typical commercial vegetable farms.
The caveat, though, is that this productivity comes in part from intensive human labor. On a commercial farm, crops are usually grown one at a time and tended with specialized equipment—you can’t plant wheat and carrots in the same field because they’re harvested in totally different ways. Crops also have to be spaced out to make room for where the equipment drives, reducing the amount of land that’s actually producing food.
An urban farm, by contrast, can grow all kinds of crops packed closely together because they’re harvested by hand. That’s part of the reason why Bousselot’s tiny rooftop garden in Denver is so productive. That crop diversity also means you can harvest different plants at different times—tomatoes in August, pumpkins in October—so the supply of food is more broadly distributed. Even though Bousselot has already harvested over 200 pounds of food, she still has two months left to go.
That requires human labor instead of a machine. So while urban farming can have a higher yield than traditional agriculture, it’s not necessarily as efficient. “But that inefficiency could easily change,” says Robert McDougall, an agricultural scientist at the independent research company Cesar Australia, who led the Australia study. “The people I studied were people who carried out urban farming mostly for recreational purposes, and so weren’t really interested in working as efficiently as possible. And they weren’t necessarily using the most efficient sources of materials.”
Take water, for example. Cities are currently designed to be impervious to rain, quickly draining it off streets to keep roads and buildings from flooding. But some urban areas are now transitioning into “sponge cities,” designed to safely soak up rain and store it for later use. In Los Angeles, for instance, officials are experimenting with roadside green spaces, where water seeps underground and into storage tanks. Rurbanized cities of the future could tap into that water source to grow food, and the gardens themselves could act like sponges, collecting rainwater to prevent local flooding.
Better municipal composting programs could also provide urban farmers with mulch so they don’t have to rely on synthetic fertilizers, which are terrible for the environment. “Were the gardeners I studied more tapped into these various sources of materials that were available within the environment around them,” says McDougall, “they could have easily carried out their farming in a much more sustainable fashion.”
Urban farms attract a host of pollinators like bees, McDougall has found. These insects, along with other pollinators like birds, could help boost biodiversity.
Cities also need all the green spaces they can get to counter the “urban heat island effect,” or the tendency for the built environment to absorb more of the sun’s energy than parks and forests do. Temperatures in urban areas can be 20 degrees Fahrenheit hotter than surrounding rural ones, where the plentiful vegetation releases water vapor—cooling the area as the plants essentially sweat. Bringing more plant life into cities will help cool things off and save lives during extreme heat events.
Urban agriculture can help insulate individual cities against food shocks, like if a particular mass-produced crop fails—which is increasingly likely as climate change spawns longer, more intense droughts. “You depend less on globalized supply chains,” says environmental scientist Florian Payen, who authored that review paper on yields. (He’s now at Scotland’s Rural College, but did the research while at Lancaster University.) “And so you are maybe less vulnerable to all of the different things like we’ve seen with Covid, or with climate change, that can impact the supply chain.”
Urban agriculture should also, in theory, reduce some of the emissions associated with conventional farming, which uses carbon-spewing machinery and requires shipping food vast distances to customers. But there isn’t much data to back that up yet, Payen says.
“The evidence so far is not really conclusive as to whether producing in urban areas for urban dwellers actually is associated with a lower carbon footprint than rural production,” says Payen. “And that is really based on the fact that there are lots of different ways of producing the food, and lots of different modes of transportation.” Wheat production, for example, is highly mechanized and relies on massive harvesting vehicles. And different crops travel different distances to get to market.
Those calculations focus primarily on the emissions from heavy machinery and long-distance trucking and shipping. But Elizabeth Sawin, founder and director of the Multisolving Institute, which promotes interventions that fix multiple problems at once, sees adding farms as a way to subtract a different source of emissions: cars. “Don’t underestimate how much of the square footage of our cities is devoted to the automobile, like highways or parking,” she says. “As we open up more space for living with things like public transportation and dense housing, that could become space for growing food.” Obliterating asphalt and planting seeds would transform cities from car-centric to people-centric systems.
In Denver, Bousselot is experimenting with solar panels to not only increase food security, but energy security as well. The idea, known as agrivoltaics, is to grow crops under rooftop solar panels that generate free, abundant energy for the building beneath them. The green roof also acts like insulation for the structure, reducing its cooling needs, while the partial shade the panels provide for the plants can significantly boost yields. (Too much sun is bad for certain crops. For example, other researchers have found that peppers produce three times as much fruit under solar panels than in full sun.) It’s also warmer up on a roof, and Bousselot has seen tomatoes grow faster, reaching harvest sooner.
Her Denver rooftop also seems to protect its crops from pathogenic fungi. “Up on the green roof, because of the high-wind, high-solar-radiation conditions, we have very, very little issue with that,” says Bousselot. “So I think there’s a ton of potential for selecting crops that would produce even higher, potentially, on a rooftop compared to the same place on the ground.”
But while rurbanization has enticing benefits, it has some inherent challenges, namely the cost of building farms in cities—whether on rooftops or at ground level. Urban real estate is much more expensive than rural land, so community gardeners are up against investors trying to turn empty spaces into money—and even against affordable developments aimed at alleviating the severe housing crises in many cities. And while rooftop real estate is less competitive, you can’t just slap a bunch of crops on a roof—those projects require engineering to account for the extra weight and moisture of the soil.
But the beauty of rurbanization is that agriculture and buildings don’t have to compete for space. Urban land is limited, which means that high-yielding, fast-growing, space-efficient crops work great, says Anastasia Cole Plakias, cofounder and chief impact officer of Brooklyn Grange, which operates the world’s largest rooftop soil farms. “That said, we approach the design of our own urban farms, as well as those we build for clients, with the consideration of the unique character of the community in which we’re building it,” says Plakias. “Urban farms should nourish urban communities, and the properties valued by one community might vary from another even in the same city.”
A hand-tended garden on a side lot doesn’t need a tremendous amount of space to make a tremendous amount of food. New developments could incorporate solar roofs from the start—they would have more upfront costs but produce free energy and food to sell in the long run.
No one is suggesting that urban agriculture will provide city-dwellers with 100 percent of the food they need to survive. Bousselot imagines it more as a collaboration, with commercial farmers churning out land-intensive and machine-harvested cereals like rice and wheat while urban gardeners grow nutrient-dense, hand-harvested vegetables like leafy greens—both creating jobs and reducing the length of the supply chain for perishable foods.
It would also provide something less quantifiable than crop yields: a renewed sense of community, says Sawin. “That’s a source of local connectivity that will ripple beyond just the food that’s produced,” she says. “People then have social networks for everything from sharing childcare to sharing resources to helping one another through, possibly, shocks and destabilization.”
Wired, 8 September 2022