A trail guide in Costa Rica might stop to flick the thorns on an acacia shrub. The thorns are wide and hollow and grow in pairs, like a demonic two-fingered peace sign. And when the guide agitates them, ants scurry out. This particular ant, Pseudomyrmex ferruginea, is a hard-biting acacia defender. It protects the plant from other insects, bigger animals and well-meaning trail guides. In return, the acacia feeds the ants a protein from its leaflets and nectar from its stalk. The acacia and its ant army are a textbook example of mutualism between species. And they represent the principle behind a new concept for urban farming: bug-like greenhouses perched on the roofs of skyscrapers.

They are rooftop farms, and they could represent a future segment of agriculture. Like their biological analogs, the greenhouses tap into the buildings below them, giving and taking in a kind of mechanical symbiosis. By recycling air and water, the two drink together, breathe together and regulate each other's temperature.

About 15 percent of the world's food is grown in cities, according to the U.S. Department of Agriculture. And that could grow, as cities strive for efficiency and reduce fuel consumption. Urban farms solve the prime source of waste—in fuel, pollutants and time—in the food industry: transportation. The average piece of food may travel 1300 to 1500 miles from farm to plate, according to studies cited by the Leopold Center for Sustainable Agriculture. So far, however, no urban farm design has gone mainstream. This is because they are costly to develop and maintain (since they are in high-rent areas) and the product—food—is already made cheaply elsewhere.

Natalie Jeremijenko, an aerospace engineer and environmental health professor at New York University, came up with a rooftop design to solve these common problems for urban farming. Her fixtures may be more economical than other urban farm concepts because they take up real estate that otherwise goes unused, and unlike other urban farm designs, they can pack in the plants, because everything, from the integrated systems to their bubble shape, is a slave to efficiency.

The Engineering Challenge

Not all roofs can support the hundreds or thousands of pounds of soil and water that a farm needs. That was a major obstacle in Viraj Puri's hunt for a rooftop to cultivate. Puri runs Gotham Greens, a startup that's trying to become New York City's first commercial rooftop farming operation. Finding the appropriate site is the first thing he mentions when listing the challenges. "You have to look at the structural composition of the building and line that up with what your operations are going to be," he says.

Jeremijenko's design sidesteps this issue with legs. The steel stilts splayed out underneath distribute the structure's weight to the building's load-bearing walls. And the farms weigh less because they grow in hydroponic, soil-free trays.

The curved shape of the farms optimizes sun exposure and doesn't require moving parts or grow lights, unlike many greenhouse designs. "The building doesn't have to rotate to follow the sun," says Jeremy Edmiston, principal at SYSTEMarchitects in Manhattan and co-designer of the Urban Space Station, as Jeremijenko calls the design. "There's enough change within the shape of the building to allow for variations."

The streamlined form also fares well on windy rooftops. A series of computer models show that decreasing wind resistance helps keep the farm intact. "The wind wants to blow the thing off the roof," Edmiston says. "So, much of the structure is about holding it down rather than holding it up." To get a streamlined shape, Jeremijenko's design incorporates a skin of Ethylene tetrafluoroethylene (ETFE) stretched over curved ribs of steel. ETFE is a supertough, translucent polymer used to cover stadiums and other big spaces.

Beneath its skin, the greenhouse is linked to the building below, sharing energy, air and water. Imagine homes and offices where garden-fresh breezes waft through the vents. The breezes may do more than improve the scent. Plants cull carbon dioxide and increase the oxygen content in air, and some species can filter other harmful gases, such as formaldehyde, as well.

Besides the air, the farms would also recycle and purify gray water, which is wastewater from sinks, bathtubs and drinking fountains. Jeremijenko is experimenting with retrofits of a building's upper two stories. They would circulate water and air through the farm and back again for people to use.

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Vertical Farms, the Other Urban Farm

The Urban Space Station has so far taken a back seat to another urban farming concept: the vertical farm. In this approach, an entire skyscraper is dedicated to agriculture, and it doubles as a water-treatment and waste-recycling facility.

Vertical farms are designed to produce a higher volume of food than rooftop farms, but it's not clear if they would do it cheaply. The premium on the building space, Jeremijenko says, is why vertical farms do not have a viable business model. "It takes a stock market to build a high-rise," she says. In other words, it may be tough to recoup the rent through crop sales, especially since high-rise crops would compete on the shelf with food that grows on cheap, building-free flatland farms.

Jeremijenko has nothing against farming upward—in fact, she has designed a vertical farm herself. But hers is built around a fire escape on an otherwise occupied building (it's still code-compliant, she says).

Dickson Despommier, a Columbia University professor who has championed the vertical farm design, has a different economic argument. He has said that costs could change as priorities change. Populations are rising, and so might both food prices and the cost of farmland. And future generations may prefer to pay more for vertically grown crops if they free up land for wildlife, he says.

Despommier likes most aspects of Jeremijenko's rooftop design. "It's a greenhouse that can be built on the roof; that's a great idea," he says. He points out that they can trap heat in the winter, and, he says, the retrofits required for water recycling would not have to be very sophisticated.

To Jeremijenko, a key difference between her concepts and others is that the farm is integrated: It can make life more pleasant for the people who live below it. The principles of mutualism is rare in modern architecture, but it can tackle unsexy problems like efficiency and cost. In other words, the extraordinary curves in this rooftop design are not paying homage to Frank Gehry. Likes ants living in acacias, they serve a purpose.

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