(courtesy http://www.greenroofs.com)

Until recently, when I thought of the word “farm” I pictured, you know, a farm–grazing cows, big tractors, an old broken down barn, rows of tomatoes and eggplants, a gruff, ruddy-complexioned man in plaid shirt, scraggly beard, and mud-encased boots.  I never ever thought of concrete, bus exhaust, sealants, wind, or structural capacity. That is until I met Casey Townsend, a young and serious farmer, whose Northampton Square Rooftop Garden, NSRG, is on the top of a four-story parking garage wedged between Boston Medical Center (BMC), Boston Emergency Medical Services, and two residential high-rise towers. This 6,500 square foot parcel of tilled 14-inch deep soil is expected to yield 2500 pounds of strawberries, rhubarb, bunching onions, scallions, tomatoes, blueberries, cucumbers, zucchini, and lots of other vegetables—like daikon.

“I have never grown daikon as well on a land-based farm as I have on this roof,” said Townsend dressed in a black ski cap and green parka on a cold April day. “They came out about 14 inches long. They were gorgeous.” Townsend’s farm might be in the middle of Boston but his veggies think they are further south, say, in the zone of Providence, Rhode Island. “If the seed package says 45 days, I can grow it in 40,” he said. He plants his peppers and eggplant in concrete cubes and the residual heat comes off the roof and surrounding buildings creating a microclimate. This allows him to grow varieties that need more heat, like collards, and start them earlier in the season.

The Northampton Square Rooftop Garden has three sections: traditional organic, biodynamic where Townsend plants with the cycles of the moon, and bio-intensive where by constantly improving and maintaining the fertility of the soil, he can yield a lot from a little space. Most of his produce goes to the Preventative Food Pantry at Boston Medical Center, a corridor and elevator walk away. No fossil fuel, no food miles, and still warm from the sun. You can’t get more local then that.

Mohamed Hage, the founder of Lufa Farms in Montreal, one of the first large-scale rooftop farms coined this type of farming as “Agricultural 2.0.” Rooftop farms range from small residential plots to large-scale commercial or high-tech greenhouse farms. Regardless of the size, “Green roofs are popping up across America,” said Lauren Mandel, Project Manager at Roofmeadow–a Philadelphia-based green roof firm–and author of Eat Up: The Inside Scoop on Rooftop Agriculture.

Spicy mustard, bitter chicories, and sweet lettuces burst from the rooftops of New York City these days. Gotham Greens has a 15,000-rooftop garden in Greenpoint, Brooklyn. And they are slated to open a 60,000-foot garden in Queens. The Grange just added chickens to their 65,000 square foot farm over Brooklyn’s Navy Yard. Whole Foods is planning a 20,000-foot greenhouse atop their Gowanus store and another on top of a Lynnfield, Massachusetts store. Higher Ground Farm is opening this spring on top of the 55,000-foot Boston Design Center. There’s a farm atop the Community Building and Restoration Building in Milwaukee, Wisconsin. And the list goes on.

Mandel attributes the boom to an overall awareness made possible by social media and online articles. This made it The Next New Thing to start a rooftop farm. Once word spread, it drove innovation for better soils, irrigation systems, and green power. When Mandel started writing her book, three years ago, she wrote about rooftop farming in the future tense–“Rooftop gardens will …” she wrote. But with their popularity, she’s had to change every will to a have.

But the boom isn’t just a hipster farmer in Brooklyn with his plaid shirt and muck boots; these farms are addressing a need. “In America, over an acre of farmland is lost every minute,” said Mandel’s book. And the website of Feeding Cities–a conference held at the University of Pennsylvania to examine urbanization and food security—predicts that “By 2050, 7 out of every 10 humans will live in cities.” And as Townsend points out, “as oil prices rise through the roof it will be harder to transport food.”

Hyper-local farming has many benefits. No food miles: those blueberries don’t have to travel from Southern Chile to your plate in a New England winter. Fewer airplanes and trucks means less fossil fuel burned. Rooftop farms create insulation for their buildings in the winter and absorb sunlight in the summer reducing bills by 30 percent some sources suggest. If there are enough of them in a city, it can reduce the overall urban heat island effect—the higher temperatures in cities due to the black roofs and asphalt. Rooftop farms can transform a hot, empty roof into something beautiful and productive. They can create sustainable agricultural environments in urban centers. And, there’s the economics for the farmers. People pay more for produce in the cities. “You don’t have to be a struggling farmer,” said Townsend. “You can get an increased income growing near the city.”

But sky-high farming isn’t as simple as hauling some dirt to your rooftop and tossing in some seeds. It requires planning and is expensive. The cheapest Mandel has heard of is $5 a square foot for a Brooklyn farm. But that farm had volunteers and many donations so might not be an accurate cost. Most soil-based farms, she said, cost about $20 per square foot. But if you’re building a high-tech greenhouse, for hydroponic farming, then the skycould be the limit.

Farmers, on average, according to Mark Winterer, founder of ReCover Green Roofs in Somerville, Massachusetts, make $3 a square foot. But the bigger the farm, the more they can make because of economies of scale. “The people that can afford it are visionaries,” Winterer said. “They put a big value on the marketing of it. But it doesn’t make dollars and sense.” But once marijuana is legalized, “that will be huge for green roofs,” he predicts. Farmers can charge more for pot then they can for carrots.

Not every building was meant to hold forty pounds a square foot of dirt and water. Urban agriculturalist Sandra Fairbank and Chef Michael Leviton found this out when planning their farm atop a garage in Cambridge. Part of the roof can sustain 10 inches of soil, enough for tomato plants. But other parts can only support three inches of soil. That’s fine for microgreens but not for Townsend’s daikons.

Roofs also need to be totally waterproof and sealed with a hydrophobic membrane. Otherwise, umbrellas will be needed in the rooms below as rooftop farms, like all farms, need a lot of water. Townsend waters for 2 to 2 ½ hours a day. His farm gets extra dry because of the sandwich effect with the parking garage below and all of the air above.

And then there’s the theft. Land-based farms don’t usually have vegetable bandits in suits looking for a sun-ripened, juicy tomato to liven up their dull sandwich. But at Townsend’s farm, accessible to the surrounding buildings, he does. To confuse the bandits, he plants tomato varieties they won’t recognize. “Partly it’s education. But, if they don’t know what it is they won’t steal it. All my tomatoes are heirloom–none of them turn red. People keep looking at them, ‘oh, they’re still green…still green.’ So they don’t pick them. I probably lost 200 pounds of produce last year. But if I was at a land- based farm I would lose stuff to deer, turkey, or woodchucks.”