People repurpose old shipping containers for lots of things. But Shawn Cooney may have found the greenest use yet. On a vacant lot near Boston's Logan Airport, Cooney is using four former freight containers ... to grow some 30,000 heads of lettuce, herbs and other leafy greens. I'm not really a farmer," said the 61-year-old Cooney, who ran software companies before starting Corner Stalk farms in 2013. "But it's more interesting than a desk job." If 30,000 heads of lettuce sounds like a lot, it is - and it's the reason why he's able to run a successful farm in one of the country's most expensive cities. The containers come from Freight Farms, a Boston-based startup that outfits the boxes with lights, growing racks and irrigation systems - creating what are essentially super efficient growing machines. The boxes themselves are former freezer containers that were used to ship meat, so they're insulated against the heat and cold. Inside, the plants get light from LEDs and there's no soil. The roots are instead placed in a peat moss base that gets a dollop of nutrient-rich water every 12 minutes. The entire container, floor to ceiling, is filled with plants in a totally self-contained operation. And it churns out the plants. Cooney said he harvests 4,000 to 6,000 plants a week - roughly 80 times the number he'd get from a similar amount of space on a conventional farm. The plants are sold to a wholesaler, which distributes them to mostly high-end restaurants in the Boston area.