When Eric Toensmeier and Jonathan Bates moved into a duplex in a run-down part of Holyoke, Massachusetts, the tenth-of-an-acre lot was barren ground and bad soil, peppered with broken pieces of concrete, asphalt, and brick. The two friends got to work designing what would become not just another urban farm, but a "permaculture paradise" replete with perennial broccoli, paw paws, bananas, and moringa--all told, more than two hundred low-maintenance...