The city has long been a beacon of opportunity, where folks flock to make it big. But metropolises the world over are wasting a major opportunity — many, many square feet of it: Flat rooftops are painted white, when really they should be green. Not, mind you, shades of mint green, forest green, or lime green, but with the lushness of actual plants. Adding vegetation to roofs — even if it’s just a coating of grass, moss, and succulents — bestows many overlapping, reinforcing benefits not only on a building’s occupants and owner, but on the surrounding community. Like parks on the ground, gardens in the sky reduce local temperatures and help prevent flooding, all while improving urban biodiversity and feeding pollinators like bees.…