Ever since Malaysian ecologist-architect Ken Yeang introduced the concept in the 1990’s, living walls and rooftops have become an increasingly common sight in both residential and commercial buildings. In addition to looking good, planted exteriors also help cut energy costs, and, in the case of rooftop farms, provide urban dwellers with homegrown produce. Now, Stefano Boeri wants to take green architecture to the next level with “Forest Cities” to combat China’s air pollution woes.