I assume rooftop gardening is a completely different (and good) idea. There you have the sunlight already, just being reflected or absorbed as waste heat, it's

I assume rooftop gardening is a completely different (and good) idea. There you have the sunlight already, just being reflected or absorbed as waste heat, it's simply a materials (weight) and cost or species issue. (also it's easier to design for one than retro-fit one on).