Many is the time I've been in a big city for one conference or another, and have looked across acre upon acre of black tar rooftop, thinking, "Look at all that space that could hold solar panels and help power those buildings."
Horticulturalists at Oregon State University looked across the same urban roofscape and thought, "That could be a meadow."
Rooftop gardens have been around for a long time, but those meant to be human-centered gardens, with tables and chairs and trees to shade them, generally require extra structural support to hold up all the dirt that's required. OSU researchers are investigating extensive gardens with low-growing plants, some requiring only a few inches of lightweight growing medium, which can happily co-exist with ordinary rooftop structures. Add a rainwater collection system to store water for dry summers, and you have a sustainable garden that can help mitigate runoff, clean the air, and mitigate the effects of putting concrete structures where wild nature used to be.
The full article is here: Greening the Concrete Jungle