A green roof is any kind of vegetation that is intentionally
grown on the roof of a building or structure. Usually it takes
the form of a rooftop garden. A green roof can be fairly simple
and involve only ground cover plants that are self-sustaining,
or it can include irrigation systems, trees, and space for
people. Poster of Peggy Notebaert Nature Museum extensive green
roof (photo credit: © Conservation Design Forum Searle Chemistry
Laboratory's extensive green roof (bottom right corner)