Hildegard of Bingen - a Christian mystic, writer, composer, saint and herbalist - called it "viriditas", the physical, mental and spiritual healing power of having greenery around you. Every decade or so since, someone "discovers" that very tangible power again, or gives it a new name, or publishes a study showing that more urban trees in public areas equals less graffiti.
The most recent version is in an article I've just read in New Scientist. It focused not just on "green is good for us", but on the scientific reasons why, as well as exactly what kind of "green power" works best. Apparently we humans are happiest, and the healing power strongest, with scattered park-like trees and lots of different smaller plants and flowers.
