I have remained on the sidelines with regard to promoting green roofs. I'm hesitant partly because many people in the multifamily industry have this saying that goes "flat roofs always leak".
In my own experience, the most frequent and worst leaks occur at the low spots where ponding occurs. A pond on your roof causes dust in the air to stick to the surface of the water. Over time, this dust builds up and starts looking like dirt, or soil. Seeds get blown into this muck, and sprout. I know this is a poorly designed ad-hoc green roof, but the fact remains that leaks never start in the dry spots on flat roofs.
I guess some sort of roof garden would be nice if you also had a roof deck and actually used it.
I'll let Dr. Joe L., every building scientist's hero, explain it:
"Green roofs? Grass and dirt are not energy efficient.Work with me here. Which saves more energy—2 inches of dirt or 2 inches of insulation? Which saves more energy—grass or a white colored membrane? Which is more expensive and does not save energy—grass and dirt or insulation and a white colored membrane? Which needs to be watered to keep the grass from dying and blowing away? But they are beautiful and look cool. And that apparently is more important than cost and energy savings. Okay, I can live with the beautiful and looking cool argument if that is in fact the argument—but don’t clutter it with half-truths such as heat island effects and water run-off. There are other (better) ways to deal with each."
He goes on to admit he won't win the argument, so he provides the proper drawing detail that works.
In semi-arid Denver, I'll posit that the money spent for the water needed to keep your roof alive will be far more than the dollar value of any perceived benefit.