Green grass is pretty close to an 18 percent gray.
No! Greenery (and landscapes in general) is pretty dark. That's why many people speak of "Sunny 11" instead of "Sunny 16". If you don't believe me, don't argue, just try with a light meter that allows both incident and reflected measurements. If the subject is a lanscape, the exposure from reflected measurement will be approx. 1 stop more than from incident (looking at light same as scene, i.e. opposite direction compared with reflected measurement).