I have never tried one on a rock like this.
I guess the moss holds the dirt on, but I wonder how you get the dirt to stay on the rock before the moss gets a chance to settle down. Also, how often do you water? July-September around here the sun shines hard many hours and highs will be 105F or so, with lows over 90F (that would be 40C and 30C in the rest of the world). Some of the trees in pots need twice a day, and it looks like one of these would need more than that.