dirtdorphins, If there is a crevice in the rock or you can make one with a grinder or chisel you can squeeze them in so that pressure holds them there. If it's directly on rock Make sure no water will collect in it. I use stone dust that I get from a gravel pit and lay it on the surface then put the rollers on it. I plant them with no roots. If they establish their own roots on the rock they'll grow best that way. You could make a temporary dam out of clay or get creative. The fall is an excellent time for the plants to root. Good luck.
