Grow bags work well when planting in boulders. The bags will sit better among the rocks. I did a few of mine last year on a rocky slope and used contractor garbage bags inside of burlap sacs... The burlap serves 2 purposes: it looks more natural than plastic and it shields the roots from the heat that would build up from direct sun on black bags. Roots hate heat! The rocks where I had mine were dark grey so I took a sample rock and bought matching spray paint to paint the burlap so it would blend better. Hope this helps