One edge of my folks' property in MA was littered with boulders from glacial action. Several had huge cracks in them that had been filled in over time with leaf litter and tiny pieces of gneiss that was the major rock in our area. It screamed to be planted.
In the shadier ones I planted some Iris cristata and they really loved it. I'm betting they're still there. In one with about half sun I planted some of the old tectorums (not wanting to use my good semps!) and they also grew quite happily. The cracks were about 6-8" deep and didn't seem to pond in rain or heavy snow melt.
I don't think anyone would think of semps as exotic invasives. The old tectorum is sterile or nearly so.
Kevin
