Does it really work good for you, Martin? Frank? I experimented a bit this morning, but wasn't very satisfied. Still had objects (little test sticks in this case) growing through rocks. I put a color adjust between the stone shader and merge input, a distribution shader+fractal into A, set it to highest, normal mix (I also tried others) @0.5, but still sticks out of rock. Played with gamma, black point and white point, which altered the distribution somewhat, but never completely away from the rocks. Perhaps it depends on the rock size as well, in case the stones 'erupt' from a certain (tiny) point. Then small stones would of course work better than large ones. I'll experiment a bit more...
---Dune