All great suggestions Verdun. I have found alot of rock roses in my garden get damaged by the frost and can have die back, they are lovely though. With regard to Cerastiums, our Cerastium matt has never sown any seed at all, It grows outwards sure and I use my shears to trim it back to the edge of the path, I never trim the top of the plant and it never produces any seed pods where the flowers were. I wouldn't go as far as to use the word thug when describing it, It looks beautiful cascading over a wall smothered in white flowers in early summer. If it does self seed, free plants to give away or plant elsewhere!