I have a single plant of this rooted into the sand of a neglected alpine bench. I should have moved it many months ago but forgot about it and it is now in full flower, 18" or so high. It is nominally in a 2 1/2" square pot (it was the only seed/plant that survived from Nargs seed) but is now obviously rooted well through the 1" layer of sand. I can pull it all up and cut the pot off and try to keep it going, but when would be the best time to try this removal. It will be eventually grown in the open garden.
I am in the southern UK.