Posted: 21/08/2014 at 07:00
I've never grown them, but a friend did - she had the lots of frost-free greenhouse space as she was the head gardener at a stately home - I would leave them in the pot but lift it and bring indoors before the first frosts - she did it when bringing in pots of pelagoniums and lifting the dahlias.
I remember her saying that they hate root disturbance so she just brought the pots in for the winter and put them out again in the spring when the summer bedding went out. As long as they were frost free they were happy. She didn't water them or anything. Just left them alone.