Posted: 10/09/2014 at 00:55
Dahlias growing in pots are going to be vulnerable to freezing if left outdoors, and if the tubers freeze they will rot and die.
I leave my dahlias in the ground, mulch them well and hope for a mild winter.
If my dahlias were in pots I would wait until the first frost blackened the plant, cut them back to a few inches, put them on their sides to let any water in the hollow stems drain away and then store them somewhere frost free, keeping the compost pretty dry, until next year when they could be planted out after the threat of frost has passed. (If you have the space in a greenhouse/conservatory etc, you can bring them into growth earlier, but you cannot plant them out until the risk of frost has passed)