My pots outside here in norwich and even some in the shed, have frozen pretty solid, so it's not ideal for the dahlia. I would treat them as being ok and move them like mentioned above because they can be surprisingly hardy. I kept them in pots many times over winter and simply tip them out in the spring and repot in new compost before watering a little to get them growing. If you have somewhere frost free to keep them then you can knock them out of the pot now and remove the soil to check the tubers which will make them less liable to rot over winter.
If all else fails they are cheap and easy to buy bare root. local garden centres usually do 3 tubers for about £5 around here, which you just pot on and start yourself.
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I kept them in pots many times over winter and simply tip them out in the spring and repot in new compost before watering a little to get them growing. If you have somewhere frost free to keep them then you can knock them out of the pot now and remove the soil to check the tubers which will make them less liable to rot over winter.
If all else fails they are cheap and easy to buy bare root. local garden centres usually do 3 tubers for about £5 around here, which you just pot on and start yourself.