terracotta pots. Frost proofing.

If you can get hold of British made ones, the chances are they are more likely to be frost proof.
I think it has something to do with the firing temperature, but not being a potter, I cant be sure. :D :rolleyes:

 

Use heavy duty bubblewrap INSIDE the pot to create an expansion gap. Use more bubble wrap laid on top of the compost to limit the amount of rain getting into pot in winter also to protect roots from the cold.