Seen on the Blackrock/Riverside Tour of Gardens here in Buffalo. This serious gardener keeps all his columnar apple trees in pots to form a vertical orchard against a wall that keeps the heat on them, producing prodigiously. He drags them into his garage for the winters to protect them. I have one columnar apple tree (in the ground) in my front yard. If I had five, not sure I'd want to bring them in each year. Bringing them in probably protects the planters from winter ice more than the trees themselves.