I live in Zone 3 (or 4, depending on whose maps you're using). That's -30 to -40 most winters. The trick (as I see it) is to grow in the ground OR in enormous raised beds. Most of my beds are 4'x8'x16" deep. I've never had a loss due to the cold. Pots (unless they are sunk completely in the ground) freeze harder than the surrounding soil if they are left out on their own. Sinking them in the ground moderates the temps. People would not think so, but the plants can tell the difference.
