Answer: It is very difficult to overwinter plants in containers outdoors in your cold winter climate. (A much warmer climate of say winter hardiness zone 7 combined with a sheltered location and a large container providing ample insulation to the roots seems to be about the bottom line for good chances of success.) Sun exposure as well as wind exposure add to the difficulty of overwintering spruce's in containers. The plants become stressed and often do not survive. The best things to do are provide wind protection and insulate the roots as best you can (possibly wrap with bubble wrap or other insulating material or heap mulch around the pots), place them in a shaded spot up against a wall -- to reduce reflected heat -- and also protect them from wind and then make sure to keep the soil damp any time it is not frozen. Best wishes with your garden!
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