I will admit that it has crossed my mind that a couple of small, shapely spruces would look wonderful flanking my front door where the fig trees stand in the summer. The size of their pots calls for trees larger than the jumbo "treetops" that are sold locally for winter decoration. However, it is just a passing thought because I know it cannot be - at least not without expensive, labor-intensive life support that only the very rich and environmentally insensitive would attempt.
Why is that? In many places south of here people grow evergreens and other trees and shrubs in tubs. Spruces are perfectly hardy here in the landscape, so why can't they be grown in tubs? Tree roots in the ground can reach below the frost line and continue the metabolic work of keeping the tree alive even after the top seems to be frozen. Trees in tubs don't have that protection. In our climate, with winter minimum temperatures that can reach -30 degrees their roots freeze and they die.
What about insulating the pot? Trees grow in planters in downtown Minneapolis. What about them? Trees in planters that are open to the ground beneath allow the roots to go below the frost line as roots in the ground do. If they don't have connection to the ground, the container must be heated to keep the roots from freezing. That temperature cannot be too high either. If it is, the tree's protective dormancy may be broken and then the top will die from cold as if the tree were not hardy here at all.
Of course, if you have the room, you might opt to bring the whole tub/tree combo into the warmth of your home whenever the outdoor temperatures required it, but the warmth inside would break the tree's dormancy and then it could no longer go back out until warm spring temperatures arrive. It's a catch-22: what would seem to be the answer creates a greater problem.
Temperate evergreens - those native to climates like ours, must have their winter dormancy to survive. Experts in Japanese bonsai must make special arrangements for this cold period without freezing the roots of their cherished works of art. At the Como Park Conservatory there is a special room for the bonsai collection that simulates cold outdoor temperatures without freezing their roots.
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