Answer: The narrowest selection would be arborvitae. They won't need root pruning as they have relatively small root systems because they grow tall and remain about 2' in diameter, even in maturity. I would not recommend unpotting every 6 months to root prune. The plants will become stressed and will eventually fail. A second suggestion is Indian Laurel (ficus). This, too is relatively narrow and can be sheared to maintain the desired shape. Neither of these shrubs produces flowers. Hope one of them works for you!
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