Answer: Unfortunately, it is difficult to grow shrubs in containers year round in your cold climate. The reason is that the roots are not as well insulated from the cold as they would be if planted in the ground. This means they often do not survive the winter, or become so stressed that they do not look very good the following year. And, the root disturbance caused by planting and growing the annual flowers will be an additional stress on the shrub.
If you want to try, use the biggest possible container such as a half whiskey barrel size as a minimum to allow for more insulation around the roots and to minimize the effects of the spring and fall freeze/thaw cycles. You might be able to grow dwarf Alberta spruce -- but based on my experience please do not count on them surviving. I wish I could be more encouraging.
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