Answer: You can fill containers with all kinds of flowering annuals and perennials, however, in a cold winter climate such as yours it will be very difficult to overwinter plants outdoors in a container. The reason is that the roots are not as well insulated as they would be in the ground. The extreme cold can damage the roots, as can the freeze/thaw caused by oscillating temperatures in spring or fall. You might be able to grow something hardy like a dwarf Alberta spruce for year around color, but it is a fairly basic evergreen so I am not sure that means your design goal. I'm sorry I can't be more encouraging in that respect, but there's no reason you can't fill your containers with lots of colorful plants every spring and enjoy them until the first frost of the season. Best wishes with your plants.
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