Answer: Although mimosa trees are most reliable through zone 7, they do occasionally survive in zone 6 and every once in a while into zone 5. It depends partly on the microclimate where they are planted, and it seems also that certain individual trees have better cold tolerance than others. So it is good that you took a seedling from a tree known to survive your cold winters, rather than a seedling from a southern tree growing in a warmer climate.
It is however difficult to maintain these as container plants, especially in terms of having them survive the winter. If you have several, you might experiment by planting one or two in the spot where you would like to have the tree and mulching heavily for the winter. Then set the others in their containers in a sheltered location such as a garage for the winter. Keep the soil just barely moist and be very patient in the spring -- mimosas typically leaf out very late.
As far as maintaining a larger shade tree in a container, this is not really feasible. The roots of a tree usually cover as much space as the canopy and thus quickly become cramped when constrained in a container. This then restricts the growth of the tree. Also, container trees are subject to temperature fluctuations of the soil because the roots are not as well insulated as they would be in the ground. This is very stressful on the tree.
If you want to experiment, you might be able to grow a crabapple tree in a large container such as a half barrel. It would need careful attention to watering any time the ground is not frozen and it will do better with protection from the winter wind. Your local professional nursery staff may also have some suggestions based on a more detailed understanding of the growing conditions on your patio.
Good luck with your trees!
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