Answer: Overwintering container plants can be a challenge! Your main risk is that the tree will dry out and/or freeze to death during the cold weather. Although poplars are hardy in your area, growing in an above-ground container does not afford the roots the same sort of protection they would have growing in the ground. You may wish to try to insulate the roots in some way. (Keep in mind, too, that if your pot is terra cotta it will be damaged when the moist soil freezes and expands.) Maintaining soil moisture is still very important, too -- wind causes plants to dry out quickly as you've no doubt noticed.
With regard to your particular tree, you should probably plan on eventually moving it to a larger container such as a half barrel. You may find that the tree does break in the wind as poplars are notoriously brittle, if you feel it needs a stake, make sure that the wind blows the tree against the stake rather than away from it. Pruning the top will make it look unnatural as this tree normally grows in an extremely tall columnar form. If you are forced to maintain it at a short height, I would suggest cutting out the leader and allowing multiple leaders to develop from the base. This would however alter the form to more of a shrub rather than a tree.
The tree will be provided with additional nutrients when you repot it; once it is in its largest possible container you will need to replenish the soil about every other year. In addition, follow a regular "maintenance" type low level fertilization program during the growing season. A balanced water soluble fertilizer and perhaps a top dressing of compost would be fine; another option would be the slow release fertilizers. If you can find one including micronutrients so much the better. Some gardeners also report excellent success using a kelp-based formula on long-term container plantings. Trees and shrubs in containers do not need the frequent feedings associated with annual flowers, and in fact overfertilizing would cause the tree to grow even faster -- which is perhaps not what you want. In any case the tree will tell you (by looking unhealthy) if it lacks nutrients.
Good luck with your tree!
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