Answer: Hydrangeas can be grown year round in containers, but in your climate it would be nearly impossible to keep them outside all year. Unfortunately, in your zone (6A or the coldest part of zone 6) it is somewhat difficult to grow hydrangeas successfully in the ground due to winter cold. In a container, the winter cold is even more damaging to plants because the roots are not as well insulated as they would be when planted in the ground.
So you would need to grow one of the hardier varieties, possibly one that can bloom on both old and new wood such as "Endless Summer" and take special measures for winter protection. You would need to insulate the roots, and keep the plant shaded in the winter to try to prevent the soil temperature from oscillating. Sometimes it works to heap mulch around the container and then wrap the top of the plant in burlap, stuff that with a dry insulating material such as straw or oak leaves. Cover that in plastic to keep it dry but leave gaps to allow air to circulate and prevent condensation. You would also need to shelter it from wind. And, check the soil and make sure it is slightly moist any time it is not frozen.
During the growing season hydrangeas require some direct sun to bloom, but you would want to avoid excessive heat caused by sun reflecting off the building as could happen on a southwestern exposure. You may have to experiment and see if you can make this work. Good luck with your project!
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