Well there's no guarantees when it comes to keeping tender plants alive during bad winters. I would be sure to mulch your hydrangea at the base with shredded bark or leaves to help insulate the root system. Boxwoods are more difficult because they are evergreen in nature so can dry out from winter winds. With a boxwood, I would consider using a burlap screen around the plant (but not touching) to keep the winds from hitting the foliage). I would also consider using a product such as Freeze Pruf which is designed to protect foliage during cold weather.