The best time to transplant hydrangeas is after the leaves have fallen off. The plants are then dormant and will have a much better chance of survival.
The dormancy period for hydrangeas varies from climate to climate. The dormant period is in early winter, late November or December, but can be as late as January or February if the ground is not frozen. However, the frozen ground makes it difficult to remove the rootball of the hydrangea, an important part of transplanting. Once transplanted, the hydrangea only needs one deep watering until warmer weather. If transplanted during the flowering season, the plants will need watered more often.