The main reasons to prune a hydrangea are to improve the appearance of the plant and weed out the dead canes. Pruning hydrangeas is a two-step process, including top-down pruning and cutting back to the ground.
Pruning Dead Canes
When weeding out dead canes, it is best to clip them back entirely to the ground level, according to garden advice columnist C.L. Fornari of the Garden Lady website. Sit on the ground next to the hydrangea bush and cut the canes at the soil level with pruning shears. After weeding out dead canes, determine if the bush needs reshaping. If so, prune one third of the canes at the soil level.
Another way to reshape the hydrangea is to prune the bush using the top-down method. After weeding out dead canes, C.L. Fornari recommends shaping the bush with top-down pruning. Starting at the highest point of the bush, clip the canes just above the second cluster of blooms.
When to Prune
According to North Carolina State University, no set rules indicate the best time to prune hydrangeas. However, clipping off new growth may affect the following year's blooms. C.L. Fornari recommends pruning in the spring, just as the bush begins new growth, because gardeners can easily differentiate between dead and live canes at that time.