Smooth hydrangea, Hydrangea arborescens, can be pruned hard in late winter when it is dormant. To rejuvenate an overgrown bush, cut 1/3 of the largest, oldest branches to the ground each year. This type of hydrangea also responds well to being entirely cut down to several inches before new growth begins. https://hortnews.extension.iastate.edu/2017/03/pruning-hydrangeas
Answered on August 11, 2018
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