Hydrangea paniculata or “panicle” hydrangeas and hydrangea arborescens or “smooth” or “wild” hydrangeas can be pruned in late winter or early spring. I try to attend to mine around St. Patrick’s Day.
Both panicle and smooth hydrangeas bloom on new wood that begins growing in the middle of spring, so before that starts, in Zone 7b, the middle of March is a good time.
Do you have to prune panicle and smooth hydrangeas? No. Should you? Probably. When I bring home one of these varieties, I usually leave it grow for a about 3 years to let it get established. If you don’t prune them, they can get leggy or droopy and the newest and thinest twigs will not support a large bloom very well.
I was taught that if you leave these hydrangeas alone you will get a lot of smaller blossoms. Prune back by one third or even one half,and remove thin, twiggy branches you will be rewarded with fewer, but much larger inflorescence. I prefer the larger blooms.
I know people who have mowed down their smooth hydrangeas, like the popular Annabelle, and they come back! My panicle hydrangeas include Limelight, Little Limelight, Little Quickfire, Bobo, Vanilla Strawberry, Strawberry Sundae, and Pinky Winky. My Smooth hydrangeas include Incrediball, Annabelle and Haas Halo.
So, while it looks drastic, don’t be afraid to cut your panicle and arborescens hydrangea back in March or early April. Remove any small twiggy branches, or any new growth that grows toward the center of the plant. I also remove thin branches that lay along the ground. New wood will grow off of old wood. It is the old wood that will serve as the support system for all the new growth.
As the Master Gardener Minute states, do not prune big leaf macrophylla, serrata or oakleaf hydrangeas. They rarely need pruning and if done for sizing or shaping are best pruned in late summer after blossoms begin to fade.