If a snowball hydrangea traveled to the large town, slimmed down and dressed in frilly white, it may resemble a “Unique” hydrangea (Hydrangea paniculata “Unique”). “Unique” is an elegant tree, using its elongated 8-inch blossom clusters that open in the shade of pale cream and mature slowly into seashell pink. The showy “Unique” hydrangea is also the hardiest of its species, however, thriving in U.S. Department of Agriculture plant hardiness zones 3 though 8. The quantity of pruning required depends upon the way you want the plant to look. If you don’t need a big bush, you will have to prune this tree early and often because it can grow quickly to 10 feet high and wide.
Snip off the dead blossoms as they begin to fade if you like a tidy garden. Make the cut just below the flower head.
Cut off any dead, broken or diseased stems as soon as you become aware of these. Use garden clippers, and wipe the blade between cuts with a cloth dipped in denatured alcohol. Remove all dead stems at their point of origin. Cut broken or diseased hydrangea stems at least 6 inches into wood.
Prune “Unique” hydrangeas otherwise in late winter or early spring before the leaves appear. All these shrubs blossom on the current year’s increase, so you can prune them anytime from late summer until early spring. But it’s easier to see what you’re doing when the leaves have dropped.
Prune out some of the oldest canes at ground level each spring if you want the plant a little shorter and slimmer. Select the taller canes to accomplish gentle size reduction. The remaining stems will become slimmer and produce more leaves and blossoms.
Cut the plant to the ground if your “Unique” hydrangea is overly tall or producing small blossoms. If you remove each stem at ground level during dormancy, the tree will create much larger, more vigorous blossoms. Take care, because the abundance of flowers might be sufficient to tip the slim new canes.
Cut the hydrangea stems back hard, alternatively, but leave a foundation frame of 18 to 24 inches of canes. “Unique” hydrangeas will expand somewhat taller with this frame intact, but they will not flop over in a heavy rain.