To reduce the size of a hydrangea that blooms on new wood, cut off about one-third of each stem in late fall or early spring before it begins to leaf out. If your hydrangea blooms on old wood, prune right after it has bloomed when the flowers are fading.
Hydrangeas that bloom on new wood can be safely pruned in late fall once the plants have gone dormant or in early spring. Next year's flower buds won't be formed until late spring the same year they bloom, so there is no risk of removing the buds if you prune in fall or spring.
Reason #3 – You Have a Panicle or Smooth Hydrangea
Panicle Hydrangeas (H. paniculata) include PeeGees, and Smooth Hydrangeas (H. arborescence) include Annabelles. These new wood bloomers can be pruned to the ground each fall and come back in the spring with lush foliage and beautiful blooms.
Prune back stems to just above a fat bud — called a heading cut — in fall, late winter or spring. These plants have conical-shaped flower heads. I recommend leaving the dry, tan flower heads on the plant to provide some winter interest in your landscape, so I wait to prune these until late winter or spring.
In late winter or early spring, these shrubs can be cut all the way back to the ground. Smooth hydrangeas will produce much larger blooms if pruned hard like this each year, but many gardeners opt for smaller blooms on sturdier stems.
To prepare hydrangeas for winter in colder areas, add a thick layer of mulch around the base of the plants to protect the crown and roots from freezing temperatures.
Bottom-line: Panicle-type hydrangeas should be pruned in late winter or early spring. While they could be pruned in fall, we highly advise waiting until late winter/early spring to reduce risk of injury.
However, stop deadheading hydrangea shrubs in mid to late fall, leaving any spent blooms in place. This not only provides winter interest, but also ensures you don't remove the buds that will become flowers next spring.
While some plants bloom on new growth, others primarily set flower buds on old wood. Regardless, it is best to wait to prune all hydrangeas until spring. In the fall, hydrangeas (and all trees and shrubs) are in the process of going dormant. They do not produce very much new growth until the following spring.
So these shrubs should be pruned immediately after their flowers fade. Conversely, if the shrub flowers on new wood (stems developed during the current season), its buds are set within the season, so the shrub should be pruned in early spring before that new growth emerges.
Old Wood Bloomers
Hydrangeas that bloom on old wood do not need pruning and are better off for it.
Mophead hydrangeas look lovely in the fall when their blue, purple and pink flowers contrast with the fall foliage. Brown flowers can be snipped off if they annoy you, but you don't have to remove them. Look for flowers like this to dry or for making Hydrangea Wreaths.
Regardless of the type of hydrangeas you're growing—and whether it blooms on old or new wood—the cutting process is the same. Start by using clean, sharp shears, then make your cut. "Cut stems above a node, and include at least two leaf groups on the cut stem," says Godshalk.
The primary reasons hydrangeas don't bloom are incorrect pruning, bud damage due to winter and/or early spring weather, location and too much fertilizer. Hydrangea varieties can be of the type that blooms on old wood, new wood or both. Old wood is the current year's growth and new wood is next year's (spring) growth.
After a day or so of being cut, the blooms begin to wilt. About three days later, they are completely done. But guess what I just learned (from an instagram conversation)? Hydrangeas take in water from their PETALS!
Without going through the deadheading process, hydrangeas will not produce as many flowers and the few produced may not grow as big to their full potential.
"Stop deadheading in the fall, when bigleaf hydrangeas produce their last flush of flowers, to enjoy the dried blooms throughout the winter," she says. "These can be removed to help produce healthy buds in the spring."