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.
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.
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.
New Wood Bloomers
It is easy to grow these hydrangeas because they bloom every year regardless of how they are cared for or treated. They can be pruned to the ground in the fall and they will emerge in the spring with bountiful blooms.
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.
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.
Wrap with winter protection.
During the winter, make a DIY A-frame wire cage out of burlap and chicken wire to protect your hydrangea plant. This cage will protect it from winter winds, heavy snowfall, and cold temperatures.
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.
Mulch Matters
Weird winters with little snowfall and drastic temperature swings are detrimental to plants. To give hydrangeas their best chance at success, apply a layer of chunky mulch around the base of the plant. Decorative mulch is helpful but we recommend straw, marsh hay, or fallen leaves.
Oakleaf hydrangeas, Hydrangea quercifolia: This native hydrangea features cone-shaped white blooms that turn a beautiful shade of russet in late summer. It, too, blooms on old wood so should not be pruned until after flowering.
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.
Hydrangeas that bloom on new wood require pruning in late winter or early spring. Prune to shape, cutting back to about two feet. The pruning promotes new, sturdy growth, which provides the blooms next season.
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.
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.
"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."
Smooth Hydrangeas should be pruned in late winter/early spring, and they can be cut down to the ground if you wish. Pruning will encourage new growth in these varieties and can help increase flowers.
If it's blue, or blooms in summer
Most of the other hydrangeas should be pruned in summer, once they have finished blooming. Most of these bloom on what's called "old wood" — growth from the year before. If you prune them in early spring, you risk cuting off the dormant flower buds.
A popular cultivar of the repeat bloomers is 'Endless Summer' but there are many newer repeat blooming cultivars available in the trade. Sometimes bigleaf hydrangeas become overgrown and need to be trimmed. However, too much pruning will greatly reduce or eliminate flowering.
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.
Many customers ask why their hydrangeas aren't blooming. 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.
Some gardeners report success in turning their hydrangeas blue by applying coffee grounds to the soil. The coffee grounds make the soil more acidic, allowing the hydrangea to more easily absorb aluminum.