To rejuvenate the hydrangea, remove up to 1/3 of the older living stems down to the ground each summer. This will revitalize the plant. If necessary to control the size of the plant, cut back before late July to allow for buds to develop. Usually the plant will return immediately to its former size.
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.
Generally, it is safe to prune as much as one-third off of your hydrangea that blooms on new wood.
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.
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.
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.
Hydrangeas do not require strict reqular pruning; simply keep them healthy by removing dead wood and they will grow and flower well.
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.
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.
Old Wood Bloomers
Hydrangeas that bloom on old wood do not need pruning and are better off for it.
So the bottom line is that if a hydrangea is too large for the location where it has been growing, the best thing would be to move it and plant a smaller variety in that spot.
So, in October or November, simply cut all branches back to about 6 to 8 inches (15 to 20 cm) from the top of the trunk. Once you've gotten your hydrangea tree back into a more winter-resistant shape, start pruning early in the spring rather than the fall.
Big Leaf Hydrangeas, Hydrangea maculata, do not tolerate frosts well and should be covered to prevent leaf damage and flower loss.
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.
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.
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.
It's best to start adding coffee grounds to the soil months before the blooming season begins, ideally in the late fall. You can repeat the process with your typical fertilizing schedule. With a little caffeine and a lot of patience, your hydrangeas should reward your efforts come spring with the boldest blue globes.
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.
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.
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.