Some hydrangea species bloom on last-year's growth, which is often called “old wood.” This old wood contains the flower buds that will open with the coming summer. Pruning in winter or spring would remove the flower buds, leaving us with a bloomless plant this year.
Cut these stems close to the ground. Remember buds for blooms are produced on old wood and the more old wood you remove the less floral display in the spring and summer. 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.
You can tell dead hydrangea branches from live ones by the color and denseness of the wood. Dead wood will look more gray and be lighter and less dense than live wood. There's no need to worry about getting rid of any dead wood now.
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.
If you don't prune hydrangeas then they can eventually resemble a tangled mass of woody stems, and the flowers will become smaller and less showy.
Check your plant when it's blooming; you should be able to see and feel a bit of difference between old wood and new wood. Old wood should be stiffer, tougher, thicker, and more grey or brown, while new wood will be much more flexible, soft, and should still have a greenish tinge.
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 do not require strict reqular pruning; simply keep them healthy by removing dead wood and they will grow and flower well.
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.
When you deadhead hydrangeas, you aren't harming the plants at all. Removing the spent blooms triggers flowering shrubs to stop producing seeds and instead put their energy toward root and foliage development. This makes plants stronger and healthier, so by deadheading, you'll be doing your hydrangeas a favor.
Incorrect Temperatures or Zone
Hydrangeas are somewhat hardy plants but don't handle extreme cold well. For species that bloom on old wood, extreme cold over winter will kill off the buds, preventing any blooms from forming the following season.
Some hydrangeas change color based on pH; some hydrangeas bloom on fresh growth from this year called new wood, and some flower on the growth they put out last year called old wood. It all depends on the species of hydrangea, so it's helpful to know what you have and keep the plant info card handy.
But when to prune them? Prune fall blooming hydrangeas, or old wood bloomers, after they bloom in the summer. If you prune old wooded hydrangeas in fall, you are cutting off next seasons blooms. Summer blooming hydrangeas, or those that bloom on new wood, are pruned in the fall, after they stop blooming.
Old Wood Bloomers
Hydrangeas that bloom on old wood do not need pruning and are better off for it.
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.
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.
Smooth hydrangeas bloom on new wood. Plants are commonly cut back to the ground in late winter to early spring to encourage abundant blooms and maintain plants at a manageable size.
Generally, it is safe to prune as much as one-third off of your hydrangea that blooms on new wood.
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.
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.