“How do I prune my hydrangeas?” is a question I get frequently. In my last column, I talked about possible reasons, other than bad pruning, for non-blooming bigleaf (mophead) hydrangeas. Now I’ll turn to the actual pruning guidelines. And if it’s all too much for you, there are hydrangea alternatives that are very easy to prune.
The simplest approach to pruning bigleaf hydrangeas is to do nothing. I have seen some beautiful examples of flowering hydrangeas that obviously have never been pruned – the crowded mass of stems indicates that. However, I’ve also noticed that over the years, the number and size of blooms diminish. So for larger and more abundant flowers, regular pruning is recommended.
When to prune?
My recommendation is mid-spring, after the threat of hard frost, which would be late April or early May. Hydrangeas set flower buds on “old” or mature wood, meaning the buds begin to develop in mid- to late-summer on existing stems but don’t flower until the following summer. By mid-spring, the buds have begun to expand and it is easy to see what has survived the winter, what has not and which buds look the healthiest and plumpest. However, it does require a bit of extra care to not damage or break off the tender buds while you’re working.
The only pruning I would do in late summer or fall is deadheading, or removing spent flowers. If you don’t like the look of the fading brown flowerheads, cut them off just below the set of leaves that often surrounds the flower.
What to prune?
Spring pruning should consist of three steps:
• First, remove stems that are completely dead, as well as stems that are crossing, rubbing or lying flat on the ground. Hydrangea stems can sometimes appear hollow and dead when they’re not, so if in doubt, scratch a bit of bark to see if the cambium just under is green or not. Cut back damaged or broken stems to a set of healthy buds.
• Second, remove a few of the oldest stems completely – cut them off at the base. This helps to maintain the size and form of the shrub and also encourages new stems to grow. Younger stems have more flower buds than older stems.
• Third, tip prune the remaining stems. Hydrangea flower buds form at the terminal end of the stem. However, there are also flower buds present farther down the stem, even to the base, although some of these may not be visible. Look for flower buds that are about 1/2 inch in length, large and plump, ranging from shiny green to reddish brown in color.
If the tip of the stem has died back, as it often does in our winters, prune back to the next set of plump healthy buds. This will allow those lower flower buds to develop.
If the tip of the stem has a healthy bud, you can leave it for flowering. But because it exhibits what is called “apical dominance” over buds lower on the stem, those lower buds will not open. If you remove the tip bud, you remove that apical dominance. This allows more of those lower flower buds to develop fully, which means more flowers.
So how much of that stem to remove? Commercial nurseries, growing for a compact flowering plant that’s easy to ship, prune to half the length of the stem. In the garden, if you tip prune stems back to a pair of healthy buds, you’ll be removing anywhere from about 6 to 12 inches of stem.
If your hydrangea is hopelessly overgrown, you can completely renovate it by cutting all the stems to near ground level in the spring. Hydrangeas resprout freely from the base, so you will get a flush of new stems. They may need to be thinned out some as they develop, and they will not flower until the following year. If you do this, it’s a good idea to apply some slow-release fertilizer to strengthen the new growth.
If this pruning regimen seems like a lot of trouble, try some of the newer “reblooming” varieties of bigleaf hydrangea, such as Endless Summer™, which produce new flower buds all season long on current wood. They will bloom despite winter weather, spring frost or bad pruning.
There are also other types of hydrangeas that are very easy to prune. The native smooth hydrangea, Hydrangea arborescens (‘Annabelle’ or ‘Hills of Snow’), blooms on new wood. It can be cut back to ground level in spring and still put on a terrific show of abundant white flowers in June.
Panicle hydrangea, Hydrangea paniculata, is another hydrangea that blooms later in the summer on new wood; it can also be pruned back hard in the spring. There are many lovely varieties of this durable old-fashioned hydrangea, such as ‘Grandiflora,’ ‘Tardiva,’ ‘Chantilly Lace’ and ‘Limelight.’
Annette MaCoy is the consumer horticulture extension educator and master gardener coordinator for Penn State Cooperative Extension in Cumberland County. If you have garden questions, contact Annette at 240-6500 or by e-mail at firstname.lastname@example.org.