Don’t let the science of pruning hydrangeas scare you

If I had to guess what the most frequently asked gardening question is, I’d say it’s “How should I prune my hydrangeas?”

It’s a topic I’ve covered in this space before and that I believe is generally made out to be far more complicated than it needs to be, but now there is a new wrinkle.

Before we confuse the issue, let’s break down the hydrangea pruning basics. For panicle hydrangeas (Hydrangea paniculata), which include varieties like Limelight, PeeGee and Quick Fire, you should prune about a third of the plant, removing any spindly stems skinnier than a pencil  and cutting to just above a leaf. Any time between now and when leaf buds start opening is fine, and if you need to cut back more, an established panicle hydrangea will tolerate it just fine.

With the native smooth hydrangea (H. aborescens), the most well-known of which is Annabelle, conventional wisdom holds that you can prune to different heights, including as low as 6 inches any time from fall to early spring.

Both of these varieties bloom on the current year’s growth, so pruning in late winter or early spring will not negatively impact flowering. The same is not true of mountain hydrangeas (H. serrata) or big leaf hydrangeas (H. macryophylla), such as Endless Summer, which shouldn’t be pruned other than to remove dead stems. If necessary, a light pruning can be done immediately after they flower.

The aforementioned wrinkle applies to smooth hydrangeas, which are prone to flopping when their large flowers get wet. The affliction applies to even newer varieties such as Incrediball, which were bred for sturdier stems. Various remedies for floppy stem syndrome include pruning at different heights, growing them through a staking grid or planting them inside a sturdy boxwood hedge to hold them upright.

As part of its trial of smooth hydrangeas, the Mt. Cuba Center botanic garden in Delaware trialed multiple pruning methods and tracked them over several years, and what it found was a whole new method of pruning. The trial found that plants that were cut back to between 6 and 8 inches remained more compact, bloomed a couple weeks later and had fewer but much larger flowers and were more sun tolerant than unpruned plants.

But there was no difference in stem strength and they were just as prone to flopping. And, even worse, after three years of this treatment, plants seemed to suffer from burnout and produced fewer flowers.

The happy medium for smooth hydrangea pruning, according to the Mt. Cuba Center, is a hybrid approach of cutting back up to one-third of the oldest stems all the way to the ground each year. This keeps the plants flowering well and slightly more compact and leaves a structure of old stems that can help support flowers.

And if all that makes your head spin, just know that the best thing about smooth hydrangeas is that no matter what pruning approach you take, you’ll still get flowers, and in that regard they offer the best solution to that age-old question of how to prune hydrangeas — any way you want.