Hydrangea paniculata ‘Limelight’ pruned to form a tree. Photo: http://www.fast-growing-trees.com

Question: I have a hydrangea tree, but its branches are too long, so when it rains, the branches and flowers bend down to the ground. Can I cut branches back and, if so, at what period?

J.Y. Simard

Answer: Sure you can prune it, but to understand how and why, it is worth explaining what a hydrangea tree is.

A Real Shrub With a Radical Bod Mod

A hydrangea tree is in fact a panicled or PG hydrangea (Hydrangea paniculata) specially pruned to take on a treelike shape. In the summer, it forms at the tip of its branches large elongated clusters of white or lime-green flowers, depending on the cultivar, which gradually become pink or even reddish through the fall. It’s pretty much the only hydrangea used this way: most other species don’t have stems strong enough to make good trees.

Normally, the panicled hydrangea grows as a large multi-stemmed shrub. It produces a profusion of branches from its base and takes on an upright, spreading shape over time, reaching up to 15 feet (5 m) in height and diameter after 15 to 20 years. It’s clearly a shrub; in its natural form, it looks nothing like a tree.

To form a hydrangea tree, the nurseryman chooses a young specimen with a robust central branch and begins to “prune it into submission.” He removes any other branches and also secondary branches that grow on the branch selected to be the future trunk. Staking is often needed to keep the trunk rigidly vertical during the first few years. When the trunk has reached the desired height (usually about 4 or 5 feet/1.2-1.5 m), he begins pruning the top of the plant too. That forces the hydrangea to produce multiple branches at the top of a central stem: its new trunk.

And there you go! A hydrangea on a trunk, somewhat like a living lollipop!

Producing a hydrangea tree is time-consuming and adds greatly to the cost of the plant, so it’s quite a pricey item. 

You can also prune a hydrangea into a tree shape on your own if you want, but it will take a few years. 

Regular Pruning Is Required

If you want to maintain a hydrangea tree, you’ll need to be ready to prune it regularly, otherwise not only do the branches lengthen to the point they start to bend under their own weight, as you have noticed, they can even snap off, especially under the additional weight of heavy snow.

In your case, since your hydrangea tree is overgrown and the branches already bend under their own weight, it would be wise to prune it before winter. If not, there is a risk that they snap off in snowy or icy weather, 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. That’s because the dried flower heads of the panicled hydrangea have ornamental value over the winter, so you’ll want to keep them until spring.

Therefore, every year, make a habit of cutting back all the branches 6 to 8 inches (15 to 20 cm) from the trunk early in the spring, before the leaves unfurl. This will give your tree the appearance of a ball of foliage capped with flowers at the top of a short trunk, usually the desired effect. Since panicled hydrangeas flower from new growth produced starting in mid-spring, it will bloom abundantly even after a severe early spring pruning.

To maintain its treelike appearance, with a well-defined “trunk,” also remove any suckers that appear at the base of the plant as well as any growth that appears on the trunk itself. You can do this kind of pruning in any season.

Hydrangeas for Laidback Gardeners

Giving any living plant an unnatural shape is always going to require extra maintenance, so might I suggest that the more laidback gardener might prefer to abstain from hydrangea trees and grow them instead the way they want to grow, as large shrubs. Give the plant lots of room (most panicled hydrangeas are very big plants) and just let it do its thing. Some, like Pinky Winky (‘Dvppinky’) have naturally sturdy, erect stems that don’t tend to flop, while the old-fashioned but once very popular PG hydrangea (H. paniculata ‘Grandiflora’), is huge and floppy and—what can I say?—just gorgeous even as it splays out over half your garden. If space is at a premium, there are now smaller panicled hydrangeas, like ‘Bobo’ and ‘Little Lamb’ that won’t take up as much room. And flower color, panicle size and flowering season also vary from one cultivar to the next. It’s up to you to choose the panicled hydrangea cultivar that best corresponds to your tastes and your needs!

Panicled hydrangeas grow best in full sun or very light shade in just about any well-drained soil. They’re cold hardy and adapted to zones 3 to 8.