When to Prune Which Hydrangea Species

They have a long bloom period, do not have a lot of pest problems (deer may be the worst), and the flowers are attractive even after they are done blooming. They also make good dried flowers. And new hydrangea cultivars are introduced every year, increasing the palette of possibilities for gardeners.

One of the more frequent calls that our Penn State Master Gardeners get on the Garden Hotline is, "Why doesn't my hydrangea bloom?" Chances are, you might also get that question when working on customers' properties. The answer starts with which species of hydrangea your customer is talking about.

Bigleaf hydrangea (Hydrangea macrophylla) is one of the most popular. They are grown for their pink or blue flowers, depending on the availability of aluminum in the soil, which is dependent on soil pH. There are white-flowered bigleaf hydrangeas whose flower color does not change with the soil pH – they are always white. They may have mophead flowers – the big, showy puffballs of sterile flowers, or they may have lacecap flowers – fertile flowers in the center surrounded by showy, sterile flowers. They begin setting next year's flowers when they finish blooming; that is, they bloom on old wood.

This has been a problem in the past because while the plants are winter hardy in Zones 6–9, they can be killed to the ground in a cold winter, which means no blooms the following summer. Even a moderately severe winter can kill or damage the flower buds. If they are pruned in fall or heavily browsed by deer, the result is the same—few or no flowers.

That all changed with the discovery of a bigleaf hydrangea that flowers on old and new wood in 1998. After much study and breeding work by Bailey Nurseries and Dr. Michael Dirr, Emeritus Professor of Horticulture, University of Georgia, Endless Summer^™ hydrangea was introduced in 2004. Bailey has introduced other bigleaf hydrangeas with the same flower power since then, including Twist-n-Shout®, BloomStruck®, Summer Crush™, and Blushing Bride.

Oakleaf hydrangea (Hydrangea quercifolia) is hardy in Zones 5–9 and is less likely to be damaged by winter's cold. But they also bloom on old wood, so if they are pruned at the wrong time or browsed by Bambi, flowering is reduced.

Smooth hydrangea (Hydrangea arborescens) blooms on new wood, which means they bloom on the current season's growth. While they may be browsed by deer, they can be cut to the ground in late winter or very early spring and still produce a decent show of blossoms. The straight species is native to much of the eastern United States and blooms with lacecap-type white flowers. There are named cultivars, such as 'Annabelle', that have large mophead-type white flowers. More recent introductions have pink flowers, such as Invincibelle® Spirit.

Finally, peegee hydrangea (Hydrangea paniculata) also blooms on new wood and can be cut to the ground in late winter or very early spring. They can be grown as shrubs or trained to a single-trunk tree form. While nothing is deer-proof, peegee hydrangeas do not seem to be browsed as often as other species of hydrangea.