Getting hydrangeas to bloom is a risky business

Every June, some gardens are bedecked with the glorious, huge, fluffy pink blooms of big-leaved hydrangea. And some aren’t.

“‘Why didn’t my hydrangea bloom?’ is one of the questions we get most often at this time of year,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at the Morton Arboretum in Lisle. The answer lies in the fact that there are several species of hydrangea shrubs, and some are more winter-hardy than others.

Panicled hydrangea (Hydrangea paniculata) and wild hydrangea (Hydrangea arborescens), which are both common in Midwestern gardens, bloom reliably because they wait until spring, after the threat of damaging cold is past, to develop their flower buds.

Big-leaved hydrangea (Hydrangea macrophylla), which is also popular, is different. It blooms on old wood, meaning that it forms its flower buds on existing stems in autumn and they must survive a grim Chicago winter in order to bloom the next summer. That’s a risky business.

“It just takes one bitter cold night to kill the flower buds,” Yiesla said. “Sometimes entire stems are killed, and the whole shrub dies back down to the ground.”

The roots, insulated by soil, will usually survive, and the hydrangea shrub will sprout leafy new stems in spring. But those new stems will have no flower buds.

In USDA winter hardiness Zone 5, which includes most of the Chicago area, and in the colder Zones 4 and 3, big-leaved hydrangeas are not considered hardy — at least not for flowering purposes. “If we get a mild winter, they may bloom. But you can’t count on it,” Yiesla said.

Yet gardeners just love those fluffy blooms, so plant breeders have tried to provide them to cold-climate gardeners by introducing a number of cultivated varieties, or cultivars, that bloom on both old and new wood. They still develop some flower buds in fall, but they also develop buds on new stems that sprout in the spring or even later in the summer.

“The idea is that even if the overwintering buds are killed, the plant still is capable of producing some flowers,” Yiesla said.

The first cultivar to make this claim, Endless Summer (Hydrangea macrophylla ‘Bailmer’), has turned out not to bloom consistently in Chicago, she said. Other reblooming cultivars that seem to be more reliable bloomers include Blushing Bride and Twist-n-Shout big-leaved hydrangeas.

“They are good replacements for Endless Summer,” she said. “Or try one of the pink-blooming cultivars of Hydrangea paniculata or Hydrangea arborescens.”

Ah, yes, pink. That’s the other common question about big-leaved hydrangeas: Why are the blooms pink instead of blue? Why don’t we have the blue hydrangeas that are an iconic flower in garden books, magazines and websites?

“It’s because of our alkaline soil,” Yiesla said. In the Northeast (where most of those books and magazines are published), the soil tends to be acid, which has the effect of making big-leaved hydrangeas bloom in blue. In the Chicago area and the Great Plains, where most soil is much more alkaline, the blooms will be pink. “That’s just how the biochemistry of this particular species works,” Yiesla said.

It’s possible, with considerable effort, to use sulfur additives to lower the pH of the soil in which a big-leaved hydrangea is growing to encourage it to have blue flowers. “It’s a whole lot easier just to enjoy the pink,” she said.

Other hydrangeas to consider for Chicago gardens include climbing hydrangea (Hydrangea petiolaris) and oak-leaved hydrangea (Hydrangea quercifolia), which has lovely fall color and reddish peeling bark that adds winter interest. Both species bloom on old wood, however, which means their overwintering flower buds can be killed by winter cold.

You can see most of these hydrangea species — but not big-leaved hydrangea — in the Grand Garden at the Arboretum.

For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.