Getting hydrangeas to thrive in the Chicago area is risky

Getting hydrangeas to thrive in the Chicago area is risky

Every June, some gardens are adorned with gorgeous, huge pink hydrangeas with large leaves. Some are not.

“‘Why isn’t my hydrangea blooming?’” “It's one of the questions we get a lot this time of year,” said Sharon Yesla, a plant knowledge specialist at the Plant Clinic at Morton Arboretum in Lisle. The answer lies in the fact that there are several types of hydrangeas. Hydrangeas, some of which are more winter hardy than others.

panicked hydrangea (Hydrangea paniculata(and wild hydrangea)Hydrangea bushes), both common in Midwestern gardens, bloom reliably because they wait until spring, after the threat of damaging cold has passed, to develop flower buds.

bigleaf hydrangea (Hydrangea macrophylla), which is also famous, different. It blooms on old wood, meaning it forms its flower buds on existing stems in the fall and must survive Chicago's harsh winters in order to bloom next summer. This is risky business.

“It only takes one very cold night to kill flower buds,” Yiesla said. “Sometimes entire stems are killed, and the entire bush dies back to the ground.”

Roots rooted in soil usually survive, and your hydrangea bush will sprout new, leafy stems in the spring. But those new stems will not have flower buds.

In USDA winter hardiness zone 5, which includes most of the Chicago area, and in colder zones 4 and 3, largeleaf hydrangea is not considered hardy—at least not for flowering purposes. “If we get a mild winter, it might bloom. But you can't count on it,” Ysla said.

However, gardeners love those delicate flowers, so plant breeders have tried to make them available to cool-climate growers by introducing a number of cultivated varieties, or cultivars, that thrive on both old and new wood. They still develop some flower buds in the 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 is still able to produce some flowers,” Jesla said.

The first cultivar to make this claim is Endless Summer (Hydrangea macrophylla Bellmer, it turns out, doesn't always thrive in Chicago, she says. Other blooming varieties that seem more reliable include the Blushing Bride and Twist-n-Shout large-leaf hydrangea.

“They are good alternatives to Endless Summer,” she said. “Or try one of the pink flower varieties Hydrangea paniculata or Hydrangea bushes.

Oh yes, pink. This is another common question about large-leaf hydrangeas: Why are their flowers pink instead of blue? Why don't we have the blue hydrangea that is featured in garden books, magazines and websites?

“This is because of our alkaline soil,” Yesla said. In the Northeast (where most of these books and magazines are published), the soil tends to be acidic, which results in blue large-leaf hydrangea blooms. In the Chicago area and Great Plains, where most soils are more alkaline, the flowers will be pink. “This is how the biochemistry of this particular species works,” said Yiesla.

It is possible, with great effort, to use sulfur additives to lower the pH of the soil in which your bigleaf hydrangea grows to encourage it to have blue flowers. “It's a lot easier to enjoy pink,” she said.

Other hydrangea plants to consider in Chicago gardens include climbing hydrangea (Hydrangea petularis) and oak-leaf hydrangea (Hydrangea quercifolia), which has beautiful fall color and exfoliating, reddish bark that adds winter interest. However, both species thrive on old wood, which means that overwintered flower buds can be killed by the winter cold.

You can see most of these hydrangeas — but not the bigleaf hydrangea — in the arboretum's Great Garden.

For advice on trees and plants, contact the Plant Clinic at The Morton Arboretum (630-719-2424, Beth Potts is a staff writer at The Arboretum.

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