Hartley Magazine

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Hardy enough for you?

Panicle hydrangea (Hydrangea paniculata, shown with purple coneflowers) blooms reliably in Chicago, because it doesn’t form its flower buds until winter has passed.

In the hottest July on record, you might expect me to be sweating about watering my plants enough so they can keep cool. But thunderstorms have been taking care of that. Instead, what I’m steamed about is winter hardiness. Brrrr!

The issue of hardiness has been on display the summer in my hydrangeas. I have a number of varieties of bigleaf hydrangea (Hydrangea macrophylla). These are the ones with the big, round flower clusters, blue in acidic soils out East but pink in the alkaline soils of the Midwest. With all the rain this spring and summer, the plants have been unusually tall and lush, with broad, bright green leaves–and not a single bloom.

I know why: a three-day blast of cold air back in January, when it was 18 degrees below zero F (-28 C), which is pretty cold even for Chicago. It froze the buds right off my bigleaf hydrangeas and killed them almost to the ground.

Technically, I guess you could say these plants are hardy—their roots survived, after all, and they’ve put out plenty of stems and leaves. But bigleaf hydrangeas produce their flower buds in fall and must preserve them until the following summer. Those buds may be sufficiently insulated for winters in Korea and Japan, where the plants are native, but they don’t have the kind of outerwear you need for a Chicago January. Most years, only a few buds survive; this year, none.

Meanwhile, across the yard, other species of hydrangea are blooming their heads off. The big, round, white flower clusters of smooth hydrangea (Hydrangea arborescens) have been especially stunning this year. The pointy, rocketship blooms of panicle hydrangea (Hydrangea paniculata) are just getting started.

Why have these shrubs able to bloom despite that harrowing winter blast? Because unlike their cousin, the bigleaf hydrangea, they wait to develop their flower buds until spring. Winter cold may cause a few twigs to die back, but it can’t harm flower buds that don’t exist yet.

Bigleaf hydrangea (Hydrangea macrophylla) is spectacular when it flowers. But it’s unreliable because the flower buds, formed in early fall, often don’t survive cold winters.

That difference in timing makes smooth hydrangea and panicled hydrangea truly winter-hardy for the purposes of my garden: They not only survive the winter but bloom every year. Bigleaf hydrangeas only bloom when we luck out and get a mild winter, so for practical purposes, they’re not hardy, even if their roots survive.

A reasonable person might ask why I have three or four bigleaf hydrangea shrubs in my garden when I know perfectly well they’re too tender. I don’t have a good answer to that, except when they do bloom, they’re so spectacular.

Maybe this fall I’ll stop clinging to those memories, yank up the plants, and give the space to something I can count on. Or maybe not.