The huge, pink mophead blooms of bigleaf hydrangea are one of the joys of summer for Chicago gardeners. What a disappointment when those blooms fail to fluff!
“Bigleaf hydrangeas just aren’t entirely reliable in our climate, because sometimes the flower buds don’t make it through the winter,” said Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle.
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Buds of bigleaf hydrangea (Hydrangea macrophylla) are formed in late summer and must survive all winter to bloom the following summer. “They aren’t especially well protected, so even a single day of severe cold is enough to kill them,” Janoski said. Dead buds will appear blackened, and they will never open. The hydrangea may produce abundant green leaves in spring but no blooms.
Gardeners can be confused because some other familiar species of hydrangea do bloom reliably: smooth hydrangea (Hydrangea arborescens), including the popular Annabelle cultivated variety, and panicle hydrangea (Hydrangea paniculata), including the Tardiva variety. “These hardy hydrangeas don’t form their flower buds until spring, so the buds are never at risk from winter cold,” Janoski said.
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In gardening parlance, shrubs that set their flower buds on stems formed in the previous year and overwinter them are said to “bloom on old wood.” Those that grow their flower buds on new twigs in the spring or summer of the same year they bloom are said to “bloom on new wood.” Roses are a classic example.
[ Garden guide: Everything you need to know about growing hydrangeas ]
A number of hydrangea species are iffy because they bloom on old wood, such as oakleaf hydrangea (Hydrangea quercifolia), climbing hydrangea (Hydrangea petiolaris) and mountain hydrangea (Hydrangea serrata).
In some climates with more equable winters, such as the Atlantic states where weather is moderated by the nearby ocean, these plants’ buds are less likely to be killed. The volatile and often harsh Chicago climate is more risky for them. After a mild winter they may bloom abundantly, but after a cold winter — or even a mostly mild winter with a bud-killing cold spell — they may not bloom at all.
To add to the confusion, some newer varieties of bigleaf hydrangea, such as the Endless Summer and Let’s Dance series, and mountain hydrangea, such as the Tuff Stuff series, have been bred and selected to bloom to varying degrees on both old and new wood.
“Even if the buds on old wood are damaged over the winter, these varieties can grow some new wood in spring that will set flower buds,” Janoski said. “You may not get as many blooms as you would after a mild winter, but you’ll likely get some.”
There’s another common reason that hydrangeas fail to bloom: “Sometimes the flower buds get cut off when the gardener is pruning in winter,” she said.
If you prune a plant in winter that blooms on old wood, such as bigleaf hydrangea, forsythia, lilac, mock-orange, or ninebark, you’ll be removing the flower buds along with branches. “To be safe, wait to prune a spring- or summer-flowering shrub until after it has bloomed,” Janoski said.
Plants that bloom on new wood, such as smooth hydrangea and panicle hydrangea, can be pruned in winter with impunity.
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How can you know what to do? “Identify your plants,” she said. It’s not enough just to know you have a hydrangea, because different species of hydrangea grow differently and need different care. “You need to find out exactly which species of hydrangea you have.”
The Plant Clinic can help you identify your plants and find out how to care for them so you get the most beauty and satisfaction from your garden.
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.
