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When to prune shrubs that bloom in spring, summer — and why hydrangeas are

By Beth Botts, Morton Arboretum Chicago Tribune | May 29, 2021

When to prune shrubs that bloom in sprin
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Spring-flowering shrubs such as lilacs should be pruned soon after they finish blooming in order to maximize next year’s flower show. (Beth Botts/Morton Arboretum)


As summer approaches, it’s time to finish pruning trees and shrubs that flowered in spring.

“If you wait too long after they bloom, you’ll diminish next year’s flower display,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum.


Most spring-blooming trees and shrubs, such as flowering crabapple, lilac, magnolia, viburnum, forsythia, ninebark, mock orange, rhododendron and witch-hazel, begin forming next year’s flower buds within a few weeks after they finish flowering. If you prune them after those buds have formed, you will be cutting off potential flowers.


To maximize their bloom, a simple rule is to prune all spring-blooming shrubs within two to three weeks after they finish flowering. Other shrubs that don’t flower in spring are often pruned in winter when they are dormant.


The difference in timing is due to the plants’ biology. In order to be ready to flower in spring, many species of trees and shrubs prepare the year before. They begin forming their flower buds soon after blooming and carry them through the winter. These plants are said to flower on “old wood” — twigs that were new the previous year and are now in their second year.


Most shrubs that bloom in summer flower on “new wood” — twigs that are new this year. Because of their later bloom time, these plants have time to start from scratch in spring. They include roses, summer-blooming spirea, and rose of Sharon.


Since they don’t have flower buds in winter, pruning these shrubs during cold months while they are dormant will not diminish their summer bloom.


Some shrubs are sold with labels that say when they should be pruned. Otherwise, the best clue is the bloom time. Prune spring-flowering shrubs shortly after they finish flowering. Prune most plants that flower in June or later any time, including the winter dormant season.


Hydrangeas are a tricky exception. They seem to cause the most confusion. That’s because although all hydrangea species bloom during the summer and have similar flower clusters, some species bloom on new wood and some bloom on old wood.


Oakleaf hydrangea (Hydrangea quercifolia) flowers on old wood. So does big-leaf hydrangea (Hydrangea macrophylla), which has large globe-shaped flower heads that can be blue in acidic soil. Because it is not quite hardy here and its flower buds often do not survive cold winters, big-leaf hydrangea can fail to bloom and be a disappointment to Chicago gardeners. To add to the confusion, some newer cultivars have been bred to bloom on both old and new wood.


Other popular hydrangea species bloom on new wood. They include smooth hydrangea (Hydrangea arborescens), including the popular Annabelle cultivar, and panicle hydrangea (Hydrangea paniculata), including the Limelight cultivar. Because they form their flower buds in spring, they can be pruned in winter with no loss of bloom.


To know when to prune a hydrangea, you need to know which species you have. This can be difficult, since many popular hydrangeas are sold under trademarks such as Endless Summer or Quick Fire rather than the formal names of the cultivated varieties, which would indicate the species.


Sometimes you really have to hunt on a plant label to find the actual species and cultivar name. The Plant Clinic can also help you identify hydrangeas or other shrubs, working from photographs and other clues.


What if your early-blooming forsythia or witch-hazel needs pruning, and you didn’t get it done promptly? Pruning the plant now won’t do it any harm. It just won’t have as many flowers in spring as it might have had. If you prune at the right time next spring, you’ll see more bloom the following year.


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









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