Gardening Home: Hydrangea pruning
The park gives children hands-on experience
After an effort by volunteers and businesses this spring, the large courtyard at Goode K-8 School in York became a place for children to experience nature, grow food and learn about wildlife.
Frequently asked gardening questions are: “Why didn’t my hydrangea bloom this year?” The most likely answer is timely pruning. So when is the right time to prune your hydrangea? It generally depends on when the plant sets flower buds, and is determined by whether the hydrangea blooms on old wood or new wood.
There are five types of hydrangea found in American gardens:
- Large leaf hydrangea (H. Macrophila) Both Mauvehead and Lacecap, bloom on old wood, usually blue or pink, sometimes white;
- Climbing hydrangea (H. Anomalies) It blooms on old wood and has white flowers;
- Oakleaf hydrangea (H. cercifolia) blooms on old wood, white flowers, leaves that resemble oak leaves;
- Panic hydrangea or Peegee (H. paniculata) blooms on new wood, white flowers;
- Soft hydrangea (H. afforestation) It blooms on new wood and has white flowers.
Another class of large hydrangea is the modern repeat-flowering hydrangea in the group called ‘Endless Summer’. These mopheads thrive on the current season’s growth and last year’s growth. If the buds are killed by frost or pruned at the wrong time, the buds will regenerate and bloom as usual.
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If you know what species are in your garden, you’re well on your way to knowing when to prune. If you don’t know which type you have, consider when your hydrangea blooms. Those that bloom on old wood usually begin in early summer and finish by mid-summer (sparse flowers may appear then). They form flower buds the next year in late summer or early fall. Therefore, to reduce the risk of removing these buds and next year’s flowers, prune when the flowers begin to wilt, in June or July, before “old wood” develops in late summer and fall. The earlier you prune after flowering, the faster the shrub can recover, producing larger and larger flowers the following season. Pruning from late fall forward will limit flowering.
Oakleaf hydrangea flowers are produced on old wood. It blooms in June, but the flowers remain throughout the summer, gradually changing from white to pink, and finally to tan in winter. Pruning them in July means cutting off the flowers but pruning them in late fall means cutting off next year’s flowers, so prune only what is necessary to maintain size and shape after flowering.
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Because they need to grow and form buds in the same year they bloom, hydrangeas that flower on new wood generally begin blooming later than those on old wood. Flowering begins in mid-summer and continues until the first frost. Although they should be pruned before breaking dormancy in the spring or while dormant in the fall, they tolerate if pruning is not done at a certain time, as long as you avoid pruning when the flower buds are opening. Both panicked hydrangeas and smooth hydrangeas, including ‘Annabelle’ and ‘Samantha’, flower on new wood, so they can be pruned back heavily after a fall frost and still bloom the following summer. In fact, some gardeners cut ‘Annabelle’ to the ground each winter in order to have flower heads on three to four feet straight the following summer.
Smooth hydrangeas will produce much larger flowers if pruned heavily each year, but because larger flowers may fail, many gardeners choose smaller flowers on sturdier stems. To reduce overhang, leave a frame for old growth by cutting the stems to a height of 18 to 24 inches. This provides a strong framework to support new growth. Cut these stems in late winter before new growth begins.
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Not all hydrangeas need pruning, but many do. As hydrangeas become old and woody, they can produce smaller flowers. Regular removal of a few of the oldest canes at the soil line can keep the shrub producing large, abundant flowers. Keep the shrub from becoming too tall by removing the tallest canes. Shape or thin out the shrubs by taking no more than a third of the length of the oldest branches each year.
Another problem when hydrangeas don’t bloom is that they may have been pruned too much the previous year. If they are pruned too much in the summer, they tend to die back more than they normally would, and you will end up waiting a year for them to flower. If your hydrangea dies during harsh winters, trim back dead branches in early spring when you can see where the dead wood is. This will prevent accidental pruning.
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Other reasons for the lack of flowers
Didn’t you prune your hydrangea this year and it still hasn’t bloomed? Lack of flowers in hydrangeas can be the same as with other plants: planted in the wrong place, too much nitrogen fertilizer, or extreme weather. It can be too wet, too dry, too cold at the wrong time, or not cold enough at others. If a late spring cold snap kills all the buds, you won’t get any flowers until the following year. New growth that comes only from the ground is a bad sign that your hydrangea will not bloom this year. Large-leaf hydrangeas are the variety most susceptible to winter bud injury.
Interesting fact: Many headdresses and bands change color, depending on the concentration of unbound aluminum ions in the soil. The more acidic the soil, the bluer the flowers; The less acidic the soil, the pinker the flowers will be. For blue flowers, amend the soil slightly with aluminum sulfate. For pink flowers, add lime.
Linda Hanson is a Master Gardener in York County. Penn State Master Gardeners are volunteers with the Pennsylvania Cooperative Extension. For more information, contact the Master Gardener’s office at 717-840-7408 or YorkMG@psu.edu.