After the article on winter pruning of shrubs, questions concerning pruning hydrangeas have come to the Berks County Master Gardener Hotline. This article will help dispel the mystery.
Do you really need to prune your hydrangeas?
Hydrangeas do not require extensive or regular pruning, and most pruning is done for aesthetic reasons to:
• Remove dead growth: Hydrangea varieties vary in their cold hardiness. Winter and spring weather conditions may cause stem damage, such as when mild temperatures are followed by drastic or rapid chilling, or when spring frosts occur after plants begin growth. Freezes may kill only flower buds or entire stems.
• Manage mature size: There are hundreds of hydrangea varieties with a range of mature sizes. Removing entire stems to force regrowth or shortening tall stems can change the shrub’s size for that season.
• Rejuvenate old plants: Even healthy plants can experience occasional dieback of older stems. Pruning stimulates regrowth, which can freshen a plant’s appearance, and may also stimulate better flowering if the shrub is receiving enough light.
• Reduce disease severity: Several leaf spot diseases can infect hydrangeas and cause unsightly symptoms by late summer. The risk of leaf infection is higher when plants are crowded, or growth is very dense. Reduced air circulation can slow the drying of leaves after rain, dew, or irrigation, creating vulnerability to infection.
• Remove spent flowers: Aging and faded hydrangea flowers can develop appealing colors and provide winter interest. Trimming off faded flowers, called dead heading, is not necessary for plant health, though it might encourage faster reblooming for hydrangea varieties capable of repeat blooming.
You’ll need to know the correct time to prune
Pruning timing depends on the type of hydrangea you have. Hydrangea species fall into one of three categories based on how they develop their flower buds. Many hydrangeas cannot replace flower buds that were accidentally removed. If a loss of flowers is not a concern, then the timing of pruning is less critical, though still should be avoided close to winter dormancy.
The age of branch growth (wood) that produces the flower buds is the important trait.
Old wood is branch growth at least one year old: All of the branches present before hydrangeas break dormancy in spring are old wood. Hydrangeas blooming on old wood start to develop flower buds in August. The flower buds lie dormant on the stems during winter and open the following spring or summer.
Prune just after the flowers fade: It is suggested to keep pruning to a minimum on old wood except for removing dead or diseased branches. Bigleaf and mountain hydrangeas are members of this category.
Oakleaf hydrangea can be pruned to the same height or left taller, especially for large-maturing varieties. Climbing hydrangeas are not routinely pruned but fall into this category.
Examples of hydrangeas that bloom on old wood
Remember: These are general characteristics, and there are many cultivars within each type with variations in color, size and bloom shape.
• Bigleaf hydrangeas (Hydrangea macrophylla) have exceptionally large, long, serrated dark green leaves (up to 8 inches), and they bloom for an extended period through mid-to-late summer. They have rounded, petal -dense mophead blooms or flattened, petal-ringed lacecap blooms. The flower color is affected by soil pH; acid soils cause flowers to be blue; alkaline soil causes pink flowers.
• Mountain hydrangeas (H. serrata) look like a smaller, more compact version of bigleaf hydrangeas. Their lace-cap flowers vary in color depending on soil pH. Their small size (2 to 4 feet) means that pruning is not needed very often.
• Oakleaf hydrangeas (H. quercifolia), as the name suggests, have leaves that resemble those of oak trees. It is an upright, broad-rounded, suckering, multi-stemmed, deciduous shrub that typically grows 4-6 feet (less frequently to 8 fee) tall. Their flowers, which bloom early in the season, are cone-shaped and start out cream or green in color, gradually becoming pink.
• Climbing hydrangea (Hydrangea anomala subsp. petiolaris) are very vigorous climbing vines (as much as 40 feet) with white flowers that appear in spring to early summer. The flowers form flattened clusters up to 8 inches wide.
New wood is branch growth produced in the current growing season. Hydrangeas blooming on new wood develop flower buds just prior to opening them, either in spring or summer. Overwintering stems do not have any dormant flower buds on them because they have not yet formed.
Prune in late winter or early spring. Smooth hydrangea can be trimmed to about 12 inches off the ground. Panicle hydrangea can be pruned to the same height or left taller, especially for large-maturing varieties.
Examples of hydrangeas that bloom on new wood
• Smooth hydrangeas (H. arborescens) can have mophead or lace cap summer flowers, primarily white, but some newer cultivars offer pink or green hues. The leaves are smooth and heart shaped. The most common garden variety is ‘Annabelle,’ easily identified by its huge snowball-shaped flowers.
• Panicle hydrangeas (H. paniculata) is the final species to flower and includes an old variety commonly called peegee or P.G. hydrangea, which some gardeners use as a common name to refer to the entire panicle group. They have large cone-shaped flower panicles, that are white or green when they first bloom, gradually turning pink. The leaves are oval shaped with serrated edges.
Remontant is the term used for shrubs capable of reblooming by producing flower buds on both old and new wood. The old wood buds open first, since they were already fully formed at the start of the season, followed by new wood flowers that keep forming well into the summer. Hydrangea varieties bred with this trait benefit gardeners.
Prune either in early spring or after the first flush of flowers has faded. Early-season pruning will delay the start of flowering, and mid-season pruning might interrupt or shorten the overall reblooming window, so the timing is a personal preference.
Pruning height can vary but should remove as little wood as possible to avoid delaying new blooms. Some big leaf and mountain hydrangeas fall into this category.
Here are some general pruning techniques
Thinning dense plants can improve air circulation through the foliage, which lowers the risk of disease outbreak like leaf spot. Since this approach removes entire stems, the timing of flowering is less important. Hydrangeas benefiting most from this technique would be the bigleaf and mountain types.
Heading back stems to reduce their length will restrain the size of an overgrown plant. When shrubs break dormancy in spring, you will be able to see if any stems have dead tips from winter damage. After their removal, you can then determine how far back to trim the remaining growth. Any hydrangea species can be pruned with this technique.
Renewal pruning can refresh a hydrangea that has become too sparse from weather damage, or which develop an irregular shape over time. Regrowth should produce a more even growth habit if the plant is not too heavily shaded or crowded on one side. Since this process removes most of the old wood, the plant might not flower well (if at all) for a year or longer while it recovers. Any hydrangea species can be pruned with this technique.
Dead-heading removes only spent flowers at the branch tips (no further down the branch) and can be performed any time after flowers fade for any hydrangea type. It is not necessary to remove spent flowers to maintain plant health.
Alternatives to pruning
Rather than regularly pruning an overgrown hydrangea too large for its space, a simpler solution is to either transplant it to another location or replace it with a variety that naturally stays more compact.
Breeding efforts in recent years have resulted in a plethora of hydrangea varieties in every major species group, so there are multiple options for customizing the desired mature size of the plant with little or no pruning adjustments. Remember Right Plant-Right Place.
If you would like further information on pruning hydrangeas or other gardening topics, contact the Penn State Extension Master Gardeners of Berks County at berksmg@psu.edu or phone the office at 610-378-1327.
Dee Dee Kerscher is a Penn State Extension Master Gardener Volunteer and Horticulture Program Assistant.
Spotted lanternfly update
Now that spring is here, many people will be planting new trees. You may have heard that spotted lanternflies will kill young trees and plants. This is not necessarily true.
Consequences of direct feeding damage by nymphs and adults to the host trees vary greatly by host species, number of SLF feeding and environmental conditions. While SLF feeding can stress plants and cause localized branch damage, it is not known to kill plants except for Tree of Heaven, black walnut saplings and grapevines.
Sooty mold — which can grow on large quantities of the pest’s sugary excrement — has been known to kill groundcover plants, particularly immediately below large populations of spotted lanternflies in trees.
Be on the lookout for the first stage of the tiny black nymphs that may be hatching starting in April. Though the number of SLF has been decreasing in recent years, we still need to be vigilant and monitor and destroy the invaders to keep them from returning again.
For the latest information on spotted lanternfly, visit extension.psu.edu/spotted-lanternfly or contact the spotted lanternfly hotline, 888-4BAD-FLY.
