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The Homeowners Column
Demystifying hydrangea pruning
March 31, 2016
State Master Gardener Coordinator
Every spring we stare forlornly at our super-sized six-foot tall shrubs. We reminisce about the plant label that stated, "Grows to three feet." In our great soil and generally adequate moisture plants often exceed their usual height. Regular pruning can correct gargantuan growth glitches. Pruning can also promote new plant growth, encourage flowering, remove diseased or dead limbs and help control insect and disease problems.
However, timing is important, especially when pruning flowering shrubs. Shrubs produce flower buds during different months of the year depending on the time of year they bloom. Early blooming shrubs develop their flower buds during the summer and fall of the previous year. This is often called "blooming on old wood." Therefore as a general rule, shrubs that flower before June 15 should be pruned soon after flowering.
Pruning early blooming shrubs in late summer, fall or early spring will remove the flower buds and therefore the flowers. Typically the shrubs don't die but the flowers are lost for that season. Shrubs that should be pruned soon after flowering include deutzia, lilac, flowering quince, kerria, mockorange, weigela, forsythia, some hydrangeas, rhododendron, viburnum, some spirea (bridalwreath, 'Snowmound' and Vanhoutte), St. johnswort and redtwig dogwood.
Hydrangeas are a tough group to know how to prune due to the myriad of species and numerous cultivars. Some species bloom on old wood and some on new wood. Cultivars such as Endless Summer™ bloom on both old and new wood. With hydrangeas, be sure to save the plant label. If you know the name of the hydrangea, it will take the mystery out of pruning.
Smooth hydrangea (Hydrangea arborescens) and Panicle hydrangea (Hydrangea paniculata) develop their flower buds on new stems (new wood). Translation, these can be cut back severely in the spring to maintain size and they will still provide flowers. Also, after exceptionally cold winters where stems are killed to the ground, new stems will still produce flowers. In our climate and soils Smooth hydrangea and Panicle hydrangea are typically the easiest species to grow.
Smooth hydrangea cultivars include: 'Annabelle'; Invincibelle Spirit®; Incrediball®;
Common Panicle hydrangea cultivars include: 'Fire and Ice'; Bobo®; 'Bombshell'; 'Grandiflora' (Pee Gee); 'Little Lamb'; Quick Fire®; 'Limelight'; 'Pee Wee'; Pinky Winky®; 'Praecox'; 'Tardiva'.
Hydrangeas that bloom predominantly on old wood include: Bigleaf hydrangea (Hydrangea macrophylla); Oakleaf hydrangea (Hydrangea quercifolia); Mountain hydrangea (Hydrangea serrata) and Climbing hydrangea (Hydrangea anomala subsp. petiolaris).
Bigleaf hydrangeas may be sold as landscape plants or florist plants. Their blue or pink flower color is triggered by the amount of aluminum available in the soil. Few other hydrangeas respond in this way to aluminum. Bigleaf hydrangeas may have the traditional bodacious mophead flowers or lace cap flowers, composed of tiny disk shaped fertile flowers in the center surrounded by much larger sterile male flowers in the outer ring.
Hydrangeas that bloom on old wood may be difficult to get to rebloom in cold climates. If they experience severe winter injury and were killed to the ground, new stems produce only leaves and no flowers.
Bigleaf hydrangea cultivars include: Abracadabra® series, 'All Summer Beauty'; Cityline® series, Let's Dance® series. Some Bigleaf cultivars such as Blushing Bride™, 'Twist and Shout' and Wedding Gown™ bloom on both old and new stems so blooms are still produced.
Oakleaf hydrangea cultivars include: Gatsby™ series; Snowflake™; 'Snow Queen'; 'Munchkin'; 'Pee Wee' and 'Ruby Slippers'.
With hydrangeas that bloom on old wood enjoy the flowers as they fade then prune stems or remove just the old flower heads. Be sure to do any pruning before September.
No clue what type of hydrangea you have? Remove only dead wood in spring and delay any major pruning until you determine flowering time then prune accordingly next year.
Check out some great videos on pruning shrubs at http://www.youtube.com/user/UIExtension