Each year at the Master Gardener Diagnostic Clinic, a well-trained group of volunteers takes hundreds of calls from Yakima gardeners. A frequent concern: hydrangeas.
One question they get over and over: “I prune my big leaf hydrangeas every spring, and I never get any flowers. What am I doing wrong”?
Pruning hydrangeas is a subject that perplexes rookie and experienced gardeners alike. Many well-meaning gardeners reduce their hydrangea’s blooming potential through simple pruning errors. Fortunately, hydrangeas are really easy to care for, as long as you understand their growth habits.
By the beginning of summer, the big leaf hydrangea is well into gearing up for next year’s flowers by developing flower buds on this year’s stems. If those buds are killed or damaged over the winter, or if you prune them off, the hydrangea’s flowering is reduced (lower buds along the stem have the potential to develop flowers) or eliminated. While pruning isn’t necessary for big leaf hydrangeas to perform, the best time to remove dead canes or rejuvenate an older plant is before they set these new buds. If done the following spring, you’ll be removing the flower buds, which means no flowers that year.
Before you even think about grabbing your pruning tools, you need to identify which species you’re growing. This determines how, when and even if it needs pruning. There are several species of hydrangeas commonly grown in Yakima, and they have very different growth habits and pruning requirements. While some plants bloom on new wood, others primarily set flower buds on old wood. Stems are called old wood if they have been on the plant since the summer before the current season. New wood are stems that develop during the current season.
There are two main groups of hydrangeas:
Group 1: Those that bloom on last year’s growth, or old wood, and should be pruned in late summer:
• Oakleaf hydrangeas (H. quercifolia).
• Bigleaf hydrangeas (H. macrophylla).
• Mountain hydrangeas (H. serrata).
• Climbing hydrangeas (H. petiolaris).
Group 2: Those that bloom on new growth, or new wood, should be pruned in late winter to early spring:
• Smooth hydrangeas (H. arborescens).
• Panicle hydrangeas (H. paniculata).
There are also new varieties called “reblooming” or “remontant” that bloom on both old and new wood. These types don’t require pruning at all, just maintenance for plant health such as cutting out dead, diseased, damaged or crossing branches.
The timing and degree of pruning is determined by which group the plant belongs to.
How to prune Group 1
• Buds for next year’s flowers form as days grow shorter and temperatures cooler during late summer and fall.
• Typically, only a trimming is needed to maintain shape, size and a healthy plant. Cut out dead, diseased or broken branches. Otherwise, harsh pruning should be avoided.
• Trimming should be done immediately after flowering stops in summer, probably no later than Aug. 1. If you haven’t done it yet, wait until next year. Other than carefully removing the faded flowers, do not prune in fall, winter or spring, or you could be cutting off new buds.
• When you cut for flower arrangements before August, cut long stems back to buds at the axil of the leaves. When you cut back the blooms after the first of August, it would be safest to remove them with a very short stem so not to disturb any developing buds for next summer.
• Cold winter temperatures and drying winds can also kill flower buds, leaving plants with lush foliage but no sign of flowers.
How to prune Group 2
• Flower buds form on current year’s growth.
• Prune in early spring, just as leaves are beginning to show.
• Cut branches back by one-half to one-third, cutting just above a node.
• Next, remove any weak or spindly branches.
• For H. arborescens, minimal pruning promotes large vigorous shrubs with numerous, smaller flower heads. Hard pruning 12 to 18 inches from the ground or even all the way to the ground, will produce fewer, but larger, flower heads that may flop without propping.
• For H. paniculata, in order to create a strong framework, prune out surrounding smaller wood, leaving the larger stems.
The best advice for hydrangeas is to consider their mature size. Locate them in an area they won’t outgrow and require heavy pruning to keep them in bounds. Oklahoma State University Extension has a great video at www.youtube.com/watch?v=zaQ4AkhQZBI if you have pruning questions.
Have a general gardening question? Ask a Master Gardener by calling 509-574-1604.
• Carol Barany and her husband, John, found paradise on 1 1/3 acres just west of Franklin Park, where they raised three children and became Master Gardeners. Contact her at firstname.lastname@example.org.