Amy Dabbs: How to properly prune hyndrangeas
Q I prune my mophead hydrangeas religiously every spring and I never get any flowers! What am I doing wrong?
A: Mophead or French hydrangeas (H. macrophylla var. macrophylla) are some of the easiest, most reliable shrubs gardeners can grow. They flourish without fertilizer, chemicals or even pruning for that matter. Their showy white, pink and blue blossoms put on a stunning display each year. But ill-timed pruning is often to blame for a lack of blooms.
Other reasons hydrangeas do not flower include late season cold snaps that freeze flower buds and too much sun, which zaps the plants' energy away from bud production.
It makes me sad to see these pretty Southern belles drooping in the heat of the day, so I have moved mine into deeper shade to avoid the need to water daily. As for late season cold snaps, all we can do is watch the weather and provide protection by covering the canes with frost blankets when needed.
Macrophylla means “big leaf” which is evident in the fat leaf buds that emerge on bare canes in early spring. Warm weather forces them to reveal their lush foliage and shortly thereafter, big happy flowers emerge in colors reflecting soil pH: Acidic soils produce blue flowers while alkaline soils produce pink flowers and a neutral pH yields a purple or mauve color somewhere in between.
While we turn our attention to summer vacation, these garden work horses immediately start gearing up for next year's flowers by developing their buds for next season. While pruning is not necessary for big leaf hydrangeas to perform, immediately after flowering is the best time to remove dead canes or rejuvenate an older plant.
When to prune
Ideally, avoid pruning big-leaf hydrangeas after Aug. 1 to avoid inadvertently removing flowers for the next season. I say “ideally” because sometimes late season pruning is unavoidable. These vigorous growers can easily overtake their intended place in the garden. In this case, forewarned is forearmed. If you must prune in winter or spring after buds have formed, take care to remove no more than a third of the canes to ensure at least a few flowers for next season.
If your hydrangea has overstepped its bounds, you might consider pruning it back now and moving it to a more appropriate location. While it's generally not recommended to move woody ornamental shrubs in the summer, I have found that hydrangeas are quite forgiving as long as they receive plenty of water and are shaded from harsh summer sun.
Mopheads aren't the only hydrangeas that should be pruned immediately after flowering. Lacecap hydrangeas (H. macrophylla var. normalis) should be treated just like their botanical kissing cousins. Personally, I think these refined beauties are underutilized in home landscapes. Their flat, lacy flower heads make them easier to incorporate into the landscape than their blowsy counterparts. Both appreciate a bit of shade from the afternoon sun and look great in mixed borders or massed together along a woodland edge.
Gardeners who yearn for hydrangeas but have too much sun for the big leaf types should consider the creamy white flowers and distinctively shaped foliage of the oakleaf hydrangea (Hydrangea quercifolia). Native to the southeastern United States, oakleaf hydrangeas can withstand more sun and require less water than their Asian relatives mentioned above. Soil should drain well. “Wet feet” will result in root rot, but otherwise, they require almost no attention once established. These hardy deciduous shrubs should also be pruned soon after blooming so they can develop flower buds for the following year. Their flower color is not affected by soil pH. Along with gorgeous flowers, they also provide spectacular fall color and attractive exfoliating bark in winter. There are a few cultivars available including Snowflake, Snow Queen and Harmony.
An easier option
If you feel like you will never get the hang of pruning hydrangeas, renowned plant breeder Michael Dirr has developed a group of “everblooming” hydrangeas that break all the old rules. Endless Summer is a widely available mophead type that will bloom on both old and new growth so gardeners can prune anytime (except while in bud) without fear of losing flowers.
If you have ever purchased a hydrangea only to have the color of the flower change without warning, Blushing Bride is here to help. It starts out white and matures to a lovely pink regardless of soil chemistry.
Lacecap lovers will enjoy the cool blues of Twist and Shout that can also be relied on to flower on old or new wood.
For more home horticulture tips and information, go to the Clemson University Home & Garden Information Center at clemson.edu/extension/hgic.
Amy L. Dabbs is the urban horticulture extension agent and tri-county Master Gardener coordinator for the Clemson University Cooperative Extension. Send questions to gardening@postand courier.com.