Try to have a discussion about hydrangea among your gardening friends, and you’re likely to start a rather lively debate. What type to grow, when to prune, why doesn’t it flower and how can I change the flower color from pink to blue are among the most frequently asked hydrangea questions.
There are many different species of hydrangea, not to mention cultivars of those species. So, these questions are a bit difficult to answer, unless you know which specific plant is being discussed. Most hydrangeas do well in full sun or partial shade. However, they thrive best in cool, moist soil. So if your site is hot and dry, it would be best to aim toward afternoon shade.
The main question that must be answered to know how to prune a hydrangea is whether it blooms on current year’s growth or previous year’s growth. Those that bloom on new year’s growth are likely to bloom in summer and can be pruned in late winter or early spring. They are generally less likely to suffer winter and spring frost injury.
Those that bloom on old wood produce their flower buds in late summer for the following season’s bloom in spring. Pruning during fall, winter or early spring will remove flower buds and reduce or prevent blooming that season. These hydrangeas are more likely to suffer winter injury and spring frosts that reduce or completely wipe out the blooms for that season.
The hardiest of the hydrangea species is Panicle Hydrangea, H. paniculata. Panicle Hydrangea is generally a large shrub that blooms on new wood, with white flowers in midsummer that change to a faint purplish-pink as they age. The paniculatas do not require a lot of pruning but can be thinned in late winter or early spring. There are a number of interesting cultivars on the market, such as ‘Limelight,’ with 12-inch, cone-shaped flower clusters, beginning lime green and changing to greenish-white to pink. ‘Pinky Winky’ has huge 12-16-inch flower clusters (white changing to deep pink) on sturdy, upright stems.
Smooth hydrangea, H. arborescens, bears large clusters of blooms that start out pale greenish, changing to white and then drying to papery brown. The flowers form on new wood so, like the paniculata, pruning can be done in late winter or early spring. The cultivar ‘Annabelle’ has been quite popular for its very showy blooms but can become rather weak-wooded and unkempt with age. It is probably best to treat this plant more as an herbaceous perennial, since it flowers on new growth; cut it back severely to about 4-6 inches in late winter. ‘Incrediball’ is a new Annabelle type but its stronger, thicker stems bear huge 12-inch flower clusters, starting out lime green, opening to white, then fading to lime green again. ‘White Dome’ has lovely, dome-shaped flower clusters in white. ‘Invincibelle Spirit’ is a new pink-flowered Annabelle type with some repeat blooming through summer. A portion of Invincibelle’s sales is being donated to The Breast Cancer Research Foundation.
My personal favorite species is the Oakleaf Hydrangea, H. quercifolia, for its outstanding dramatic foliage shaped like an oak leaf, with outstanding fall color, cinnamon peeling bark visible in winter, as well as showy blooms in late spring and early summer on old wood. It is generally a large, coarse shrub; however, some compact cultivars are available. It does tend to colonize a bit, so it is best planted where its volunteers will be welcome. The flower buds may be killed in severe winters, but the foliage is still a good show. Unfortunately, it is also prized snacking for deer. ‘Alice’ has large white flowers maturing to a rose pink but, even more outstanding for fall foliage color, changing from deep green to bronze, maroon and purple. ‘Snow Queen’ has upright, white flower clusters maturing to pink and burgundy fall foliage. ‘Little Honey’ is a compact plant with yellow foliage in spring, chartreuse in summer, then changing to red in fall. ‘Snowflake’ has large, pendulous flower clusters opening in white and maturing to purple-pink and burgundy fall-colored foliage. ‘Pee Wee’ and ‘Sikes Dwarf’ are good compact shrubs with good flowering and fall color.
Climbing hydrangea, H. anomala petiolaris, is a striking climbing vine with fragrant flowers on current season’s growth and interesting cinnamon peeling bark. Although it can be slow to get started, once established, it will grow profuse foliage that clings to any type of support. ‘Skyland Giant’ has large white flower clusters. ‘Firefly’ has variegated foliage, green and bright yellow in spring maturing to green and chartreuse in summer.
This brings us to the Bigleaf Hydrangea, H. macrophylla, certainly the most popular hydrangea across the United States. This is the plant with huge flower clusters whose color can be pink or blue, depending on the availability of aluminum in the soil, which is usually a function of soil pH (blue flowers in acidic soil, pink flowers in alkaline soil).
Southern Indiana gardeners have had some luck with this plant, but unfortunately for most Indiana gardeners, this particular species does not flower reliably in our area. It normally blooms on previous year’s growth, and, because it breaks dormancy very early, its flower buds are most often killed in USDA hardiness zone 5. The vegetative buds often survive, or new shoots sprout from the roots if killed back to the ground, forming a tidy little foliage plant, but alas, no blooms.
However, there are a few relatively new cultivars that are able to bloom on current season’s growth as well as old wood. So if these are killed back in winter, there is still some flowering to enjoy during most summers. ‘Endless Summer’ is probably the most well known of these. Others include ‘All Summer Beauty,’ ‘Penny Mac,’ ‘David Ramsey’ and the new ‘Let’s Dance’ series. If these re-bloomers aren’t killed back to the ground, flowering will be the showiest on old wood. So they are best left un-pruned other than to remove dead or damaged stems.