Hydrangeas in most home gardens belong to four different species and each of these has slightly different requirements when it comes to pruning.
The four main types of hydrangeas grown in NE Oklahoma are:
Hydrangea quercifolia, with the common name of Oakleaf hydrangea
Hydrangea macrophylla, which are the familiar bushy large-leafed garden type
Hydrangea paniculata, mostly called panicle hydrangea
Hydrangea arborescens, often called smooth or wild hydrangea
The Oakleaf hydrangea is usually the sturdiest of the hydrangeas, takes the full morning sun, has beautiful white blooms, waxy leaves, and brilliant fall color. The Oakleaf hydrangea is an Oklahoma Proven “shrub” that was picked as the first representative shrub picked for the OK Proven Program, started in 1999. See: http://oklahomaproven.okstate.edu/gallery.html.
In the spring, you can prune out any deadwood, but these also bloom on old wood, so any pruning of living tissue now would remove flowers. Oakleaf hydrangeas require very little pruning. If you wish to improve the shape of the plant, prune it in the summer after the plant finishes flowering.
The second and most diverse group of hydrangeas, Large leaf or Garden hydrangea, is the most common hydrangea planted as a shade-type ornamental, and the type your grandmother grew in her garden. There are two main types of these species: “mopheads” and lace-caps. Big-leaf hydrangeas bloom on old wood, meaning they bloom on stems produced during last season.
Pruning in the spring would remove the flower buds, leaving us with a bloomless plant this year. Instead, prune big-leaf hydrangeas in the summer, after they finish blooming and strong new shoots are developing from the crown or base of the plant. Remove the weaker shoots, both old and new, by cutting them at the base. You’ll want to leave several stems of old productive wood, as well as strong new stems that will flower next season. As the plant ages, gardeners can remove up to one-third of the stems each season to keep the plant productive.
Many newer varieties of these type hydrangeas, like “Endless Summer” (Blue), and “Glowing Embers” (Dark Pink) bloom the color they start out as. The older varieties are the ones that bloom bluer when adding sulfur, and pinker when adding ag lime.
Another commonly planted hydrangea (especially in the eastern U.S.) is Hydrangea paniculata, also called panicle hydrangeas. Panicle hydrangea (H. paniculata) is the most cold hardy member of the genus. It can be reliably grown in USDA cold-hardiness zones 4 to 7. Native to Asia, it grows 10 to 15 feet tall. Large creamy-white flowers, which are borne in 6- to 18-inch long panicles, are produced in mid-summer.
As flowers mature, they may turn pink. Panicle hydrangeas can be lightly pruned to maintain an ideal shape, particularly those of the cultivar ‘Grandiflora’ (’Pee Gee’), are sometime pruned into a tree form and grown as a specimen plant. Panicle hydrangea is also suitable for use in a mixed border or as a deciduous hedge. The pruning method also will vary depending upon the cultivar being grown.
A newer variety Little Lime™ Hydrangea (Hydrangea paniculata ‘Jane’) is a newer dwarf version of Limelight, but reaching a mature size of only 3 to 5 feet. Obviously, it requires much less pruning to maintain a compact form. We have a larger woody cultivar toward the back of the garden called Pinky Winky™. This cultivar matures at about 6 to 8 feet tall.
It responds very well to hard pruning. Cutting it back by one-third to one-half will encourage larger flower production. All panicle varieties are best pruned in early spring, as they bloom on new wood.
The last main type of hydrangeas belongs to the species Hydrangea arborescens. These are often called smooth hydrangeas, or wild hydrangeas, because the plant is native to the southern U.S., including parts of Oklahoma. Smooth hydrangeas also are known as “Annabelle,” which is actually a cultivar of Hydrangea arborescens.
Several new introductions have been made in recent years, including Incrediball® hydrangea, which has massive flower heads up to a foot in diameter. Smooth hydrangeas bloom on what is called new wood or new growth, which is the new growth produced in the spring.
To encourage abundant blooms and keep plants to a manageable size, smooth hydrangeas are typically cut back to the ground in late winter to early spring. If a larger shrub is desired, cut back some of the stems to the ground, and leave others at varying lengths, from 1 to 2 feet.
(John E. Haase is the OSU Horticulturist for Rogers County Extension Service.)