Stillwater News Press

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March 15, 2014

Grow! 03-16-14

Hydrangea pruning depends on variety

Hydrangeas are an elegant addition to the garden. Last year we added several new varieties to our collections at The Botanic Garden at Oklahoma State University. The hydrangeas in our gardens belong to four different species and each of these has slightly different requirements when it comes to pruning.

The first group 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.

The most commonly planted hydrangea is Hydrangea paniculata, also called panicle hydrangeas. Again, this group is often incorrectly called by a common name derived from an old cultivar. The name PeeGee comes from the plant Hydrangea paniculata ‘grandiflora,’ with PeeGee standing for “paniculata grandiflora.” However, many nurserymen started adopting this name for all “paniculata” cultivars and so a new common name has evolved. Panicle hydrangeas are pruned differently from smooth hydrangeas. And, we have more options when it comes to pruning panicle hydrangeas.

Panicle hydrangeas can be lightly pruned to maintain an ideal shape, while allowing the plant to grow rather large, up to 10 or 15 feet, depending on the cultivar. Plants can be pruned more severely to encourage a compact plant form and larger flower heads. Panicle hydrangeas can be cut back to the ground, but the new growth is weak and can be unstable under the weight of the flowers. For this reason, many gardens prefer to leave a framework of old growth for support. To do this, simply cut the stems back to a height of 18 to 24 inches. This will keep stems upright during bloom. Another option is to prune your hydrangea into a tree. The panicle hydrangeas are the only species that can be pruned into a tree form with a single or multi-stemmed trunk.

The pruning method also will vary depending upon the cultivar being grown. We have two very different panicle varieties at TBG — 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 big-leaf hydrangeas, or Hydrangea macrophylla, are the most diverse of the hydrangeas, including two common types, 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.

The Oakleaf hydrangea, otherwise known as Hydrangea quercifolia, is a hardy shrub grown as much for its lush foliage as its flowers. 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.

Kimberly Toscano is assistant extension specialist and host of the OETA television program Oklahoma Gardening, which airs at 11 a.m. Saturdays and 3:30 p.m. Sundays, and assistant director of The Botanic Garden at Oklahoma State University.

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