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By Tom Woods
A lot of gardeners grow hydrangeas and this time of year they begin to ask questions about how and when to prune these plants.
First, we need to consider the different species of hydrangea that are available in this area. If you have a Hydrangea mycrophylla, Big Leaf or French Hydrangea as they are commonly called, these plants flower largely on old wood or stems that were produced last season. Consequently, when we have a spring freeze in April like in 2007, these plants won’t flower.
Big Leaf Hydrangea should be pruned after blooming and before August. A common cultivar of the Hydrangea mycrophylla is NikKo Blue. Hydrangea quercifobe or Oak Leaf hydrangea also bloom on last years’ growth and should be pruned after flowering, as well.
Hydrangea arborescens or Annabelle is a native cultivar that flowers on new wood produced in the spring. These cultivars should be pruned before the new growth begins in the early spring. The Peegee or Panicle, which is Hydrangea paniculatte, is another species found in this area that flowers on new growth. Limelight is another popular cultivar of this species.
If you don’t know the species, and also don’t know whether the plant blooms on new or old wood, you can still safely prune the plant using the following technique. Begin by cutting off the old flower heads and then cut off the old dead stems. Some of these old dead stems may go all the way to the ground.
This pruning technique is appropriate for both old wood and new wood flowering Hydrangea and is done after the flower heads lose their color and die. You can also do thinning and heading back of healthy foliage at this time. This would be done to control the plants size or protrusion into a walk or path. Or course, you could be cutting off potential flowers, but plant control sometimes trumps flowers in the spring.
A rule of thumb that can be used to decide whether a plant flowers on new or old wood is the June 1 rule. If a plant flowers before June 1, it more often than not means the plant is flowering on old wood; if it flowers after June 1, it is flowering on new wood. Like all rules of thumb, there are exceptions; Big Leaf hydrangea are an exception to this rule. They flower after June 1 on old wood.
It is sometimes necessary on older plants to do what is called rejuvenation pruning. March is a good time to consider this pruning technique. Older plants that have become too large for the site or that are not flowering as much as they did when younger can be heavily/rejuvenation pruned to stimulate new growth and flowering energy as well as regain control over plant size.
Begin this pruning by cutting all the old flower heads and dead wood first. Then select one-third of the remaining branches and remove them to the ground. This is a three-year process, one-third of the branches removed each year for three years removes all of the original plant and stimulates new growth from the ground that will be smaller and more vigorous.
Good luck with your hydrangea pruning. If you need more information on pruning hydrangea, contact the Master Gardeners at the Brunswick Cooperative Extension at 253-2610.