One of the questions I get asked most often is when to prune hydrangeas. Knowing what variety or species of hydrangea you have is a must.
Hydrangeas are divided into those that bloom on old wood and those that bloom on new wood.
What is “old” and “new” wood? Hydrangea stems are called old wood if they have been on the plant since the summer before the current season. When it comes to pruning, these varieties should only have the spent blossoms removed. They should be planted where they can grow to their expected size so pruning is non-existent or minimal. New wood are stems that develop during the current season. These varieties should be pruned late fall to early spring before they begin to form new flower buds.
I will begin with Smooth Hydrangea (Hydrangea arborescens). These varieties, such as Incrediball and Annabelle, have white, cream or pink flowers. They bloom on new wood.
One of the most common hydrangeas found in the landscape, Bigleaf Hydrangea (Hydrangea macrophylla), blooms on old wood. You will find this hydrangea available in Mophead and Lacecap varieties which will bloom blue (low soil pH) or pink (high soil pH). Some of the newer, repeat blooming varieties produce blooms on old and new wood.
Oakleaf Hydrangea (Hydrangea Quercifolia) blooms on old wood and prefers morning sun and a little afternoon shade. This hydrangea blooms in June to July sporting a cream panicle, grape cluster like flower. It has wonderful foliage texture and beautiful fall color.
Panicle Hydrangea (Hydrangea paniculata) include varieties such as Limelight and Vanilla Strawberry. These have a somewhat cone shaped flower and are super cold tolerant. They are also tolerant of sun and heat and you will commonly find them grafted into tree forms. Flowers form on new wood.
Mountain Hydrangea (Hydrangea serrata) is similar to H. macrophylla, the Bigleaf Hydrangea but is more cold hardy and is reblooming.
Winter protection is necessary for H. macrophylla and H. quercifolia since these species produce their flowers on old wood. Protect these hydrangeas in late fall after the leaves drop with four to six inches of winter mulch and by wrapping the plant in burlap or spraying the stems with Wilt-Pruf. This will protect the older wood and buds for the next growing season’s bloom cycle. If pruning is done on these hydrangeas, it can be done in early spring removing only winter damaged, weak growth down to the uppermost buds being produced.
In mid-summer through fall, hydrangeas should be dead-headed, removing the spent flower and a short stalk down to the next leaf set or new bud growth. With H. paniculata and H. arborescens, you can prune heavily in late winter or early spring, taking the stems down to six to 12inches or back to any node producing new growth. Any older hydrangea will benefit from removal of one-quarter of the oldest branches down to the ground to improve air circulation and strengthen existing growth. Older branches usually have a silver color. Hmmm, kind of like this older woman’s hair!
As always, happy gardening!
Karen Weiland is an advanced master gardener at Purdue Extension in LaGrange County, Ind.