Hydrangeas are a beloved landscape plant, especially so with the newer hybrids. There are more than 100 different varieties, including the climbing hydrangea, large oak-leafed varieties, red-stemmed choices and hydrangeas with pure white blooms. Because there are oodles of cultivars it’s helpful to categorize hydrangeas in one of five groupings: Bigleaf hydrangea (H. macrophylla); Climbing hydrangea (H. anomala); Oakleaf hydrangea (H. quercifolia); Panicled or Peegee hydrangea (H. paniculata); and Smooth hydrangea (H. arborescens).
The prevalent hydrangea is the big-leaf hydrangea, Hydrangea macrophylla. This “older” cultivar is typically pink or blue, depending on soil pH. Acidic soils produce blue flowers and alkaline soils produce pink petals. Aluminum uptake from the soil to the roots is responsible for color change.
The general rule for pruning hydrangeas is to trim spring blooming shrubs after they flower and summer blooming shrubs in late winter. Remember azaleas? Hybrid azaleas bloom in the spring and then without delay set flowering buds for next year, so pruning “post bloom” is the best option. Summer flowering plants bloom on wood that grows in the summer, so by pruning in the winter, you avoid removing any flowering buds.
Hydrangea species vary in the type of wood they bloom on, making pruning decisions a bit more difficult. With this in mind, it’s important to know the category type your hydrangea falls into and go from there.
Hydrangea macrophylla - French or Bigleaf Hydrangeas: There are two main reasons why bigleaf hydrangeas fail to bloom. The cultivars were developed for areas with a warmer climate than geared for Jackson and Swain counties, or the hydrangeas were pruned at the wrong time of year. Bigleaf hydrangeas set flower buds on old wood. Old wood develops in late summer and fall. When pruning is done any time after flower buds form, the next summer’s blooms are removed. The safest time to prune bigleaf hydrangeas is immediately after flowering.
The color of Bigleaf Hydrangea flowers are directly affected by pH. Acid soils will cause Bigleaf Hydrangeas to bloom blue, and alkaline soils will cause the plant to bloom pink.
Hydrangea quericifolia: The flowers on Oakleaf hydrangeas are produced on old wood from last year’s growth. Typically flowers bloom in June, but persist through the summer and gradually change from white to pink, and eventually to a tan in the winter. If needed, prune after flowering to maintain a desired size and shape. Winter-killed or other dead wood can be removed any time.
Hydrangea paniculata & Hydrangea arborescens: Both panicled and smooth hydrangea such as Annabelle bloom on new wood, or wood grown in spring and early summer. These can be pruned after fall frost and still flower the following summer. Some growers cut their “Annabelles” to the ground each winter in order to have flower heads on three to four foot straight stems the next summer.
Climbing hydrangeas are vigorous growers that bloom on old wood. The only necessary pruning is to remove unwanted stray stems for growth control. This may need to be repeated several times in the season as the vine quickly produces new stems. To avoid reducing bloom, prune them after blooming.
Western North Carolina often experiences “early spring” only to have new growth hit by a cold snap! Pruning at the correct time may not ensure hydrangea blooms each year.
Christy Bredenkamp is NCSU Horticulture Extension Agent for Jackson County.