There is nothing more stunning than hydrangeas in full bloom, but nothing more disappointing to a gardener who has fertilized, watered, and pruned hydrangeas than one that refuses to bloom. According to Richard Bir, NCSU Extension Specialist (retired), lack of blooms on hydrangeas is sometimes the same as with other plants: too much sun or shade, too much nitrogen fertilizer, too wet, too dry, too cold at the wrong time or not enough cold at others. But most often it is caused by improper pruning.
To know when to prune hydrangeas, it is necessary to know their species. Five species are found in American gardens: Bigleaf hydrangea (H. macrophylla); Climbing hydrangea (H. anomala); Oakleaf hydrangea (H. quercifolia); Panicled or Peegee hydrangea (H. paniculata); and Smooth hydrangea H. arborescens).
The most spectacular of these species are the bigleaf hydrangeas, the blooms of which are divided into mopheads and lacecaps. The blooms of mopheads, including the popular “Nikko Blue,” consist of mounded to globular inflorescences. Lacecaps have flatter inflorescences, usually with a central core of fertile flowers surrounded by showier sterile flowers. Both mopheads and lacecaps offer blooms in a variety of colors, and many change their color, depending on the concentration of unbonded aluminum ions in the soil. The more acid the soil, the bluer the flowers; the less acid the soil, the pinker the flowers. For blue flowers, amend the soil slightly by use of elemental sulfur or aluminum sulfate. For pink flowers, add lime.
There are two predominant causes when bigleaf hydrangeas fail to bloom. The cultivar was developed for areas with a warmer climate than is found in western North Carolina, or they were pruned at the wrong time. 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. Thus the safest time to prune bigleaf hydrangeas is immediately after they have bloomed.
Both panicled hydrangea and smooth hydrangea, including ‘Annabelle’ and ‘Samantha,’ flower on new wood or current season’s growth, so they can be severely pruned after fall frost and still flower the following summer. In fact, 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.
Oakleaf hydrangeas flowers are produced on old wood from last year’s growth. It blooms in June, but the flowers 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. Winterkilled or other dead wood can be removed any time.
The climbing hydrangea is a vigorous grower that blooms on old wood. The only pruning needed is to remove unwanted stray stems to control its growth. 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.
Since Macon County often experiences signs of an early spring, only to have new growth hit by an extreme cold snap, pruning at the correct time will not always guarantee hydrangea blooms.