Dick BirNorth Carolina State UniversityMountain Research & Extension CenterFletcher, NC
I have been asked to put in writing my opinions on the proper way to prune hydrangeas . . . not parrot what I have read somewhere. In other words, what is my experience? Most of the hydrangea pruning confusion, I think, comes from folks who have never grown hydrangeas wanting to lump them all into one pruning rule, i.e., prune in winter or prune immediately following flowering or prune to the ground or never needs pruning. Unfortunately, each of these pruning rules can be correct for some hydrangea species and cultivars. It is a big genus.
The most straightforward hydrangeas to prune are the mostly white flowered, extremely tough and hardy Hydrangea paniculata and Hydrangea arborescens cultivars. H. paniculata cultivars, among others, include 'Brussels Lace,' 'Kyushu,' 'Pee Gee,' 'Pee Wee,' 'Pink Diamond,' 'Unique' and 'White Moth.' H. arborescens cultivars include 'Annabelle' and 'Hills of Snow.' All selections of these two species flower on new wood. Therefore, they can be cut back immediately after flowering and may rebloom (for H. arborescens in long growing seasons) or they can be cut back in the winter and still be expected to flower the following year. How far you cut them back is strictly personal preference. Unless you live in a restricted community with pruning covenants, there are no pruning police. You do not HAVE to prune H. arborescens or H. paniculata at all but it makes for a neater plant and, if you want long stems for fresh or dried flowers, pruning back hard encourages long stems on older plants.
The next group seems to cause the most confusion. This includes our native oak leaf hydrangea, H. quercifolia, as well as the plants that were sometimes called the 'French' hydrangeas, H. macrophylla, a few decades ago. This species has the capacity to have flowers that are pink, blue or white depending upon the cultivar and the acidity of the soil plus the availability of aluminum. These are the big leaf hydrangeas. H. macrophylla var. macrophylla are mopheads but are also called hortensias. H. macrophylla var. normalis produces very similar plants but lacecap blooms. H. macrophylla var. serrata are the mountain hydrangeas which were mostly lacecaps when I first encountered them but many more cultivars are being introduced from Japan with some cultivars lacecap, some mopheads and some that will have both types of flowers on the same plant as with the cultivar 'Preziosa.' Mountain hydrangeas are allegedly hardier than the other H. macrophylla and, in my experience, have flowers that are less gaudy than many of the H. macrophylla var. macrophylla.
These species, the oak leaf and big leaf hydrangeas, are alleged to flower on buds that were formed the previous season. Therefore, if you prune them in late fall, winter or early spring, you will be cutting off that season's flowers. If drastic pruning is required, do it immediately after flowering. The best and safest way to prune them is to remove some older stems but leave most stems so that you are removing about one-third of the growth each year. That said, my experience is that these plants have not all read the books. We have cultivars in our H. macrophylla trials that continue to produce new flowers for months on growth that is feet from any buds that could seemingly have been formed the previous season. It seems unlikely that these flower buds opening in September were formed the previous September. Many of these cultivars are also killed nearly to the earth every winter but regrow from buds an inch or two above the earth and flower a few feet above the ground. In nurseries I see quart liners put in five gallon pots that have full five gallon, flowering plants of cultivars like 'Nikko Blue' by fall. These flowers all appear to be on current season growth to me.
My advice with these is to do what you feel like doing when it comes to pruning. If cautious, prune by only removing some growth each winter. Since most of these shrubs don't seem to demand pruning in order to be attractive, don't prune unless the plants really call for it. In the mountains of NC, after years of observing over a hundred cultivars of macrophyllas, I am convinced that when plants fail to flower in our fields it is usually due to a late frost killing new growth and a slow recovery.
I have never seen any need to prune the climbing hydrangeas, including their cousin Schizophragma hydrangeoides ('Moonlight' is the only cultivar I know). In my experience, they are very slow growing and the thought of cutting them back is foreign to me. They are clingers so do not need to be pruned to make them climb.
I am just beginning to learn about Hydrangea aspera and relatives. In the Heronswood Nursery catalog it is suggested that you prune heavily for the first three years after planting in order to produce more branches near the ground that will result in a fuller, more attractive plant. However, that has not been necessary with the plants we have grown. I suspect this is because we have grown them with too much light and not enough water for best growth. Specimens we have moved to shadier locations have become rangy and might benefit from more vigorous pruning.