Homeowners frequently ask NYBG's Plant Information Office to help them identify the hydrangeas growing in their garden so they can learn how to care for them properly. Most have characteristics that make them easy to identify. Observe your plant for one growing season, making note of the color, shape and frequency of blooms, as well as leaf and plant size. Recognizing the type of hydrangea is also the key to understanding the correct pruning routine for your plant. The timing for pruning depends upon whether it blooms old or new wood. You are trying to avoid cold weather damage and cutting off buds for the next season's flowers by pruning at the wrong time of year. And there are also some hydrangeas that do not really need to be pruned, unless you would like to reduce the size or re-shape the plant.
Here are the types of hydrangeas you are most apt to find in a garden in the New York area, how to recognize them and pruning information.
These are the classic mop-head and lace-cap hydrangeas and usually flower in blue, pink or purple (not white). Their leaves are thick, crisp, shiny, and coarsely toothed. Lace-caps differ from mop-heads in having smaller florets in the center of the bloom; these are fertile flowers. Lace-caps may be either Hydrangea macrophylla or H. serrata, which are more compact plants with smaller leaves and flowers. Mop-head hydrangeas are also sometimes called hortensia hydrangeas.
These hydrangeas bloom once with long lasting flowers in mid-summer, unless they are an ever-blooming variety (see below). Some good H. macrophylla cultivars include 'Ami Pasquier', 'Blue Wave', 'Générale Vicomtesse de Vibraye', 'Lilacina', 'Mme Emile de Mouillère', 'Mousseline' and 'Veitchii'.
These types bloom on old wood (stems that have been on the bush since the summer before). The plants can be left unpruned in their early years and will flower freely but after three to five years may need some rejuvenation to avoid twiggy growth. They can be pruned in the summer as soon as the flowers have started to fade. Pruning after the end of July runs the risk of removing the buds that will produce flowers next spring. A bush can be trimmed by cutting back stems close to a node. If the plant is overgrown or producing fewer flowers and twiggy new growth, prune back about 1/3 of the older stems almost to the ground each summer to encourage strong new stems.
Note that there is a small group of mop-heads that will bloom no matter when they are pruned; these are called “ever-blooming” (see below).
Newer H. macrophylla cultivars have been introduced in recent years that flower on both old (last season's) and new (this season's) stems. These are sometimes referred to as ever-blooming or remontant hydrangeas. They share the characteristics of the bigleaf hydrangeas described above but flower almost continuously throughout the season.
These ever-blooming cultivars include: Hydrangea macrophylla 'All Summer Beauty', 'David Ramsey', 'Decatur Blue', 'Endless Summer', 'Mini Penny', 'Oak Hill', and 'PennyMac', Hydrangea serrata 'Blue Deckle', and 'Coerulea Lace'.
In general, they will not need regular pruning as long as they are in a spot that gets the light and water they need. The safest practice is to clip back individual flowers right after they bloom but you can prune back in late winter too and still have flowers in somewhat reduced quantities and later in the season. The older wood and its buds will be clipped away but the plant will produce new growth with flower buds in the spring.
If you need to do a reshaping pruning, typically that is done by cutting back just a third of the branches at a time over a period of three years, so that the plant stays strong and flowering. These plants do have a habit of suffering some dieback in harsh winters in our area. If that happens to you, a gentle clipping of branches back to a pair of healthy buds can be done but if you need to do heavy clipping you will lose flowers.
Many gardeners enjoy having the spent flower heads in the garden over the winter for a bit of structure and interest. It is part of the year-round beauty of the plant.
The oak-leaf hydrangea (Hydrangea quercifolia) is becoming a staple of the east coast garden and it is hardy to zone 4. This is beautiful North American native can be distinguished by the leaves which resemble those of red oaks. They have ornate, peeling bark, large-flowering panicles that age well, and wonderful red fall leaf color. The blooms open white and turn pink as they age. It thrives in partial shade, but can tolerate quite a bit of sun.
Some good cultivars are 'Alice', 'Amethyst', 'Pee Wee', 'Snow Flake', and 'Snow Queen'.
Oak-leaf hydrangea flowers on old wood and should be pruned immediately after flowering, before the end of July. It is a low-maintenance shrub that needs only a little pruning; simply remove dead wood and cut back a few stems when necessary to maintain a full, healthy plant.
Another hardy hydrangea is the smooth hydrangea (Hydrangea arborescens). An East coast native, this hydrangea does best in partial shade to full sun. it grows from 4 to 6 feet tall and up to 6 feet wide. Smooth hydrangeas often have very large flowerheads made up of numerous small individual flowers (often called snow balls) that bloom in June and July. Most flowers open green and turn white in 2 - 3 weeks. Their leaves are thinner and floppier than mop-heads and lace-caps. Some popular varieties are 'Annabelle', 'Grandiflora', ' Incrediball®' and 'White Dome™' and newer varieties in the Invincibelle® series include unusual pink and green colored flowers. There are lace-cap types as well, such as the pollinator favorite 'Haas Halo'.
Smooth hydrangeas flower on new wood. You can cut them back in late fall, or early spring where winters are colder.
Panicle hydrangeas (Hydrangea paniculata) have long been a staple of the home garden. There are a great variety, in sizes from small shrubs to small trees. Bloom shapes vary but are often at least slightly cone-shaped. Their leaves are thinner and rougher than many other hydrangeas, and characteristically 3 leaves grow from a node in a whorl. They may have a solid cone of florets like the popular H. paniculata 'Grandiflora' or a lacy mixture of fertile and infertile florets, like the cultivars 'Tardiva' and 'Pinky-winky™'.
Panicle hydrangeas tend to flower later in the season than other species. If yours are blooming in late August and early September with pointed flowerheads they are probably panicle hydrangeas. The large flower panicles can range from white to pale pink to lime green and the color takes on a red tone as weather cools. The stems may be peeling and often arch under the weight of the flowers. Cultivars include: 'Burgundy Lace', 'Chantilly Lace', 'Grandiflora' (also called peegee), 'Limelight', 'Little Lamb', 'Pee Wee', and the late-flowering 'Tardiva'.
These types bloom on new wood and can be pruned at any time except in the spring or summer after the new growth has started. Late winter/ early spring is best. Panicle hydrangeas can also be pruned into tree forms. For a small tree, prune the crown of the tree to 3 to 5 main branches. As with all hydrangeas, remove any weak or dead branches.
Hydrangeas for American Gardens by 
Encyclopedia of Hydrangeas by 
Hydrangeas : a gardener's guide (2004) by 
Hydrangeas : a gardeners' guide (1995) by 
