Q. how can you tell whether to prune hydrangea in fall or spring
The timing for pruning depends upon what type of hydrangea you have and whether it blooms old or new wood. You are trying to avoid 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 is a rundown of some common hydrangea types and the appropriate pruning. Hopefully you will recognize your plant(s) in one of the descriptions. If not, please feel free to send us photos of your plant at PlantInfo@NYBG.org that include the overall shape of the plant, a full leaf and a flower and we will be happy to help you determine which plant you have.
Ever-blooming hydrangeas (H. macrophylla ENDLESS SUMMER®)
The Endless Summer hydrangeas are ever-blooming because they bloom on both old and new wood, thus they have a succession of flowers.
Pruning: In general, they will not need regular pruning as long as they are in a spot that gets the light and water they need. You should not prune them now or you risk losing flowers next summer. The safest practice is to clip back individual flowers right after they bloom and not to prune after August 1st.
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 plants.
Mophead (H. macrophylla) and lacecap hydrangeas (H. macrophylla var. normalis)
These usually flower in blue, pink or purple (not white). Their leaves are thick, crisp, shiny, and coarsely toothed. Lacecaps differ from mopheads in having smaller florets in the center of the bloom (these are fertile flowers).
Pruning: these types bloom on old wood (stems that have been on the bush since the summer before). They should be pruned in the summer when the flowers have started to fade but before August. Pruning after this time removes 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, prune back about 1/3 of the older stems almost to the ground each summer. Note that there is a small group of mop-heads that will bloom no matter when they are pruned; these are called “everbloomers” (ENDLESS SUMMER® is a common variety).(see above)
Paniculata and aborescens hydrangeas (such as H. Paniculata 'Grandiflora', or peegee, and ‘Limelight’ and H. aborescens 'Annabelle')
‘Annabelle’ and related H. arborescens species: These smooth hydrangeas have very large flowers made up of a large number of small individual flowers (often called snow balls). The blooms open green and turn white in 2 - 3 weeks. Their leaves are thinner and floppier than mopheads and lacecaps.
Paniculata (H. paniculata): There are a great number of varieties of paniculate hydrangeas, varying in size from small shrubs to small trees. Bloom shapes vary but are often cone-shaped. Their leaves are smaller, thinner and rougher than other hydrangeas, and characteristically 3 leaves grow from a node in a whorl.
Pruning: These types bloom on new wood and can be pruned at any time except in the spring or summer (for peegee) after the new growth has started. Late winter is best. Paniculata hydrangeas can also be pruned into tree forms. If you're aiming to do this then the main trunk and the top branches should not be removed.
Tree of heaven hydrangea (H. serrata)
This species is sometimes confused with mophead hydrangea, but it has smaller, narrow, pointed leaves.
Pruning: Tree of heaven also blooms on new wood and should be pruned as for paniculata types.
Oakleaf hydrangeas (H. quercifolia)
These can be distinguished by their leaves that resemble those of red oaks. The blooms open white and turn pink as they age.
Pruning: Oakleaf hydrangeas rarely need pruning. If they need to be pruned for shape or size, follow the rules for mophead hydrangeas.
Climbing hydrangeas (H. anomala subsp. petiolaris)
This is the only hydrangea that grows like a vine.
Pruning: Climbing hydrangeas also need little pruning. They can be lightly pruned for shape in late summer. If more drastic pruning is needed, early spring is best.