When do I prune my Hydrangeas?
Generally speaking, prune the mopheads (H. macrophylla, the big blue ones) after they bloom in summer, but before they set next season’s buds. How? Cut out the oldest stems to the ground to make room for new ones. This one’s a trickster to normal thinking, because though it blooms relatively late, in most varieties it does so on year-old wood, so it does carry flowerbuds over the winter. Deadheading is always allowed, as flowers fade.
With Hydrangea paniculata, the ‘Pee Gee’ and ‘Tardiva’ and other panicle types, a different thinking: They bloom on new wood. Prune them in spring just before new growth begins or thereabouts. They can be cut back quite hard, and still make new wood that then gets buds and blooms. Or you can skip pruning altogether, if you like.
Where to place your cuts can be confusing on paniculatas. I have had ones I didn’t cut back enough turn to octopuses (octopi?) and really look a mess, and this is inclined to happen on older specimens that have been cut back again and again. I’ve also cut old ones back too hard and had to suffer through some gawky recovery years.
The “secret,” if there is one: To get a good-shaped plant, you will often be cutting back to a mixture of oldest (thicker) wood and younger (last year’s) wood, which is counter-intuitive. The pruned structure will look odd, but try to think only in terms of creating a framework for emerging shoots that will then be topped with flowers. Your cuts indicate to the plant where it should (please) sprout from. Let go of the fact that the wood you’re leaving will be of varying thicknesses, not some perfect-looking architecture.
As for the oakleaf types, H. quercifolia, I either don’t deadhead at all or if they need pruning I do it right after bloom–not later. If they are overgrown, take out some of the oldest stems at the base, as with the blue mopheads. When I used to grow H. arborescens ‘Annabelle,’ I cut her to the ground each year (like the panicle types, she blooms on new wood, so how much you prune is up to you).
Want to hear the contrarian view on hydrangea pruning? As with all gardening, part-art and part-science, here is one, and it’s from a very impressively credentialed individual, Dick Bir of North Carolina State University, whose point of view is summed up here.