Q: Please advise on the best time to prune hydrangeas.
A: Hydrangeas require differing pruning times; therefore, it is critical to know which type you have before you prune. In addition to using the techniques explained below, always remove any winter-damaged branches in early spring.
- The climbing hydrangea (Hydrangea anomala ssp. petiolaris) grows to 50 to 60 feet and only requires pruning to keep it within its boundaries. Prune in early March.
- The smooth hydrangea (Hydrangea arborescens) flowers on new wood and should be pruned down hard to 1 foot in early March. This species propagates itself by sending up many ground-level suckers, which may also be cut out. Failure to prune regularly results in a top-heavy shrub that flops to the ground by midseason.
- The bigleaf hydrangea (Hydrangea macrophylla) flowers on old wood and should be pruned shortly after it flowers. Gardeners should not wait too late into fall before pruning because late-season pruning can stimulate tender new growth that might be killed during winter.
- The panicle hydrangea (Hydrangea paniculata) and its cultivars also flower on new wood but don't require the drastic pruning of the smooth hydrangea. They will look best if one-third of the old growth is pruned back in late winter. If unpruned, these cultivars can reach 20 feet, which may be too large for certain landscapes.
- The oakleaf hydrangea (Hydrangea quercifolia) flowers on old wood also but is often left unpruned since its spreading habit is quite attractive. The huge, 12-inch flowers are quite ornamental and will lend a late-season interest to the autumn garden. If the terminal buds of this type are killed off during winter, the plant will not flower.