May 26, 2020
Introduction Pruning hydrangeas baffles many gardeners—novice and experienced alike—because of hydrangeas’ growth habits. Before you even think about grabbing your pruning tools, you need to identify which species you have in your garden. Many well-meaning gardeners reduce a hydrangea’s blooming potential through simple pruning errors. There are several species of hydrangeas commonly grown in New Hampshire, and all of them have very different growth habits and pruning requirements. While some plants bloom on new growth, others primarily set flower buds on old wood. Fortunately, hydrangeas are quite simple to care for if you understand their growth habits and follow these simple pruning guidelines.
Bigleaf Hydrangea (Hydrangea macrophylla) Bigleaf hydrangeas are the quintessential hydrangeas most gardeners are familiar with. Also called mopheads or lacecaps, bigleaf hydrangeas are characterized by shiny, dark green foliage and large colorful blooms. They generally bloom on old wood and can be difficult to get to flower in New Hampshire. Cold winter temperatures and drying winds often kill flower buds, leaving plants with lush foliage but no sign of flowers. Flower buds are produced at the tips of stems on old wood of the previous year. If those buds are killed or damaged over the winter, the hydrangea’s flowering potential is reduced (although lower buds along the stem can develop flowers too). Wait to prune your bigleaf hydrangeas until new growth appears in the spring. Make pruning cuts one quarter inch above the first set of live buds. Hint: stems with live buds will be green on the inside, while dead stems will be brown. Entirely dead stems should be cut flush to the base.
If you have trouble getting your bigleaf hydrangea to bloom year after year, consider replacing it with a “remontant” (reblooming) type. Several cultivars have been released in recent years that flower readily on new shoots. Even under the best of circumstances, bigleaf hydrangeas can suffer winter damage, and protection is usually required to achieve maximum blooming potential. To protect the buds, simply surround hydrangeas with a mesh cylinder, taking care not to damage any buds, and fill it up with fallen leaves. The leaves may provide enough insulation to keep flower buds alive through the winter. This winter protection should be removed only once the buds begin to expand in the spring.
Mountain hydrangea (Hydrangea serrata) Mountain hydrangeas look very similar and are closely related to bigleaf hydrangeas. Mountain hydrangeas have a more delicate branching structure and flowers that are slightly less showy than their relative. Flowers develop on old wood, so do not remove stems until new growth begins in the spring when you can determine which buds are still viable. Reblooming types are increasingly available and provide more reliable blooms. Though reported to be slightly more winter hardy than bigleaf hydrangeas, mountain hydrangeas will also benefit from winter protection measures.
Smooth hydrangea (Hydrangea arborescens) Smooth hydrangeas are native to Eastern North America and are well suited to most New Hampshire gardens.They have a mounded growth habit and white to pale pink flowers that bloom mid-summer. Unlike bigleaf and mountain hydrangeas, smooth hydrangeas bloom on new wood and will tolerate heavy pruning. Old flowers from the previous year can be removed in late winter through spring. Old stems will flower, but the flowers will be small. For larger, more robust flowers, prune stems to the ground in the spring.
Panicle hydrangea (Hydrangea paniculata) The most common hydrangea grown in New Hampshire, panicle hydrangea is incredibly cold hardy and flowers reliably season to season. It grows as an upright, low-branched small tree or large shrub whose branches tend to arch under the weight of numerous, large conical flowers. Panicle hydrangeas bloom on new growth and should be pruned in late winter to early spring before leaf emergence. Remove spent flowers and prune to improve overall plant structure and habit. The size of panicle hydrangeas can be reduced by removing a few branches in the spring. Make cuts one quarter inch above live buds or to the branch collar of stems (the slightly swollen area where the tissue of the main trunk connects with the branch tissue). Large, old shrubs can also be rejuvenated in the spring by cutting a third of the oldest stems to the ground in the first year, half the second year, and the remaining old stems the third year.
Oakleaf hydrangea (Hydrangea quercifolia) Oakleaf hydrangeas are native to the Southeastern part of the United States. Although this shrub may suffer some winter damage and flower bud loss in northern gardens, its striking foliage and exfoliating bark are enough justification to grow it. Oakleaf hydrangeas also bloom on old wood, so only remove dead or damaged branches in the spring. In general, oakleaf hydrangeas do not require much pruning. As needed, thin stems and reduce plant height after new growth has begun.
Climbing hydrangea (Hydrangea anomala subsp. petiolaris) Climbing hydrangea is a vining species that grows up structures with aerial rootlets along its stems. It can be slow to establish but grows quite vigorously once it does. Older branches have attractive exfoliating bark that peels away in cinnamon brown flakes, and large flat-topped flowers that are held away from the foliage in mid-summer. Little, if any, pruning is required. Far extending lateral branches can be pruned one quarter inch above a bud shortly after flowering to reduce the weight of the vine and prevent the plant from falling off the structure it is growing on. Though not necessary for plant health, spent flowers can be deadheaded after bloom.