Many people often wonder when the best time of year is for pruning hydrangeas. Hydrangeas are in a class of their own when it comes to pruning. Unlike other flowering shrubs, one pruning window does not exist for all types of hydrangeas. This special classification arises with hydrangeas because five different species are most readily available for home gardening. Two pruning groups are formed among these species. The two classifications that arise within hydrangeas are those that bloom on old wood and those that bloom on new wood. Each group should be pruned differently and at a different time of the year.
Pruning Oakleaf, Big Leaf, and French Hydrangeas
Oakleaf hydrangeas (H. quercifolia), Lacecap/Big Leaf hydrangeas (H. serrata), and Mophead/French hydrangeas (H. macrophylla) each bloom on old wood that was formed in the years prior to the current bloom year. The flower buds on these plants form in the late summer and new spring growth does not form a flower. Hydrangeas in this class typically bloom in early summer with flowers beginning to fade by mid-summer and overall flowering slowing. Since the flower buds for the following year are formed on the existing growth (old wood) in late summer with these types of hydrangeas, then these old wood flowering hydrangeas should be pruned just as their flowers being to fade. This will ensure that buds for the following year are not removed by pruning too late in the season.
Hydrangeas that flower on old wood typically only need a soft trim and removal of spent blooms. This should happen immediately after flower cessation to maintain plant form and overall health. If a shrub is older and is producing small blooms, then removing a few of the larger branches at the soil line will help produce larger blooms and better growth. This technique can also be used to maintain plant height by removing the tallest canes at the soil line after flower cessation. Cuts should be made flush with the trunk when pruning large canes. When pruning smaller branches cuts should be made at 45 degree angles 2/3 cm from the lower leaf stem ensuring that no more than 1/3 of the top growth is removed. When pruning always use sharp, clean pruners or loppers.
Pruning Panicle and Smooth Hydrangeas
Panicle hydrangeas (H. paniculata) and Smooth hydrangeas (H. arborescens) bloom on new growth that is formed in the spring once the plants break dormancy. The flower buds of these new wood flowering hydrangeas set on new growth that forms in the same year that they bloom. Panicle and Smooth hydrangeas typically flower later in the summer than Big Leaf, Oakleaf, and French hydrangeas because the plants in the new wood classification need to form buds after breaking dormancy; while the old wood hydrangeas already have bud set.
New wood flowering hydrangeas should be pruned in late winter or early spring to promote new growth that forms flowers. These types can be cut back to the ground, but leaving a framework of old growth 1 to 2 feet above the ground provides support for new spring growth. Keeping this framework of old growth at the base of the plant helps prevents new growth hydrangeas from flopping over after heavy rains or under the weight of heavy blooms. The blooms on Smooth hydrangeas tend to be larger when the plant is cut back harder (to the soil line). Smaller blooms on sturdier stems resulting when 12 to 24 inches of old growth is left above the soil line.
Pruning Hydrangeas at the Wrong Time of Year
Pruning hydrangeas at the wrong time of year often results in minimal or no blooms during the flowering season. The variety of a hydrangea should be determined before any pruning takes place to ensure that optimal flowering will occur. One trick for determining the pruning classification is through flower color. Big Leaf and Mophead hydrangeas (old wood flowering) typically have pink or blue flowers. Oakleaf hydrangeas (old wood flowering) have a tell-tale oak tree shaped leaf. Panicle hydrangeas (new wood flowering) have white or lime colored blooms. Smooth hydrangeas (new wood flowering) have large white blooms. Popular varieties of hydrangeas include:
Crossing branches and dead branches should always be removed to ensure plant health and longevity.