It’s January, and my hydrangeas look bedraggled and terrible. When should I prune them?
The answer will depend on which species of hydrangea you are growing. According to the American Horticultural Society Pruning & Training edited by Christopher Brickell (DK Publishing, 1996), Hydrangea macrophylla (big leaf or mophead hydrangea) should be pruned after flowering in warm climates but in colder climates it is best to leave the old flower heads on the plant over the winter, and prune in spring. This rule also applies to Hydrangea serrata. If your hydrangea is blue-flowered, it is probably H. macrophylla. In her Guide to Pruning (Sasquatch Books, 2006), local pruning expert Cass Turnbull of Plant Amnesty says this plant should not need much pruning, but if you want to remove the faded blooms, you can do this in February by looking for four or five pairs of plump buds below the old flowers, and cutting just above the lowest or second lowest set of buds.
Hydrangea paniculata should be pruned in early spring, before active growth begins.
Hydrangea arborescens needs little pruning, and any pruning at all should be done in early spring.
Oakleaf hydrangea (Hydrangea quercifolia) also needs little pruning, but may be pruned in spring.
The U.S. National Arboretum offered a good introduction, no longer available on their website, but excerpted here:
“Established bigleaf, panicle, oakleaf and smooth hydrangea plants can often benefit from regular pruning. Removing about one-third of the oldest stems each year will result in a fuller, healthier plant. This type of pruning is easiest to do in winter, since the absence of leaves makes it easier to see and reach inside plants.
Gardeners may also want to prune to control height or to remove old flower heads. The best time for this type of pruning differs between species. Bigleaf and oakleaf hydrangea, which flower on previous year’s growth, should be pruned shortly after flowering is complete. Panicle and smooth hydrangea flower on current year’s growth and can be pruned anytime from late summer until early spring. If pruning these two species in the spring, try to prune before leaves appear. Plants of H. arborescens ‘Annabelle’ have been known to produce a second flush of flowers if pruned lightly after the first flowering.
Stems of bigleaf hydrangea that have been damaged by cold should be pruned as soon as it is determined that they are dead. Watch for new growth at the base of the plant. If your plant has basal shoots that are 6 to 8 inches in length, but the upper parts of the stems are still bare, then the bare stems need to be removed. For bigleaf hydrangea plants that are subject to frequent weather-related dieback, other than removing the dead stems, you probably won’t ever need to do any other pruning–Mother Nature has been doing the work for you.”
Kitsap County Master Gardener Peg Tillery, in an article formerly available on the WSU Extension Kitsap County web site, recommends waiting until March to prune hydrangeas: “In our climate we need to wait until early March to prune roses and summer blooming hydrangeas. This way the tender new growth won’t be harmed by frosts.”
The Royal Horticultural Society provides general pruning recommendations.
Here is an excerpt from a Seattle area gardener’s response to a question about hydrangea pruning on Garden Banter, a British gardening forum:
“Different species of hydrangeas have different criteria for pruning. Some
need very little pruning at all,other than to shape, as with H.
quercifolia and H. anomala petiolaris. H. arborescens does well with the
dramatic pruning you describe. Pruning of H. paniculata would depend on if
you were training it to be upright like a tree or as a broad shrub, and need
not be dramatic pruning, just barely enough to induce new growth on which
flowers occur, though in your zone a more dramatic pruning might be needed
because of winter damage. Most hydrangeas prefer late winter pruning, but
H. macrophylla is better done in late summer when flowers are getting
scruffy and new shoots are developing.”