As mentioned in last week’s post, there are two groups of shrubs I receive more questions about than all the rest. We discussed various aspects of rose care last week, so now on to the second group: hydrangeas. Confusion still abounds, despite many recent improvements through plant breeding. Matters are not helped by some disagreement among professionals. Ask Tim Wood of Spring Meadow Nursery, a part of the Proven Winners’ brand, and you might get a slightly different answer than from The American Horticultural Society’s Pruning and Training guide. We will discuss how Wellfield horticulture staff prune four different species of hydrangea on display in the Gardens.
The questions to ask when pruning any fruiting or flowering tree and shrub include:
- When does it flower and/or set fruit?
- Does it do so on new growth or old growth? (and specifically, how old?.. since some plants like old lilacs lose flowering vigor with age).
As a general rule, spring flowering plants flower on older growth (buds set last summer), while mid-summer and fall flowering plants bloom on new or current season growth. Knowing a little bit of plant development goes a long ways towards making pruning decisions a lot easier.
Hydrangea macrophylla, or bigleaf hydrangea, carries the most confusion and hesitation among the average backyard gardener. Each year, someone asks why their hydrangeas failed to bloom, and should they cut off all the buds? The answer is: possibly. Old hydrangea varieties bloom strictly on older wood, and the biggest culprit for no growth is the weather. Late-fall freezes or extreme fluctuations in early spring warm-cool cycles can damage above-ground plant tissue.
Some will advise pruning bigleaf hydrangeas in summer right after flowering. However, this removes persistent flower heads which, if left on the plant, can provide winter interest. Wellfield staff waits for spring bud swell to occur before pruning. The first step is to cut to the ground any dead or extremely old, unproductive stems. This makes way for new growth. The second step is to cut flowered stems back to strong new buds. Stems that did not flower are left intact to hopefully bloom this season. The last step is to remove any thin, spindly, often withered-looking growth to concentrate plant resources in more productive stems.
Plant breeders removed many of these concerns starting with the Endless Summer series which grows on old and new season growth, thus eliminating much of the concern over weather and bad pruning.
Hydrangea arborescens, or smooth hydrangea, blooms on new growth. Wellfield staff cut all stems of this species back to swelling buds after removing dead and spindly material. Flopping flowering stems is a concern for older Annabelle hydrangeas. These are best cut only in half, leaving a foot or two of strong, woody stems supporting large summer blooms. Newer varieties are bred with stouter stems, eliminating flopping concerns.
Hydrangea paniculata, or panicle hydrangea, blooms on current season growth in late summer. These spiky flower heads can be handled in a couple of different ways. The first method calls for cutting out all but three or four strong stems on newly planted small plants, shortening the remaining three or four to ten to twenty-four inches. Remove previous season’s growth back to the lowest healthy pair of buds, which produces fewer, but larger, blooms. The second method varies from the first only in removing only three to six inches of old growth all over the plant. This is done to maintain a larger shrub and keep it proportional in size to others planted nearby.
Hydrangea quercifolia, or oakleaf hydrangea, blooms on older growth and requires no special pruning consideration. Wellfield staff treat oakleaf like any other summer flowering shrub; pruning in winter to maintain good structure and plant health, removing dead and weak wood along with crossing, competing stems.
Hope this helps you grow fabulous, healthy plants this season!
Josh Steffen, Horticulture Manager
Wellfield Botanic Gardens