Winter pruning of deciduous trees and bushes is hard work. It becomes easier if we spread out the work over our short eight-week dormant season.
Most deciduous trees and bushes become dormant when all the leaves have fallen. Some (hydrangeas, Chinese pistache) lose their leaves in late November and early December; others (sycamores, elms, roses) lose theirs in late December and early January. Japanese maples and crape myrtles are the last to lose their leaves and can be pruned into February if temperatures remain cool. Spring-blooming deciduous trees (dogwoods, lilacs, deciduous magnolias) are pruned right after flowering in early spring, before they flush new leaves. Trees higher than 15 feet are required, by law, to be pruned by a certified arborist.
It’s now time to prune hydrangeas and that’s a job for the home gardener. Pruning hydrangeas confuses many people. The common practice of whacking back all the hydrangea stems to the ground every winter usually results in the plant producing few, if any, flowers the following season. That’s because most older, traditional mophead and lacecap hydrangea varieties bloom on old wood, the stems that sprouted last year and the years before. There are newer hydrangea hybrids that bloom on new wood, the stems that sprout at the start of the spring season, and there are also hybrid hydrangeas that bloom on both old and new wood.
The basic technique for pruning hydrangeas so that they produce abundant flowers throughout our long summer season works well on all varieties.
Start by sticking your head into the center of the hydrangea plant. You’ll be looking for thicker older stems with dark gray wood and comparing them with newer more slender stems with lighter brown or sometimes greenish tan wood. You’re also noting crossing branches and broken or diseased stems that should be cut back to the base.
Sharpen your pruning shears, then cut out about one-third of the oldest stems. Make all these cuts back to the base of the stem. Removing a third of the oldest stems and dead, broken or diseased stems each year assures that there will always be healthy old wood on the plant. Then start pruning back each individual stem about one-third of the overall height, making your cuts just above a double leafed node if possible. Not all nodes (they look like a horizontal line around the stem) have a swelling or an immature leaf bud tip showing on each side, but most do. Double leaf nodes will produce two new stems which will each flower. When cutting flowers or deadheading hydrangeas during the bloom season, make those cuts just above a double leaf node as well to encourage continued flower production.
Hydrangeas prefer shady conditions, low PH or acid soil, and need plenty of water in summer. Our arid climate with alkaline high pH soil and water is not ideal for them. Reapply a 3- to 4-inch layer of a shade plant mulch or planting mix to insulate roots and conserve water after cleaning up pruning debris. Sprinkle a cupful of sulfur granules to help lower the soil pH around the plant’s root zone.
Send Elinor Teague plant questions at firstname.lastname@example.org or email@example.com (“plants” in the subject line).