There are four main ways to prune a shrub.
“Which style you choose will depend on the situation, the species of plant you’re dealing with, and your purposes,” said Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle.
Your aim may be to control the size of a plant. Or you may just want to tidy it up; perhaps a few branches are sticking out, or the plant has grown lopsided. On older plants, pruning can spark more vigorous growth or more blooming.
Before you make your first cut, think through your purposes and choose one of these pruning techniques.
Heading back means using sharp hand pruners to cut branches partway back, not down to the ground. This technique is most often used to shape up an unruly shrub.
Work on one branch at a time and plan ahead. “Pick your spot before you make the cut,” Janoski said. It should be just above a good bud, where a leaf will form, or just above a lateral (sideways) branch. Don’t cut in the middle between buds or intersections. That will leave unsightly stubs of dead wood, because the plant can only sprout new growth from a bud.
You can head back a shrub in winter, when it’s dormant, or in summer, after the spring flush of growth. “If you make heading cuts in spring when the plant is actively growing, it will stimulate more growth,” she said. “That may defeat your purpose if you’re trying to get a shrub under control.”
Renewal pruning means removing the old stems or canes, cutting close to the ground. It’s often done to encourage a shrub to grow and flower more vigorously, since younger wood grows faster and blooms more than older wood.
Renewal is usually done over a period of three to four years. Each year, in winter or very early spring while the shrub is still dormant, remove the oldest, thickest stems until you’ve cut out about one-third of the total stems. Cut the stems back to within 1 or 2 inches of the soil.
Rejuvenation pruning is major surgery, used to restore neglected old shrubs that have become too tangled to tackle one branch at a time. It means cutting the entire plant — all the stems — back to 1 or 2 inches above the soil. It’s best done in late winter or early spring.
Not every kind of shrub will respond to rejuvenation pruning, but some that will include forsythia, weigela, privet, honeysuckle, spirea and some kinds of hydrangea. “It can save some old, messy shrubs that you might otherwise remove,” Janoski said.
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Shearing is the only kind of pruning that can be done with power tools rather than hand tools. It means cutting all the twigs back to a flat plane or curved surface. It’s mainly used for formal hedges.
On a sheared hedge, the sides should always slope so the top is narrower than the base. “That allows sunlight to reach the lower branches,” Janoski said. “If the top is wide and shades the bottom, lower branches will have a hard time growing leaves. You’ll have a green muffin top above a lot of bare sticks.” The best time to shear is in late spring, when new growth is complete.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or [email protected]). Beth Botts is a staff writer at the Arboretum.