Many gardeners know that winter is a good time for pruning shrubs but hesitate because they’re not sure how to go about it.
Since different shrubs have different needs, pruning them can seem complicated, said Sharon Yiesla, plant knowledge specialist at The Morton Arboretum in Lisle.
Ideally, you’d know the species of every shrub in your yard, so you can look up how and when to prune it. You can find lists at mortonarb.org/plantadvice.
Don’t know your shrub’s name? Don’t let it stop you, Yiesla said. “You still can prune it if you know just two things about it: when it blooms, and how many stems it has.”
Bloom time is important because spring-blooming shrubs such as forsythia, mock-orange and flowering quince already have flower buds on their branches during the winter. “If you prune those plants now, you’ll be cutting off some buds,” Yiesla said.
Although pruning off buds now will reduce the flower show in spring, it won’t actually harm the shrub, Yiesla said. “Sometimes winter pruning is necessary, even for spring bloomers, if the shrub is really overgrown or out of shape.”
Shrubs that bloom in summer, such as butterfly bush, winterberry and rose of Sharon, don’t have their flower buds yet, so there’s no downside to pruning them in winter.
To figure out which branches to prune, look at the base of the shrub, Yiesla said. If you see a large cluster of stems coming out of the ground, it’s a multi-stemmed shrub. Thin out multistemmed shrubs such as spirea and lilac by removing about one-third of the largest, oldest stems, cutting them just above soil level.
Other shrubs may have a single main stem, like a short tree trunk, that branches a few inches or a foot above the ground. “Sometimes there are three or four of those fat stems,” Yiesla said, “but it’s not a whole cluster.”
The technique for pruning single-stemmed shrubs is called heading back or making heading cuts. To make a heading cut, find an overlong or awkward branch. Follow it back to where it branches off another stem. Remove it by cutting just outside that joint.
“Heading back is a way to control the size and shape of a shrub by cutting off branches, rather than entire stems,” Yiesla said. This technique can be used to shape up multistemmed shrubs too.
Two categories of shrubs should never be pruned in winter. One is formal hedges or sheared shrubs that you maintain with hand or electric hedge trimmer. The other is evergreen trees and shrubs.
Since evergreens never become completely dormant like deciduous shrubs, pruning them in winter could stimulate new growth. “Any new twig that grows in winter won’t survive,” Yiesla said. Wait to prune evergreens until very early spring, just before the growing season starts.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (mortonarb.org/plantadvice, 630-719-2424 or [email protected]
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). Beth Botts is a staff writer at the Arboretum.