Prune Evergreen Shrubs in Winter?

As acreage owners ponder the winter landscape, trying to answer the question – “Is now a good time to prune my shrubs?” – it’s wise to consider a few basic guidelines.


First, the easy one; forget about pruning evergreens.  As Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle, IL indicates: “Evergreens aren’t entirely dormant in winter, so they will respond to pruning,” she said. In a warm spell, “it might trigger them to try and start growing.” The tender new growth would be vulnerable to freezing in the next cold snap.


With this is mind, it’s best to put off pruning boxwood, yews, arborvitae and holly until they’ve finished pushing out a flush of growth in the spring.  This timing will avoid damaging these plants.


Speaking of easy, a good rule of thumb for spring blooming deciduous shrubs such as viburnum, quince, dogwood and lilac is to prune them soon after they finish blooming.  Summer blooming deciduous shrubs such as mockorange and rose of Sharon should be pruned just before growth begins in late winter.


With either spring or summer blooming deciduous shrubs, it’s best to use a thinning technique rather than a shearing approach.  This is done by simply removing a third of the oldest stems at the ground level each year.  This technique removes the plant parts that are most susceptible to borers and cankers and keeps the stems in the plant that will be most vigorous and produce the best blooms.


For the evergreens, each species should be treated differently.  The thinning technique works well, but only for yews, holly and boxwood.  Small pines and spruce can be kept in check by cutting the new growth in half each year, much like Christmas tree growers do in June.   Not much can be done to reduce the size of arborvitae, other than to shear them.  Unfortunately, shearing is tricky for many reasons.


When hedge trimmers are used to shear a plant, many indiscriminate cuts are made, without attention to the best location…similar to topping a tree under power lines.  The result is a rebound of unwieldy growth in some cases, and in others, a lack of growth altogether.  The tricky part of shearing is being aware that if the dead zone of bare wood (that all evergreens have) is cut into, there will be no regrowth, leaving bare branches for the rest of the life of the shrub.


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