Identify your evergreen shrubs before it’s time to prune them in early spring. Most species aren’t suited to the harsh shearing technique used to shape these yews. (Morton Arboretum)

When you’re outdoors pruning bare-branched trees and shrubs in winter, it may seem obvious to prune the evergreens too. That’s not a good idea, according to Julie Janoski, Plant Clinic manager at The Morton Arboretum in Lisle.

“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.

You can safely prune deciduous shrubs in January or February because they’re fully dormant. However, wait to prune evergreens until spring, Janoski said.

Remember, though, that not all evergreens are the same. The different species of trees and shrubs that are common in Chicago-area yards grow in different ways, so they require different pruning techniques.

If you don’t know what species you have, the Plant Clinic can help. “Evergreens are easy to identify in winter because they still have their green needles,” Janoski said. You’ll need to send photos or bring in samples with needles or leaves attached. Once you’ve put a name to a shrub, you can also get advice on the particular technique it needs.

“Many homeowners assume all evergreens can be pruned like yews,” Janoski said. “Unfortunately, that’s not true.”

Yews were widely planted as formal hedges in the 1950s and 1960s because they can tolerate being sheared with a hedge trimmer. They remain among the most common evergreen shrubs in city and suburban yards.

If you shear a yew, inflicting many indiscriminate cuts, it can bounce back, sprouting new twigs from buds under its bark. Even if you cut the shrub back severely, to where the shaded branches are bare in the center, a yew may be better able to respond with new growth.

However, most other species of evergreens won’t regrow at all if severely pruned. “If you prune a juniper back into that dead zone of bare wood, it will never resprout,” Janoski said. “You’ll have bare branches forever.” The same is true of older arborvitaes.

Pines need a completely different approach. Their growth can’t easily be controlled by pruning. Instead, you’ll need to pinch back the new shoots, or “candles,” that sprout at the tips of branches.

In general, prune evergreens in spring, just before their big growth spurt. This may be in April or as late as early May, depending on the species and the weather.

There are two general kinds of pruning. One approach is to prune selectively, branch by branch, to keep an evergreen’s natural overall shape.

The other is to use a hedge trimmer to shear a yew or boxwood into a formal, geometric shape or hedge. Shearing is harsh treatment, and plants will recover best during their natural time of greatest growth in spring.

Don’t assume an evergreen shrub can tolerate shearing; not all can. “The first and most critical step in any pruning is to identify the species of the tree or shrub,” Janoski said. “That will guide how you prune it and when.”