Winter is pruning season: How to trim overgrown woody shrubs

Pruning is my favorite gardening task by far, and the winter dormant season is a great time to prune trees and shrubs.

You can heavily prune overgrown deciduous woody shrubs up until new growth begins in the spring. Most gardeners, however, are hesitant to prune them aggressively enough for the best results.

Start by removing all dead wood. Prune old stems off at the ground level whenever possible, leaving young canes that are smaller in diameter. You may need to cut back the young growth if it is spindly. If there aren’t any young canes, cut the large stems back to one to two feet from the ground. This will be unsightly but if the plants are healthy, you’ll see extensive new growth from the old canes in the spring.

It is important to note that shrubs such as lilacs that formed flower buds last summer will not bloom when pruned aggressively in winter. Check these plants later in spring and cut out any dead sections in the stems. Then continue to prune out the old stems in subsequent winters until they are completely removed and replaced with healthy new stems.

It is best to keep any logs that you saved for burning in the fireplace outside until you are ready to burn them because insects can come in with the firewood. I keep a small stash of firewood piled near our mudroom door for convenience. Never treat firewood with insecticides.

You can continue cutting buckthorn out of native and cultivated garden areas during the winter. There will be less impact on herbaceous plants when this work is done on frozen ground.

Be sure to treat stumps with an herbicide such as glyphosate or triclopyr herbicide to kill the root system. Glyphosate needs to be used at a high enough concentration to work as a stump treatment. Some recommend a 50% concentration — our staff has had success with a lower concentration of around 30%.

One of the drawbacks with glyphosate is that it is water based and will freeze during cold weather and is only effective on the cut surface. Triclopyr is the preferred method because it is oil based, effective through the bark and on the cut surface and will not freeze. Follow all label instructions.

• Tim Johnson is director of horticulture at Chicago Botanic Garden, chicagobotanic.org.