Prune now for healthy shrubs and trees

Winter is a good time to prune trees and shrubs. From now until new growth begins in spring, overgrown deciduous woody shrubs can be heavily pruned to rejuvenate them.

Start by removing all dead wood. Prune old canes off at ground level whenever possible, leaving young canes that are smaller in diameter. Try not to leave stubs at the base. You may also need to cut back the young growth if it is spindly.

If there are not any young canes, cut the large canes back to 2 to 3 feet from the ground. These stubs will be unsightly for the time being, but if the plants are healthy, extensive new growth should start from the old canes in spring and fill in the plant.

Shrubs such as lilacs that formed flower buds last summer will not bloom this spring if pruned aggressively in winter.

It still is important to water any evergreens planted in containers outside during any warm spells throughout the winter. Evergreens lose water through their leaves and will be more likely to suffer winter burn if they dry out.

Continue cutting buckthorn out of native habitats or garden areas during the winter. There will be less impact on herbaceous native plants or adjacent perennial borders if this work is done on frozen ground.

Be sure to treat the cut buckthorn stumps by spraying or painting them with a systemic herbicide such as glyphosate or triclopyr to kill the root system. For best results, treat the cut stumps promptly after cutting.

To work as a stump treatment, glyphosate needs to be used at a high enough concentration. Some recommend a 50 percent concentration but staff at the Chicago Botanic Garden have had success with a concentration of around 30 percent.

Glyphosate has certain drawbacks: It is water-based and will freeze at low temperatures, and it is only effective on the cut surface. Another herbicide, triclopyr, works better in winter because it is oil based and will not freeze. It also is effective through the bark and not just on the cut surface.

Ask a good garden center or the botanic garden's Plant Information Service for help in choosing an herbicide for this use. Read and follow label directions and be careful not to let these herbicides dribble or spray on plants you do not want to kill.

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