I would like to dramatically reduce the size of my very large, overgrown, fragrant viburnums and lilacs. Can this be done successfully? If so, how should I proceed?
— Camille Jordan, Evanston
I prune plants that are overgrown very aggressively. Most gardeners hesitate to prune aggressively enough to get good results. Overgrown plants have little value in your garden, so don’t be afraid to prune. The dormant season is the best time of year to do this work, while the plants have a lot of stored energy in their roots to push up new growth in spring. With the unusually early warm weather this year, the window to complete this type of pruning is closing rapidly. Your viburnums and lilacs should respond well, as long as they are in decent health. These plants formed flower buds last year, so the heavy pruning that you will be doing this winter will remove all the flower buds. That means you should not expect flowers this year. The shrubs will flower next year, providing that you avoid pruning them after their flower buds form this year in late summer or early fall.
The first step is to try to remove all the large stems — especially if they have formed mature bark — at ground level. A small folding hand saw works well for this task and minimizes damage to any remaining young stems. Prune at ground level to avoid leaving stubs. The remaining young stems will need to be cut back in height if they are spindly and flopping over. It is difficult to give exact height recommendations without seeing a picture of your shrubs, but it is likely that you will end up cutting the remaining younger stems back to a height of 2 to 3 feet. Encourage growth from the base of the plant by cutting the shrubs back lower than you think they should be. If you do not cut them back hard enough, you will end up with spindly growth and not have a shrub that is densely branched from the base. This type of pruning is dramatic.
If your shrubs do not have young stems coming from the base of the plant, cut back the old thick stems to about 2 feet, or just above the point where there is a young stem. This can make for a rough-looking shrub, but in most situations, this leads to new growth from the old stems and base. I almost always get good results from this pruning approach. Since the goal is to remove most of the old stems, prune back thick remaining stems to new canes that develop lower on the stem over the next couple of years. Cut back any remaining young stems. Once you get these old thick stems to a couple of inches from the ground, you should be able to stop cutting them back, providing you have developed a good-looking shrub.
Monitor these shrubs in spring and cut out any dead portions of stems. You will probably have some follow-up pruning to do. Cut back any new growth to encourage the shrub to develop more densely. It may also help to fertilize your shrubs in spring and provide extra water during dry spells during the growing season.
For more plant advice, contact the Plant Information Service at the Chicago Botanic Garden at plantinfo@chicagobotanic.org. Tim Johnson is senior director of horticulture at the Chicago Botanic Garden.
