Lilacs are an icon of spring, blooming in shades from deep purple to pale lavender to white all over the Chicago area.
Yet lilac bushes can be monsters. Hardy and resilient, a lilac can live for many decades, and if it is not regularly pruned, it can become tangled, overgrown and choked with deadwood.
To tame it, prune it. “Pruning will help lilacs bloom more and resist disease,” said Sharon Yiesla, plant knowledge specialist in the Plant Clinic at The Morton Arboretum in Lisle.
The time to prune lilacs is right after they finish blooming. Like other spring-flowering shrubs such as serviceberry, chokeberry, forsythia, viburnum and weigela, lilacs form their flower buds for next year just after they bloom this year. “There’s a pruning window of three weeks or so after they finish blooming,” Yiesla said. “If you wait longer to prune them you’ll probably be cutting off next spring’s blooms.”
Left to itself, an old-fashioned lilac will grow into a large, ungainly plant up to 15 feet tall with a dense, crammed mass of stems, many of them dead. Lilac shrubs also spread through suckers — new stems that sprout from the root system and can crowd out other plants.
Pruning a lilac after blooming not only will keep it under control, but will stimulate it to have more flowers. Lilacs need full sun, and they generally bloom only where the flower buds get plenty of sunlight. On overcrowded plants, that tends to be at the top. “If you prune the shrub, you’ll get more blooms and they will be more at eye level,” Yiesla said.
Usually, lilacs need the renewal method of pruning. Plan to prune the shrub this way every year.
First, remove all stems and branches that are dead. Then remove a few of the oldest, thickest living stems, cutting them down to about 2 inches from the ground. Be careful not to prune too heavily.
To reduce the overall height of a lilac, target the tallest stems for elimination. To further control the shrub’s size, look for branching among the remaining stems. Remove the longer branch just above the place where it is attached to the main stem. Don’t cut in the middle of a stem.
Then search for outlying suckers and prune them down to the ground. You want to keep some new sprouts at the base of the plant for future growth, but removing far-flung suckers will keep the shrub from spreading out of control.
Pruning out the spent flowers is tidy, but it isn’t essential.
With the dead wood and oldest stems gone and the shrub now more open, air will be able to circulate, drying out trapped moisture that can encourage powdery mildew and other fungal diseases.
If you follow this procedure every year after the shrub’s flowers are spent, you will have a lilac bush that consistently blooms but stays in bounds.
For tree and plant advice, contact the Plant Clinic at The Morton Arboretum (630-719-2424, mortonarb.org/plant-clinic, or plantclinic@mortonarb.org). Beth Botts is a staff writer at the Arboretum.
