Pruning Flowering Shrubs
Timing is important when pruning flowering shrubs, said Nancy Pollard, a U of I Extension horticulturist.
"It can make the difference between a delightful show, and a disappointment," explained Pollard. "Spring bloomers like lilac, forsythia, viburnum or flowering crab form their flower buds during the late summer and autumn of the previous year. This is called "old wood," since it was formed last year.
"The best time to prune spring bloomers is right after you enjoy the floral display, before the flower buds form on the new summer growth. If you prune them any other time of year, like early spring or fall, you sacrifice future showy blooms."
Summer bloomers flower on the current year's new growth. Examples are spirea and butterfly bush. Prune in late autumn or early spring, before the new flower buds form later in the spring. Annabelle hydrangea (Hydrangea arborescens) and PeeGee (H. paniculata) also bloom on the current year's growth, but rarely need significant pruning. The Endless Summer hydrangea, flowers on both old and new wood.
"Prune out old flowers sparingly to maximize new blooms," she noted.
Late summer or fall bloomers like Rose of-Sharon (Hibiscus syriacus) are best pruned in early summer. Avoid pruning any plants in late summer or early fall. Late pruning encourages new leafy growth that will not mature in time to harden off before it frosts. The tender new growth on shrubs pruned too late in the year is usually damaged or killed by frost.
For more details on how to prune specific shrubs, Pollard recommended a University of Illinois publication, Pruning and Care of Trees and Shrubs . It can be ordered online (https://pubsplus.illinois.edu/U5040.html) or by calling 1-800-345-6087. It is priced at $6.75 plus shipping.
"In addition to pruning flowering shrubs, the publication discusses pruning several kinds of evergreens and other deciduous trees, hedges, and vines," she said. "It contains basic information on pruning and care of trees and shrubs, including renewal pruning, heading back, rejuvenation, insect control and avoiding damage."