Buds of spring bloomers form on "old wood," which means they form during the late summer and autumn of the previous year. Thus, Pollard says, "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 at any other time of year, such as early spring or fall, you sacrifice future showy blooms."
Summer bloomers such as spirea and butterfly bush form buds on the current year's new growth. They should be pruned in late autumn or early spring before the new flower buds form. Annabelle hydrangea (Hydrangea arborescence) 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. Pollard suggests that old flowers be pruned out sparingly to maximize new blooms.
Late summer or fall bloomers such as Rose-of-Sharon (Hibiscus syriacus) are best pruned in early summer. Pruning in late summer or early fall encourages new leafy growth that will not mature in time to harden before the first frost, which usually damages the tender new growth on shrubs pruned too late in the year.
For more pruning advice, Pollard recommended a University of Illinois publication, Pruning and Care of Trees and Shrubs ($6.75 plus shipping). It can be ordered online (URL below)or by calling 1-800-345-6087.
"In addition to pruning of flowering shrubs, the publication discusses pruning of 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."