As with other woody plants, late in the dormant season just before growth begins is usually the best time to prune shrubs. At this time, stored food reserves are least affected and cuts close over most rapidly. Without a cloak of foliage on the shrub, it is easy to observe the structure of the plant and determine which branches to thin.
What about the flowers? Pruning spring-flowering shrubs in the late dormant season removes some of the flower buds set during the previous season. When pruning is restricted to a light annual thinning, however, losses are minimal; don’t wait until a major pruning task is necessary because it will dramatically reduce the flower display.
Although it is possible to prune immediately after flowering, most shrubs bloom in spring—a busy time when many other tasks must be attended to in the garden. Pruning during the dormant season allows you to spread gardening activities throughout the year.
A small group of shrubs—including abelia, butterfly bush, bluebeard, hydrangea, roses, and some spirea—bloom in mid- or late summer on new wood produced in the spring and early summer. These must be pruned in the late dormant season to encourage new growth with flower buds. Pruning after growth begins only removes the flower buds.
By understanding how plants respond to pruning and by using good judgment, it is possible to prune lightly almost any time of the year without harming the plant. The vigor of many flowering shrubs won’t suffer if pruned just after flowering, especially if leaves have not fully developed. Limit late-summer pruning, however, because it stimulates the growth of new shoots, which can be injured by cold winter temperatures.