Late winter and early spring are ideal times for pruning many shrubs. At this time, shrubs are leafless, so you are better able to see the overall shape of the shrub and easily identify dead, damaged, diseased wood and structural defects. Because pruning stimulates new growth, late summer and fall pruning are not desirable pruning times as new growth at that time of year is subject to winter injury. (Rutgers University factsheet fs1221)
Summer blooming shrubs such as summerweet (Clethra alnifolia) and blue mist spirea (Caryopteris x clandonensis) are pruned in late winter/early spring because their flower buds form on new wood produced during the current season.
Spring flowering shrubs (those that bloom before mid-June) should be pruned after flowering. Their flower buds develop during the previous growing season on “old wood." If pruned too early, you will remove the flower buds. Examples of some shrubs that fall in this category include lilac (Syringa spp.), azalea, forsythia, fothergilla, kerria, mockorange (Philadelphus coronarius), rhododendron, and weigela.
Prune shrubs using the renewal method in which some of the largest stems of the plant are cut to the ground. New stems will arise from the base of the plant. This practice maintains the natural shape of the plant and allows more air and light to reach the interior of the shrub. Older stems often don't produce as many flowers so this will increase flowering. Don't remove more than one-third of the largest stems. There is no need use pruning paint on any of the cuts, as the wounds will naturally seal.
Selective heading cuts (removing the terminal part of a stem back to a bud) from one-third of the younger stems will help to control the size of the shrub, while still maintaining a natural shape. Do not use hedge shears to shear off all of the stems to the same height as this will destroy the natural shape of the shrub.
If you have an overgrown shrub that is no longer flowering, try rejuvenation pruning in early spring by cutting all the stems back to the ground. This may delay flowering for several years but will result in a more manageable shrub. Plants that respond well to rejuvenation include, forsythia, lilac, and spirea.
Here is a good reference with more detail and illustrations about pruning, as well as lists of spring and summer flowering shrubs: Pruning Flowering Shrubs
