A rough rule of thumb is to prune spring-blooming shrubs soon after they finish flowering because most bloom on old wood, while those that bloom in summer and fall usually bloom on new wood and can be pruned in late winter or very early spring.
Spring blooming shrubs
Like many spring-blooming shrubs, azaleas "bloom on old wood," which means they set next year's flower buds shortly after they finish blooming this year. If you wait too long to prune them, you will remove many of next year's blooms when you prune, especially if you shear your azaleas.
Other shrubs that fall into this category include forsythia (Forsythia spp.), Virginia sweetspire (Itea virginica), mock orange (Philadelphus spp.) ninebark (Physocarpus opulifolius), quince (Chaenomeles spp.), rhododendrons, including azaleas (Rhododendron spp.), spring-flowering roses (Rosa spp.), spring-blooming spirea (Spiraea prunifolia and S. x vanhouttei), lilacs (Syringa spp.), and viburnums (Viburnum spp.).
Summer blooming shrubs
Conversely, shrubs that bloom later in summer and fall tend to "bloom on new wood," which means they set flower buds on the current season's growth. These shrubs should be pruned in late winter or very early spring, before they leaf out.
Shrubs that fall into this category include butterfly bush (Buddleia spp.), sweetshrub (Calycanthus floridus), beautyberry (Callicarpa spp.), trumpet vine (Campsis radicans) summersweet (Clethra spp.), buttonbush (Cephalanthus occidentalis), bush honeysuckle (Diervilla spp.), smooth hydrangea (Hydrangea arborescens), peegee hydrangea (Hydrangea paniculata), repeat-blooming roses (Rosa spp. and hybrids), summer-blooming spirea (Spiraea x bumalda and S. japonica) and chaste-tree (Vitex spp.).
Pruning is an important cultural practice to maintain the health and appearance of flowering shrubs. Removal of dead, damaged or diseased wood reduces insect and disease problems while allowing the pruner to catch problems before they get out of hand. Keeping the center of the shrub open to sunlight and air circulation improves the growth habit of the shrub while allowing interior leaves to dry quickly after rain or heavy dew - which can reduce the incidence of disease problems. Removing crossing stems eliminates potential bark damage, reducing the chance of insect or disease problems taking advantage of that damage. Pruning also forces new growth, which in most cases produces the most colorful stems and new flowering wood for future years.
Controlling plant size is low on the list of reasons for pruning, because pruning is not a substitute for proper plant selection. Most plants have perfectly lovely natural shapes that can be enhanced and somewhat controlled through proper pruning practices; very few adapt well to shearing. Most plants stay healthy and attractive longer if allowed to grow naturally, so reserve the hedge shears for formal hedges.
The process of removing stems at their point of origin is known as thinning, while shortening a stem from the top is known as heading. Technically, shearing is just making a lot of heading cuts. Thinning cuts are preferable because they open the shrub up to sunlight and air circulation. Heading cuts result in a profusion of growth below the cut that creates a wall of growth on the outside of the shrub that blocks sun from the interior of the shrub and impedes air circulation. Even formally sheared hedges should be opened periodically to encourage new growth from inside the plants.
Shrubs with a suckering growth habit such as forsythia and lilac should have the oldest, biggest stems removed at ground level periodically. Rejuvenate badly overgrown specimens removing the biggest oldest stems at ground level. This can be done all at once if the shrub is healthy and vigorous, or it can be spread out over a three-year period if it is not by removing one-third of the overgrown stems each year. Keep the sturdiest, well-placed younger stems and remove those that are damaged, spindly or too close to one another. New suckers will sprout from the roots that will have to be similarly thinned later in summer. Hard pruning should always be done in early spring, before the shrub leafs out. It is less stressful for the plant, and you can clearly see the stems when they are leafless.