Why prune shrubs?
Pruning is part of the routine care of landscape shrubs. It can mean maintaining the specimen at a desired size and shape. reducing the overall size of a shrub, or removing ailing limbs. All of these tasks generally further the goal of keeping a flowering shrub growing in a healthy, vigorous fashion. Strong growth means bountiful bloom.
Here's an analogy for the layman gardener: Rabbits have babies; shrubs have flowers. Eventually the rabbbit will get old and tired, and stop having babies. Unpruned shrubs accumulate old branches. The unpruned shrub thinks it's too old and tired for making flowers (babies.) Pruning keeps a shrub in a prolonged juvenile state, always striving to make lots of flowers.
Why prune "after flowering"?
Shrubs that don't bloom just fade into the great greenness of spring. It's the flowers that make the statement. Blooming bushes invest a lot of energy and time into forming those flower buds. When you prune shortly after this year's flowers are done, the shrub begins the job of buildling next year's flowers on the remaining branches. With good pruning and care, you'll get as many blooms as the bush can possibly make. Like the prolific rabbits, the shrubs want to make "babies."
How soon is "soon" after flowering?
The sooner the better. When the shrub is finished blooming in spring, it will quickly push out a lot of new growth. Trimming promptly will mean less mass for you to bundle up. Better yet, the bush won't exert a lot of energy on new growth, only to have you trim it off three months later. Prompt pruning allows the plant to direct its energies towards summer growth and the eventual formation of next year's buds.
When sooner doesn't quite happen, though, assume about a two month window of pruning time. The emphasis is on making sure the bush has not gone about the job of forming new flowers for next year, on branches that you are soon to lop off. Remembering to "prune soon after flowering" gives you the right pruning timing for spring flowering (and many other flowering) shrubs.
What kind of pruning is needed?
Some shrubs have unattractive seeds heads or remnants of flowers. Lilacs fall into this category. Cutting off the "nasty bits," those ends of branches bearing all the twiggy flower remains, makes these bushes more attractive.
Next look for damaged or dead branches. Cut back to near the ground, or to a major, live branch. Try to direct growth outward by cutting above a bud facing out from the center.
Many shrubs grow multiple stems from the ground. The older stems eventuallly bloom less. For shrubs with multiple stems, take out some of the oldest every year. The pros say you can generally take out one-third to one-fifth of the canes (stems) each year. Cut them all the way to the ground. Another way to express this is "remove any canes older than five years." That rate should keep the shrub at about the same overall size. Of course, use your judgement if a specimen is trending much too big or too small.
There are twists to this cane removal guideline. Ninebark, for example, has many arching canes. As canes age, they thicken and display interesting peeling layers of bark. You'll need to let some old wood mature in order to enjoy this effect. Conversely, specimens like red-twigged dogwood have the best show on their young new stems. It's important to keep removing old branches to force bright new stems.
When routine pruning doesn't happen, shrubs may grow wildly out of bounds. These can often be "rejuvenated" with major surgery. Forsythia is my favorite example of a shrub that goes bonkers without regular attention with a cutting tool. "Rejuvenation" means cutting the entire above ground mass of the shrub to short stubs a few inches tall. The energy stored in the rootball will enable the bush to make plenty of new stems. You may need to thin this growth next year. Shrubs that naturally make new stems from ground level are ideal candidates for this job. Note, though that a few, such as azaleas, only have one stem from the ground. Don't cut that off, but do cut well into the bush, to the level of just several branches, if rejuvenation is necessary one of these.
Avoid shearing. Better yet, just don't shear flowering shrubs. Blooms don't show well on sheared twigs, and shearing creates a thick layer of crowded twigs enclosing bare stems.
My shrub is young and small. Do I still have to prune something?
Maybe not. Shrub youngsters in their first several years from the nursery may still be growing to fill their space. We want young adults, not little children, in the shrub border. But young shrubs, like young children, need to be monitored, and possibly corrected, or cared for when injured or ill. So watch young, growing shrubs for crowding uneven growth, or dead branches, and do remove seed heads even if there are only a few.
Remember to keep your pruners sharp, clean, and handy as you admire your spring blooming shrubs. Attend to pruning needs, and your shrubs will thank you with gorgeous bloom every year.