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Prune flowering shrubs after they bloom

Purple lilacs in bloom
The best time to prune spring-flowering shrubs and trees, such as lilacs, forsythia, viburnum, Japanese quince, redbud, and crabapple is within a few weeks after they bloom.  The same is true for summer-blooming shrubs, such as rose of Sharon, summersweet, weigela, or bigleaf hydrangea.
After they flower, most of these plants move quickly to form the buds that will become flowers next spring. If you wait more than three or four weeks to prune, you risk cutting off some of next year's blooms. Pruning later will only reduce the flower show, however; it will not harm the plant. 
Prune flowering shrubs selectively, using sharp bypass pruners to remove only those stems or branches that need to go.  In general, follow some simple steps for routine pruning. First, cut out dead wood -- dried-out, brittle branches that have no leaves. Remove branches that are broken or injured. Then look for branches that are crossing or rubbing against each other. Resolve the conflict by choosing one branch to keep and removing the other.
You may need to prune flowering shrubs to control their size; keep an attractive, balanced form; remove dead or diseased wood; eliminate tangled or crossing branches; or keep plants from becoming overgrown. Different species will require different treatment. See The Morton Arboretum  web page on pruning deciduous shrubs for pruning advice by species. 
Prune out branches that appear awkward or that cause any obstruction, such as sticking out into a sidewalk. 
To reduce a shrub's overall size, remove the longest branches at their bases. Cut off any unwanted suckers that have sprouted from the shrub's root system.