As a general rule, the best time to prune any bush is in the late winter or early spring, before it begins to grow again. However, if you are careful, you can prune any time. Despite common misperceptions, pruning at the wrong time will not kill your bushes, but it may postpone blooming for a season.
The worst time to prune is right after new growth appears. Food and nutrients used by new growth are stored in the roots and stems of your bushes; if you cut off the new growth, your plant may become stunted. Always wait for new foliage to appear as this is what replenishes the roots and stems.
You should also avoid pruning too late in the fall, as most plants need a chance to harden off before the stresses of winter. Pruning in the fall encourages new growth and this may not have a chance to harden off. Your best bet is to wait until later in the season when the bush is dormant.
Nature does a lot of pruning for you in the winter when branches break from the weight of snow or wind whips some stems away. Basically, you want to mimic nature, but with more control. Always cut out old, dead, damaged, or diseased wood when you prune your bushes. If new damage is done, remove it right away, as the open wounds on your bushes are an invitation to insects and diseases.
Some plants need to be pruned at other times to encourage next season's growth. For best results, always check before pruning to find out what the particular bush needs.