I often hear this question and it would be nice if all shrubs could be trimmed at the same time. However it really depends on the shrub and what you want to accomplish. Here are some general guidelines:
Spring Flowering Shrubs
Trim these after they’re done blooming but don’t wait until Fall. The flower buds form in late summer so a Fall trimming removes next Spring’s flowers. Examples are: Forsythia, Lilac, Snowball Bush, etc.
Summer Flowering Shrubs
These can be trimmed in the Fall or in the Spring. The flower buds form on new growth so generally you can trim these shrubs quite heavily if needed. Examples are: Spirea, Snowberry, Blue Mist Bluebeard, Rose of Sharon, etc.
These shrubs can be trimmed at anytime. If you like the manicured look you may need to trim these shrubs a couple of times per year. Spring trimming will encourage growth; Summer trimming slows growth; and Fall trimming is to prevent breakage from heavy snow. Examples are: Juniper, Burning Bush, Privet, Euonymus, etc.
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