Answer: You didn't specify which plants you want to prune. A general rule of thumb is to prune immediately after flowering. This is because many plants (rhododendrons, for example) spend all summer developing flower buds for the next year's display. If you prune too late in the year, or too early in the spring, you could be pruning off potential flowers.
If you submit a new question with a list of specific plants, we can give you a more specific answer.
Best wishes with your landscape!
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