Answer: The best time to prune a lilac is immediately after flowering has finished. You can safely prune off about one third of the entire plant without stressing it too much. If you need to prune more, wait until after next year's floral display and prune it back as much as one third again. By the second or third year of pruning, your lilac tree will be a more manageable size and you will not have sacrificed any flowering. Best wishes with your lilac!
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