Answer: Lilac bushes develop buds during the summer months for the following spring's bloom. Prune after this season's flowers fade by cutting back the stems that produced flowers. Cut back to a pair of leaves and new flowering shoots will develop. Annual late-spring trimming will keep your lilac compact in size and enable it to produce new shoots for the following year's bloom.
You can prune your lilac back now (in mid-summer), but you may sacrifice some of next year's flower display. If that doesn't matter to you, go ahead and prune it now. If you want to preserve next year's flower display, wait until after it finishes flowering and then prune it back.
Best wishes with your lilac!
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