Lilacs can be pruned after they've finished blooming. The flower buds for next year form in pairs where the leaves join the stems. After bloom, remove the spent flower clusters just above the points where the buds are forming. Heavy pruning will result in loss of next year's bloom. You can thin out dead and weak wood at the same time, and renovate overgrown shrubs by pruning back a few of the oldest stems to the ground.
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