Answer: Your lilacs will survive, but you probably won't get any flowers this year. Lilacs produce flower buds during the summer months for next year's display. Pruning them now, which just happens to coincide with the normal end of lilac flowering, should provide plenty of time for your plants to develop flower buds for next year. In addition, pruning your shrubs back will encourage lots of new growth and they won't be quite so leggy.
Best wishes with your lilacs!
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