SOON TIME for TRIMMING!!!!! pruning lilac bushes is IMMIDEATELY after their flowering has ceased. This allows new shoots plenty of time to develop the next season of blooms. Pruning lilacs too late can kill young developing buds. If you are pruning lilac trees or shrubs entirely to within inches of the ground, it is best to do so in early spring. New shoots will develop during the regular growing season as long as there are a few healthy shoots left. link HOW TO PRUNE!!!