Most lilacs don’t require pruning until they reach about six to eight feet ( m.) tall. The best time for pruning lilac bushes is right after their flowering has ceased. This allows new shoots plenty of time to develop the next season of blooms.
If you trim a lilac bush too late in the year, then you’ll cut off unwanted branches and next year’s blossoms. The ideal time to prune is just after the bush finishes blooming for the year. May 20, New lilac bushes don't require maintenance pruning until they are about 6 - 8' tall. The time to prune mature lilac plants is just after the flowers have faded.
They set the next season's flower buds almost immediately, so late pruning will mean sacrificing next year's flowers.