Apr 02, 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. 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.
Jul 08, If you can't stand the look of your old lilac or you just want a quicker approach, you can take the drastic measure of cutting back the entire plant to about 6 to 8 inches above the ground in the early spring. 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.