Trim lilac bushes once they reach six to eight feet in height and after they bloom. Because lilacs bloom on old growth each year, prune the current year's old growth to ensure beautiful, fragrant blooms the next year.Continue Reading
Until a lilac bush reaches six to eight feet tall, it generally doesn't need trimming. After that, it should be trimmed once blooming is over. Trimming too soon destroys the blooms still to come and may prevent blooming the following year. If the entire plant must be pruned to the ground, cut it in the early spring, as this allows new shoots to grow and develop during the growing season, with blooms returning in the following years.
To trim a lilac bush, cut away no more than one-third of the branches. Trim the entire branch off from the main stem, not just the tops. Cut away dead or diseased branches first, followed by those that cross other branches, small, thin branches and those that originate from the main stem close to the ground. If the branches are too dense to allow proper circulation to the bottom of the shrub, or don't allow enough sunlight to filter down, trim away interior branches by snipping them off the main stem.Learn more about Trees & Bushes