Answer: Lilacs should be pruned in the spring right after they bloom. First, remove any dead wood. Next, remove some of the oldest stems by cutting them at the ground, and remove any of the suckers that are smaller than a pencil, also by cutting them off at the ground. If you do this every year, you will always have vigorous new growth and the shrub will stay relatively short. You may also remove the spent flowers, although it is not strictly necessary.
An overgrown lilac may be cut to the ground all at once, or it may be renewed over several years' time by removing a third of the oldest wood in year one, half of the remaining old wood in year two, and the rest of the oldest wood in year three. Always cut it to the ground and, each year, remove the smallest suckers. Then follow an annual program as described above.
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