Answer: Lilacs are pruned once a year, if needed, immediately after blooming. Periodically remove some of the oldest stems by cutting them off very short at the base. A rule of thumb would be to take out about a fifth of them each year. This will keep the plant vigorous. Also remove any suckers that are smaller than a pencil in diameter. You may also deadhead, meaning remove the spent flowers, if you wish.
Q&A Library Searching Tips