Answer: Lilacs bloom in the spring, so prune immediately after flowering. If you prune too early, you'll sacrifice the blooms. If your plant needs rehabilitation, you can remove one third of the oldest thickest stems for three years in a row. Then, each spring after the shrub blooms, remove any suckers smaller than a pencil; also remove enough of the larger suckers and older trunks to make a well shaped shrub. Always cut as close to the ground as you can. Lilacs seem to appreciate a nice dose of fertilizer or rotted manure early in the spring, too. Good luck with your lilac!
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