How do you prune lilacs? —Mike Logan, Rosamond, CA
Prune all lilacs immediately after they finish blooming to make sure you get flowers each year. Flower buds form during the summer for blooms the following year.
Your pruning technique depends on which lilac you have and what you want it to look like.
If you have an old-fashioned common lilac (Syringa vulgaris, Zones 3 to 8) that’s too large, begin renewal pruning once flowers fade this spring. Remove about a third of the oldest, thickest stems right to the ground. Do this for three years. This method leaves some mature stems that will bloom the following spring.
If you want to restrict the width as well as the height of your lilac, thin out some suckers and small new stems each year. You must leave some, though, to mature and take the place of older stems you cut down.
Because they grow slowly, compact varieties do best when pruned lightly every few years, right after the flowers fade.
If lilacs form a hedge, keep the upper part slightly narrower than the lower portion so the base isn’t shaded by the top. Without good sunlight, foliage will be thinner at the base and only the upper parts will bloom.