Despite their short blooming season, the unmistakable look and fragrance of lilac trees inspires many gardeners to grow this garden specimen. Known as a low-maintenance plant, the lilac tree does require a little annual pruning to keep the blooms coming. To trim lilac trees you must clip the plant lightly, since it starts working on next year's blooms on this year's limbs as soon as flowering is over.
Wait for the flowering to finish on the lilac tree and watch for drying blooms. This will be your signal for trimming each year. For most lilac varieties this occurs in the early spring, but some late-blooming lilacs don't flower until closer to summer.
Clip off spent blooms one at a time by grasping the ends of branches you can reach and cutting the stem off the lilac bloom just behind the last set of flowers. Leave just the tip of the branch behind.
Clip as many blooms as you can reach safely, leaving any higher blooms alone. As the blooms dry, occasionally you can knock the dried ones off with a long stick, but this can damage other limbs and is not recommended.
Look over the plant for any old, damaged branches, and for branches that are crossed or rubbing on one another. Cut these branches off at their base 1 inch from the ground with a clean, horizontal snip only every four or five years.