The Best Ways to Trim a Lilac Bush Tree

Formative Pruning

• Start formative pruning when the shrub is first planted and continue until it's about 9-feet high. Encourage young plants to develop a bushy habit with strong shoots just above soil level; these shoots will become the framework for the plant. Cut out any weak or damaged stems the first spring after planting; keep 8 to 12 well-formed branches and remove any suckers. Remove the end one-third of the tips of the main shoots.

Maintenance Pruning

• Maintenance pruning takes over from formative pruning when the shrub is mature. Remove dead or broken branches, branches less than 12 inches from the ground, those curved or too close together and any that cross or rub against another. Remove old, bare and less vigorous branches, choose strong suckers at the base of the shrub to replace them and cut the rest. Flowers take five years to form, so do this gradually.

Deadheading

• Remove the panicles once they've finished blooming and before fruit forms, especially on shrubs with large flowers, such as the common and hybrid lilacs. The spent flowers drain nutrients from the shrub. For lilacs with small blooms, this is more labor-intensive and not necessary. Lilacs bloom from buds that formed the previous season at the tips of branches, so prune just under the panicle a couple of weeks after the blooms have finished. There will be fewer flowers next year if you cut buds after they've formed.

Rejuvenation Pruning

• Lilacs that are overgrown with lots of bare wood and few flowers can be brought back with pruning. In the late fall or winter, cut half of the oldest branches to 18 inches above the soil, and cut any thin, weak or unwanted shoots. In the second year cut thin, weak shoots and any old branches remaining from the previous year. If you know the lilac is grafted, remove any suckers when they're about 12 inches long; if it's not grafted suckers can grow as part of the rejuvenated structure.