Like other spring blooming shrubs, lilacs develop flower buds for the next year after blooming during the current year. Lilacs should be deadheaded immediately after blooming to encourage good bud
development and flowering the following spring.
Deadhead: Use a hand pruner to cut off dead flower heads down to a pair of leaves, or use a hedge trimmer for larger plantings. Shear lightly, taking off only the dead flower heads.
Renewal pruning: Renewal pruning allows more light throughout an older plant and encourages new stems to grow and flower. Newly planted lilacs usually do not need pruning for 2 to 3 years and it will take 1 or 2 years before a lilac blooms. Years later, lilac stems can grow very large and tree-like and they will tend to shade out new growth at the plant base. Use a lopper or hand saw to remove a third of the thickest stems at the base. Do this every year until all large stems are removed.
Rejuvenation pruning is a technique for smaller, densely-branched lilacs like Korean or Meyer lilacs (Syringa meyeri) that have become overgrown. In late winter, use a saw or hedge trimmer to cut the entire plant to the ground. New growth will grow from the root area in spring. The plant may not bloom or sparsely bloom for 1 or 2 years.