Lilacs produce flowers at the tip of the stems that grew the year before. Little pruning is necessary for Lilacs; to rejuvenate an old Lilac bush, cut a few of the oldest stems close to the ground. Most of the suckers should be removed, however, to have a dense lilac bush, some of the suckers should remain. To conserve the plant’s energy, cut off the seeds when bloom is over.
Azaleas and Rhododendrons:
Rhododendrons may require some light pruning from time to time. To increase flower production next year, remove the current year’s flower stem as soon as flowering is complete. Break out the flower head, being careful not to damage the developing buds.
Both Azaleas and Rhododendrons should be pruned after flowering. If severe pruning is necessary, do not cut any more than one third of the shrub in a season.