With a quick, fragrant burst of color each spring or early summer, a lilac makes a beautiful bloom for only a few weeks a year and then acts as a wonderful shade tree for the remainder of the growing season. When well taken care of, a lilac can grow for over a century and live as a foundation for any garden or yard. When you know when and how to trim lilac bushes you can ensure flowering each year and decrease the chance of disease from low air circulation.
Let the lilac plant bloom and wait for the flowers to start to die off. After blooming is the best time to prune without damaging next year's blooms no matter when your particular variety of lilac blooms.
Clip off old blooms with the pruners by cutting just behind the bloom at an angle. If your lilac is tall you may not be able to clip the highest blooms, so leave them and clip the ones you can reach.
Cut off old or damaged branches by following them either to the ground or to where they branch off from a healthy branch. Cut them off cleanly either at ground level or parallel to the healthy branch, respectively.
Prune the lilac for shape by trimming healthy branches only if it is obstructing moving objects, such as garage doors, or rubbing on the side of a building where damage could occur. Otherwise, allow the plant to grow in its natural habit.