A healthy and well-blooming lilac bush adds color, fragrance and shade to the landscape. When blooms start to look small or grow sparse, it may be time to prune. Pruning immediately following blooming in the spring is best for redirecting the plant's energy. If necessary, you can also prune lilac bushes in the fall. Pruning not only helps increase the size and number of flowers, but also improves the air circulation around your lilac to reduce the chance of disease.
Cut away any spent blooms on the ends of the branches. Clip the branch with hand pruners just behind the blooms and just after a set of leaves.
Look over the lilac bush for old, brittle branches, branches that rub against one another, and any misshapen branches. Use lopping pruners to cut away these branches as close to the ground as possible. Clip old branches only every three to five years.
Step back from the lilac and review overall shape of the lilac bush. It should be a wide "V" shape. Cut away any branches from their base that disrupt this shape or are rubbing along the side of a wall or fence. Leave behind at least five strong stalks.