Lilac bushes are an old garden favorite for their early spring blooms and sweet, intoxicating scent. Without regular prunings, a lilac bush may reach up to 30 feet tall. Lilacs are hardy bushes and don't require much work, but a yearly trimming will help keep them short and manageable and will also encourage more blooms. Lilacs should be trimmed in the summer time, after the blooms have faded and before the bush starts producing seeds.
Remove spent blossoms from your lilac bush with garden shears. Removing the flower cluster immediately after the blooms fade allows the lilac bush to put its energy into strengthening its root system and growing more leaves and flowers, rather than producing seeds.
Trim any branches that stick out or that do not complement the shape you desire for your lilac bush. Lilacs are tough and will often survive being trimmed almost right down to the ground, so as long as new shoots are allowed to come up, don't be afraid to be bold with your pruning.
Remove up to one third of the oldest, thickest stems from the interior of the lilac bush every three to five years. This will increase ventilation and allow vigorous new growth to form.
Begin a three-year plan to revitalize lilac bushes that are old, overgrown, lanky or otherwise misshapen. The first year, prune one third of the old growth of the lilac bush right down to the ground in the late spring or early summer when the lilac has finished flowering. The next year, remove another third of the old growth, and the year after remove the remaining third. Allow new growth to form while you are pruning away the older stems. Following a three-year plan ensures that you will still have lilac flowers every spring, since it takes three years for lilac wood to produce flowers.
Cut a lilac bush that is completely unmanageable down to within 6 to 8 inches of the ground in the late winter or very early spring, around March or April. That year, the roots will put out a large amount of new shoots from which a new lilac bush will grow. The following winter, trim the weakest shoots down to the ground, and thereafter prune your lilac regularly to maintain good growth form. This drastic form of lilac bush renewal will leave you without lilac flowers for a few years while the bush grows back.