The smallest varieties of lilacs reach only 4 to 8 feet in height while larger varieties can grow to 30 feet tall. The fragrant blooms range in color from white and light pink to lilac and purple. Since lilacs only bloom once a year and only for about two weeks, growing different types of lilacs can extend the flowering season. Trimming your lilac bushes correctly at the right time promotes blooming and keeps them healthy.
Trim lilac bushes immediately after they finish blooming. This should be done once a year. Use clippers to remove dead blooms and suckers growing at the bottom of the bush, but leave a few suckers to grow into stems.
Use loppers or a saw to cut back large stems growing from the center of the lilac. Remove enough to provide air circulation to inner leaves but not so many that the bush looks bare. Interior or lower leaves that don't get light will brown and fall off, giving the bush a leggy appearance.
Create a uniform shape by cutting back branches that grow faster than others or stick out from the bush. Avoid topping or cutting off the top of the bush. Leave the top rounded.
Prune old overgrown lilacs in March or early April by cutting the plant back to within 8 inches of the ground. Lilacs are hardy and the plant will grow back if not diseased. The following winter select a few healthy stalks and cut back the rest. Nip off the tops of the retained stalks just above a bud to promote bushiness.
Remove 1/3 of old overgrown lilacs once a year to rejuvenate the plant and while it continues to bloom. This method differs from step 4. Select the oldest stalks and cut them back. In year one, select another group of stalks to cut back and then again after three years. At the end of three years the plant will be almost completely new.
Eliminate a few of the oldest branches every four or five years to keep the plant vigorous.