The lilac bush, part of the syringa family of flowering plants, is kin to the olive tree. A deciduous shrub that can grow from 7 to 33 feet at maturity, older lilac bushes can be unwieldy if left to grow unchecked. Proper care of the mature lilac bush requires shaping the plant, which reduces the chance of disease and promotes flower growth. Prune and fertilize mature lilac bushes every year.
Apply a fertilizer in the spring that is low in nitrogen but high in phosphorous. According to North Dakota State University horticulturalist Ron Smith, too much nitrogen will prevent the flowers of the lilac bush from forming properly. Use a water-soluble 10-10-10 or 20-20-20 fertilizer for the bush.
Trim larger branches from the inside of the mature lilac bush to increase air circulation. Trim just after the flowers fall off the bush to increase flower production in the new year.
Cut away suckers from the bottom of the bush as close to ground level as possible. If the sucker is coming out of the main trunk, cut is as close to the trunk as you can.
Shape the bush by standing back from it and looking at the shape. Cut away any branches that stick out too far from the main bush and ruin the shape.
Eliminate one-third of the shrub at a time if the mature lilac plant is overgrown. Eliminate the oldest stalks on the plant first, cutting them down to soil level. Allow new shoots to replace the new ones. Cut over a three-year period to rejuvenate the bush with new growth.