Gardeners should only prune lilac bushes on wood that is at least 3 years old, which can present a challenge when trying to prune a mature shrub. Iowa State University recommends gradually replacing the old growth with new growth by pruning over a three-year period. This allows gardeners to continue to enjoy blooms while shortening the height of the bush and promoting the development of flowers on low-growing branches. Always prune lilacs in the late fall or early winter, before frost danger sets in for your area.
Check the branches of your lilac bush for signs of dead, diseased or damaged branches. Diseased or damaged branches are often discolored or bear physical markings or nicks. Dead wood will feel hollow to the touch.
Prepare a sanitizing solution in a bucket by mixing 1 part bleach and 10 parts water. Place the pruning tools in the bucket before begining pruning, and dip the tools back in the solution between cuts to prevent the spread of disease.
Cut off all the dead, diseased and damaged wood by pruning it off at the base.
Remove one-third of the old mature wood by cutting it off at the base of the lilac shrub. Choose old woody stems that only produce flowers very high up. Cut stems less than 3/4-inch think using anvil pruners and larger stems with lopping shears. After pruning, wait until the following year to prune again.
Repeat the prior year's pruning by removing half of the old-growth stems remaining on the bush. Again choose woody stems that produce flowers only at the top. Also, thin out some of the new growth that developed over the season. Remove weak shoots from crowded areas, leaving strong, new growth in areas spaced throughout the bush. Clumped new growth will not develop properly.
Remove the rest of the old wood in the third year. By now, the new growth should bloom in the spring. After this point, maintain a neat and vibrant lilac bush by removing some of the old wood every three to five years.