The lilac grows naturally as a bush but can be trained and shaped to grow like a small tree. Lilacs are popular for their explosion of fragrant blossoms of lavender, white, pink, blue or purple every spring. Lilacs are perennial plants that reproduce and spread by sending up shoots from their root system. While you can keep your lilac strong and shaped with regular removal of these suckers, allow the new shoots from the suckers in order to revive your old lilac tree, gradually replacing old growth with new growth.
Prune lightly to shape and remove old growth in the spring after the bloom. This way you can avoid pruning off the branches that will bear next year's flowers by only pruning off the old branches which have already flowered. Lilac flowers bloom on the 1-year-old branches.
Prune severely to revive an old tree in the winter months when the tree is dormant. Some flowering branches will be sacrificed, but if you take three years, removing only a portion of the old wood each year, you can keep your lilac blooming while you revive it.
Remove one-third of the old growth the first winter. Cut it back to the ground. Use pruning shears for smaller branches, loppers or a hand saw for larger growth.
Prune back one half of the remaining old growth the next winter. Cut it back to the ground. Thin the new growth as well, retaining some well-spaced and healthy shoots.
Remove the remaining old growth the third winter. Thin and shape the new shoots.
Keep your old lilac tree healthy and vigorous by pruning away some of the old branches every three to five years.