How to Prune Lilac Bushes

Regular Maintenance Pruning

Follow this general shrub pruning rule for lilac bushes: Cut no more than a third of the stems each year, starting with the oldest. That will help the plant remain vital with new stems developing as old stems bloom. The goal is for a lilac bush to have between 10 and 12 main stems each between 1 and 2 inches in diameter.

1. Prune Unsightly Features

   • Begin by pruning dead or diseased stems, pencil-thin suckers that are far from the bush, and twiggy growth. Cut these back all the way to ground level.
   • Use smaller hand-pruning shears for the pencil-thin, twiggy growth.
   • Tackle thicker stems with long-handled loppers.

2. Remove Any Stems Thicker Than 2 Inches in Diameter

   • Remove entire older stems to prevent your lilac from becoming too tall.
   • Avoid cutting off just the tops of long stems because this can leave the plant with an odd, unnatural shape.
   • Use a pruning saw for very large stems because mature, thick lilac stems can be tough.

3. Trim Remaining New Stems

   Trim the remaining new stem to an outward-facing bud to help your lilac fill in more and become shrubbier. Prune just beyond buds that face away from the center of the plant. This technique will cause more branching and create a denser shrub.

Rejuvenation Pruning

Older lilacs can have stems as thick as small trees and without pruning, the shrub will only flower on the top of the uppermost branches. Fortunately, rejuvenation pruning revives an old lilac in about three years. There are two approaches you can take.

Cutting Back Part of the Plant

The "1/3" rule is a less drastic approach to rejuvenating an overgrown lilac. In the spring before the branches leaf out, prune 1/3 of the oldest, thickest stems down to the ground for three consecutive years. After three years, the plant should have mostly all new shoots that will begin blooming. Then you can start regular maintenance trimming.

Cutting Back the Entire Plant

You can take the quicker and more drastic approach of cutting back the entire plant about 6 to 8 inches above the ground in the early spring. Fertilize the plant with compost or a balanced fertilizer to prompt new growth. New shoots will develop throughout the growing season and into the summer. The following spring, prune out the spindly growth and maintain the healthiest shoots while shaping the plant. Encourage branching by cutting back the remaining shoots to just above a bud. Carry on with regular maintenance pruning after this.

Working With Japanese Lilac Trees

The popular Japanese lilac tree (Syringa reticulata) grows as high as 30 feet. It has a vase-shaped crown with spreading branches and produces showy white flowers in June.

Prune this bush's tree shape to maintain an open interior with several main branches that form a vase shape. Then prune dead, diseased, or interfering or cluttering branches in the middle of the shrub after it flowers every year. If the lilac tree grows too tall, cut back individual branches to around 1 foot below the desired height to prompt dense growth at the top.

Tips for Pruning Lilacs

Dwarf lilacs, such as ‘Palibin’ Meyer lilac (Syringa meyeri ‘Palibin’) and ‘Miss Kim’ Manchurian lilac (Syringa pubescens ssp. patula ‘Miss Kim’), look similar to the common lilac but they are pruned differently. They rarely require maintenance pruning, though you can trim when necessary for shape.

Like other lilac varieties, they also can benefit from deadheading (pinching off faded blooms) to stimulate continuing flowering.

New lilac plants should begin blooming within two to five years. Deadheading the spent flowers encourages new bud development for the next spring. Once the plant has matured, you don't need to take the time to deadhead.

Some years your lilac bush will bloom magnificently and some years not so much due to weather and other factors. So don't panic if your lilac isn't as vibrant from year to year.