When and How to Prune Lilacs for a Prolific Show of Flowers Year After Year

Many plants benefit from some type of pruning, whether it's a conservative reshaping or a harder cut to rejuvenate. One plant that benefits greatly from pruning is the lilac shrub. Over time, lilacs can become unruly and overgrown. Pruning keeps the plant a manageable size, preventing overcrowding and reducing the risk of disease. The part of pruning that gardeners will appreciate most is that it encourages a more prolific blooming season, meaning more flowers for you to enjoy in the garden or cut for arrangements. Is your lilac shrub ready for a haircut? Learn when and how to prune lilacs with these tips from experts.

Benefits of Pruning Lilacs

Pruning lilacs is an essential aspect of caring for these flowers. "Pruning lilacs helps to keep them healthy by removing old wood that can choke out the lilac," says Linda Langelo, horticulture specialist at Colorado State University. "This also allows light into the interior of the lilac bush." Even more important, pruning directs energy towards flower production, rather than seed production, for a longer and more prolific growing season.

Beyond productivity, pruning lilacs also has aesthetic benefits. "Pruning lilacs improves their shape, encourages stronger blooming, prevents overgrowth, and keeps plants healthy by removing dead or diseased wood," says Kelly Funk, president of Jackson & Perkins. "Pruned lilacs tend to stay more compact and attractive in the landscape.

When to Prune Lilacs

Prune lilacs in late spring or early summer immediately after they finish blooming. "This timing lets you enjoy the current season’s flowers while avoiding the accidental removal of next year’s buds, which begin forming shortly after bloom," says Funk. "Pruning too early cuts off current blooms; pruning too late removes future ones." Pruning in early summer gives the plant enough time to heal and set new buds.

How to Prune Lilacs for Maintenance

Maintenance pruning keeps your lilac bush looking tidy by removing spent blossoms and damaged branches. It also helps prevent disease by removing crossing branches to improve airflow.

1. Remove dead, damaged, diseased, and crossing branches to open the center of the plant.
2. Trim off spent flowers, cutting just above a healthy set of leaves to promote tidy regrowth.
3. Prune 1/3 of the oldest, thickest stems down to the ground to stimulate new shoots.
4. Remove stems thicker than 2 inches in diameter to control the overall height.

How to Prune Lilacs for Rejuvenation

Rejuvenation pruning is for old or overgrown lilac bushes that have been neglected, says Langelo. Neglected bushes are overgrown with dense stems, little growth at the bottom, and few blooms. Rejuvenation pruning takes place over the course of three years. "This slow approach avoids shocking the plant while encouraging a full, blooming recovery," says Funk.

1. Right after flowering, cut 1/3 of the plant's oldest stems down to the ground.
2. Repeat this process annually for three years until all old wood has been removed and replaced by new growth.
3. In extreme cases: Prune the lilac to a height of 6 to 12 inches off the ground. It will take a few seasons to bloom again with this method.
   

Lilac Pruning Tips

While pruning is essential for the overall health of your lilacs, doing so incorrectly can cause more harm than good. Keep these expert tips in mind the next time you take pruning shears to your lilac shrub.

• Maintenance pruning is ideal: Regular seasonal pruning keeps your lilacs manageable and healthy. "It is better to do annual maintenance than rejuvenation pruning," says Langelo. "The rejuvenation is more stressful on the lilac."
• Wait for maturity to prune: There is no need to prune lilacs at a young age. Langelo recommends waiting until the shrub reaches about 6 to 8 feet tall to prune.
• Make cuts on an angle: Cut branches at a 45-degree angle to prevent the water from collecting on the open wound, says Langelo. Water that sits can encourage fungal diseases, like powdery mildew.
• Prune with sharp tools: Always use sharp, clean tools to prevent disease spread and ensure clean cuts, says Funk. If you recently pruned a diseased plant and then prune your lilac bush with the same tool before cleaning it, you risk spreading the disease.
• Cut in the right spot: Cut just above a pair of healthy buds or lateral branches to encourage outward growth, says Funk. Cutting lilacs in the wrong spot can impede future growth.
• Don't prune too late: Pruning lilacs too late means you risk cutting off next year’s buds. "Pruning in the winter can cause stress to your lilac by making it susceptible to winter damage," Langelo adds.