The intoxicating fragrance of a lilac in full bloom stirs up intense memories for most gardeners. One whiff sends me back to Lilac Sunday preparations at the Arnold Arboretum in Boston, where I once lost control of my push mower on a steep hill and watched in horror as the whirling blades careened into a cherished specimen. But lilacs are very resilient; it took just a few skilled pruning cuts to restore the shrub’s appearance.

Fortunately, lilac pruning is almost never this dramatic. These multi-stemmed shrubs all benefit from regular maintenance, and most will even respond well to heavy pruning when needed. A critical eye and a series of well-planned cuts will make a big difference in the plant’s appearance and performance, whether you are training a new plant, maintaining an established specimen, or rejuvenating an overgrown shrub that has not been touched in years.

Regardless of the species or the season, there are a few preliminary steps that will help you to approach any pruning job with confidence.

Step 1 | Assess the situation. Start any pruning project by taking the long view, stepping back about 20 feet to evaluate your subject. Be curious. Is this plant thriving or struggling? Is it blocking windows or crowding neighboring trees?

Step 2 | Set goals. Reimagine your lilac based on your initial assessment. Will you be changing the shrub’s shape? Is this a young shrub that needs training, an established plant in need of routine maintenance, or an older specimen with long, leaning stems?

Step 3 | Remove dead wood. Before you cut a single living branch, identify and remove dead and broken ones throughout the plant. Prune each dead branch back to a branch collar or to the ground.

Once the dead wood is gone, it’s time to decide whether the plant needs training, routine maintenance, or a heavier renovation. To help you decide, I’ve covered each of these three pruning strategies on the following pages.

1. Use just a few strategic cuts when establishing a new lilac.

All woody plants need some training at planting time or shortly thereafter, and all types of lilacs are no exception. Limit the number of live branches you are removing to about 15% of the shrub’s overall mass. This is because young transplants need most of their energy reserves to get established, so it’s important not to take too much. Focus your efforts on removing dead branches and stems that will never contribute to the desired shape of the lilac. When you examine stems as they emerge from the ground, you’ll see that some are already growing too close to each other or may be twisted together. These issues will only become more problematic over time. Ask yourself which branches will make a better contribution to the long-term natural habit of your lilac.

After every few cuts, step back to ensure that you are staying within your pruning limit and that you are balancing the density of the canopy. There should be some air circulation within the plant without it looking too sparse. The final step is shaping; create a shrub that is uniform but not a gumdrop. When you walk away, you want to see a transformed, more refined lilac.

2. Routine maintenance combats leggy growth

For lilacs about five years of age and older, routine regenerative pruning with a long-term mindset prevents plants from developing an awkward, gangly habit. As lilacs age, lower side branches at the base of the plant will often die off as they become shaded by taller stems. Without these lower branches, it looks like the lilac has long, bare “legs” (see photo below). Legginess also occurs when gardeners habitually remove all of the new suckering stems, retaining only older ones. 

Regenerative pruning rejuvenates a plant gradually by removing a few of the oldest main stems each year. Ask yourself which tall stems could be taken out to allow sunlight to filter down into the base of the shrub without leaving a gaping hole. Cut these chosen branches down to ground level. Focus on removing one stem at a time, and step back from the plant to reevaluate after each cut. Leave most of the suckers to grow; this new generation of stems will eventually replace future retirees. 

Your final task will be to fine-tune the shrub’s shape, emphasizing its natural, loosely vaselike habit by removing errant or overextended branches. Again, remember to periodically step back and reassess your pruning job; this will help you gauge when it’s time to stop and move on.

Regenerative pruning prioritizes long-term structural integrity over maximum flower production. Do not forgo overdue maintenance pruning because it is the wrong time of year. You can do regenerative pruning on a lilac in early spring, early summer, or early fall. Although your judicious selections will remove some flower buds, you will certainly not eliminate all of them. Making rejuvenation cuts part of your annual routine will encourage a full, lush habit where several generations of growth thrive together.

TIP: Remove large branches in pieces. Taking off the ends of an older branch in a couple of steps will allow you to tease the pieces out of the shrub without damaging neighboring stems.

3. In extreme cases, deeper cuts may be necessary

When a common lilac has been subjected to years of deferred maintenance, nearly every stem may be larger in diameter, and most of the flowers and foliage will be concentrated at the ends of the upper branches. In a case like this, a hard reset may be appropriate. This is usually not necessary with other lilac species.

Renewal pruning, or total rejuvenation, involves cutting every large, old stem to the ground. You may not have as many flowers for a few years as the shrub regrows, but the healthy new stems that emerge will put future flowers closer to eye (and nose) level. 

As you consider rejuvenation on an older lilac, you must first weigh timing and its overall health. If it is early in the growing season and the shrub is healthy and vigorous, go for it. But be aware that lilacs that are in decline may not survive renovation, particularly if you make the cuts after midsummer. 

I’d be willing to bet that most of our fondest lilac memories are from plants that were quite old at the time. Pruning young lilac shrubs early and revisiting their pruning needs throughout the years will greatly contribute to longevity and multigenerational growth. I’ve cared for many shapely specimens that are over 100 years old and still flowering beautifully.

Jen Kettell is a consulting arborist and the owner of Radiant Leaf Consulting, a firm based in Natick, Massachusetts, that provides training to green-industry professionals and homeowners.