Please note: This article was originally published on 4/6/2020 and was republished on 6/9/2021.
Common lilacs (Syringa vulgaris) are a favorite landscape shrub here in Ohio and beyond, with flowers that provide beauty and an unmistakable fragrance every spring. Other cultivars of lilacs offer different habits and uses in the landscape, but provide the same display of flowers that we all love. In order to ensure you get the most flowers on your lilac year after year, it’s important to know when and how to prune them.
When To Prune
As a general rule for all lilacs, they should be pruned immediately after they’re done flowering in the spring. Since lilacs set next year’s flower buds right after the current year’s flowers have faded, pruning later in the summer or fall will result in cutting off many or all of next year’s flowers. This rule of timing applies to the larger common lilacs as well as the cultivars that are shorter or more “shrub” like. While the “when” of pruning lilacs is fairly straightforward, the “how” gets a little trickier. To keep things simple for now, we’ll think of lilac pruning as either maintenance pruning or rejuvenation pruning.
How To: Maintenance Pruning
For any lilac shrubs that have not outgrown their space or are still producing vibrant flowers each year, regular pruning can simply consist of any shaping that you choose to do along with removal of dead, diseased, or broken stems. You can also remove spent flowers from your lilacs to help encourage a cleaner growth habit and appearance. It’s always better to do this type of pruning by hand, rather than shearing. When making cuts, try to cut back to an outward facing bud. A good pair of hand pruners is the perfect tool for this and makes for much better pruning cuts than hedge shears.
How To: Rejuvenation Pruning
If you’ve ever had an older common lilac in your landscape that went unpruned for many years, you’re probably familiar with their overgrown, unruly habit when left alone. Many people mistakenly believe that these shrubs have stopped flowering at this point. Oftentimes what’s actually happening is the flowers are being produced on just the upper portions of the shrub where the plant has reached a taller height and is exposed to sunlight. Once they’ve reached this stage, we’re often left to stare at bare, woody branches at eye level and below. For these overgrown shrubs, we can remove entire older canes or stems that are 2” in diameter or larger to encourage a rejuvenation of the shrub. We want to apply the rule of thirds when doing this type of pruning - Remove approximately one third of the older canes or stems each year for 3 years. This gives the shrub a chance to slowly transition back to a fuller, shorter shrub with more new growth filling in from the bottom. If you decide to drastically prune the entire shrub this way all at once rather than just a third of it, a little extra care like fertilization and watering will be important to encourage new growth. Note that this “all at once” approach is generally not recommended for the health of the shrub.
Your lilac flowers can be influenced by many things, including the temperature, soil conditions, even disease and insect problems, but proper pruning goes a long way to ensuring they put on a great show every spring. And remember, if you have specific questions about pruning or anything tree and shrub related, your dedicated Russell Tree Experts arborist is only an email or phone call away.
*New* Video!
To accompany the above article, Walter Reins demos how to prune lilac trees in this new video! Click below!
Walter Reins | Regional Manager, Russell Tree Experts
Walter has been an ISA Certified Arborist since 2003. He graduated from Montgomery College in Maryland with a degree in Landscape Horticulture, and has called Columbus, OH his home for nearly 20 years. Walter appreciates trees for their majesty and the critical role they play in our world.