The common lilac, Syringa vulgaris, is a classic landscape shrub, cherished for its dense clusters of highly fragrant flowers that signal the arrival of spring. Native to the Balkan Peninsula in southeastern Europe, this deciduous plant has been cultivated across North America for centuries. While its reliability and sweet scent make it a favorite, many gardeners express frustration over its persistent tendency to spread. This raises a fundamental question about its classification: is the lilac genuinely invasive, or is its spread simply an aggressive behavior confined to the garden setting?
Defining Invasive Versus Aggressive Growth
Understanding the difference between an invasive species and an aggressive plant is crucial for assessing the lilac’s ecological impact. An invasive species is a non-native organism that causes or is likely to cause economic or environmental harm, or harm to human health. These plants typically spread rapidly outside of cultivation, displacing native plant communities and disrupting local ecosystems. The common lilac is generally not classified as invasive in North America because it rarely escapes cultivation to colonize natural habitats on a widespread scale.
Lilacs are classified as an aggressive or naturalized plant, meaning the species spreads rapidly but does not pose a serious ecological threat. Aggressive plants, often called “garden thugs,” are problematic primarily within a maintained garden or landscape. They compete vigorously with nearby cultivated plants for water, nutrients, and sunlight, but their spread is usually limited to the immediate vicinity of the parent plant. The lilac’s behavior falls into this category, making its spread a nuisance to the homeowner rather than a hazard to the wider environment.
How Lilacs Spread in the Garden
The primary mechanism by which the common lilac spreads is through the production of suckers or basal shoots. Suckers are new, upright stems that emerge from adventitious buds located laterally along the plant’s root system. These shoots are clones of the parent plant, allowing the lilac to propagate itself vegetatively and form dense, clonal thickets.
Suckering is a natural survival and renewal strategy, allowing the species to replace older stems with new growth. The root system of an established lilac can extend significantly beyond the main shrub, and suckers may appear several feet away from the central trunks. The tendency to sucker is particularly pronounced in older plants and in varieties not grafted onto non-suckering rootstock.
The spread can be inadvertently encouraged by certain gardening practices. Damage to the shallow, lateral roots, such as from tilling, digging, or mowing too closely to the shrub’s base, stimulates the plant to produce more suckers. This response is a defense mechanism where the plant attempts to repair the injury and regenerate new growth. The resulting aggressive spread frustrates many gardeners.
Managing and Controlling Lilac Suckers
Effective management of lilac suckers involves consistent effort focused on eliminating the shoot before it becomes established. The most effective method for removing new growth is to pull or tear the suckers off rather than simply cutting them at ground level. Tearing the shoot away from the root collar helps remove the dormant buds at the base, which reduces the likelihood of immediate regrowth.
For a long-term solution to prevent lateral spread, physical root barriers can be installed around the shrub. These barriers, typically made of plastic or metal edging, should be placed vertically in the soil to a depth of 8 to 12 inches to contain the spreading root system. While roots may eventually grow under the barrier, this technique significantly reduces the number of suckers appearing in unwanted areas.
Gardeners seeking the lilac’s fragrance without the maintenance burden can choose specific cultivars known for low or no suckering tendencies. Varieties like the hybrid Syringa x chinensis (Chinese Lilac) or the Syringa meyeri ‘Palibin’ (Dwarf Korean Lilac) are less prone to aggressive spreading behavior. If complete removal of a shrub becomes necessary, the process requires digging out the extensive root mass to prevent remaining root fragments from continually producing new shoots.
In extreme cases, a selective herbicide may be used on isolated suckers that are far from the main plant. Caution is necessary, as applying chemical control to any attached sucker can injure or kill the entire shrub. Consistent vigilance and prompt removal of new shoots remain the most reliable, non-chemical control methods for managing the lilac’s aggressive growth.