Lilacs are beloved flowering shrubs or small trees, known for their fragrant spring blooms and hardy nature. The most common varieties of lilac do spread over time. This expansion is a natural behavior for the species, allowing the plant to renew itself and colonize favorable areas of the landscape.
The Primary Method of Spreading
The spreading behavior in lilacs is a form of asexual reproduction accomplished primarily through the growth of “suckers.” Suckers are basal shoots that emerge from the plant’s underground root system, often appearing near the base of the main shrub but sometimes several feet away. These shoots are genetically identical clones of the parent plant, making this an effective way for the lilac to propagate without relying on seeds.
Suckers arise from dormant buds located either directly on the root crown or on shallow, horizontal roots (rhizomes). When conditions are right, these buds are stimulated to grow into new shoots. The plant uses this mechanism for rejuvenation, allowing younger, more vigorous stems to replace older, less productive wood over time.
Factors Influencing Spreading Behavior
The tendency for a lilac to spread is heavily influenced by how the plant was originally propagated in the nursery. Lilacs are typically sold as either “own-root” or “grafted” stock, and this difference dictates their suckering habits.
Own-Root Lilacs
Own-root lilacs are grown from cuttings and possess a root system genetically identical to the entire shrub, including the desired flowering top growth. These plants will consistently and vigorously produce suckers. If left alone, these shoots will eventually grow into new flowering stems that are true to the parent variety. This is the classic, spreading lilac often found on old homesteads.
Grafted Lilacs
Grafted lilacs are created by joining a desired cultivar onto the rootstock of a hardier species. Any suckers that emerge from a grafted plant will originate from the rootstock below the graft union, not the decorative top portion. These rootstock suckers must be removed because they are a different variety. If allowed to grow, they will outcompete and eventually overtake the growth of the desirable cultivar.
Environmental factors also play a role in stimulating sucker growth. Any damage to the roots, such as from digging, tilling, or mowing near the base of the shrub, can trigger a stress response that increases suckering. Furthermore, a lack of direct sunlight reaching the base of the plant can encourage the growth of new shoots from the bottom, as the plant attempts to reach for light.
Controlling Unwanted Growth
Managing a lilac’s spread requires consistent action focused on removing the suckers at their point of origin. The most effective removal technique is to physically tear or cut the sucker off as far below the soil line as possible, ideally right where it connects to the root or root crown. Simply mowing or shearing the shoots at ground level will only stimulate the dormant buds at the base to grow back thicker and more quickly.
For small, young suckers, tearing them off by hand is often the best method. This action removes the dormant buds at the base, which helps discourage immediate regrowth. If the suckers are persistent, a physical root barrier can be installed. This barrier should be placed vertically into the soil to a depth of 8 to 12 inches to contain the spreading roots.
Chemical control offers another option, particularly for isolated shoots, though it must be used with caution. Products containing growth inhibitors, such as naphthalene acetate (NAA), can be applied to the base of the removed sucker to prevent the re-sprouting of new shoots. Herbicides should be applied only to suckers that are clearly detached from the main plant to avoid inadvertently injuring or killing the entire lilac shrub.