The common lilac, cherished for its sweet fragrance and beautiful spring blossoms, often causes confusion for gardeners. This popular woody perennial (Syringa) grows in diverse forms, leading many to question its botanical identity.
Some lilacs appear as dense, bushy growths, while others stand tall on a single stem, resembling a small tree. The answer to whether a lilac is a tree or a shrub lies in the specific growth patterns that define these two plant categories.
Defining the Botanical Difference Between Trees and Shrubs
The distinction between a tree and a shrub is based primarily on structure and growth habit. Both are classified as woody plants, meaning their stems are persistent and maintain tissue above ground year-round.
The defining characteristic of a tree is a single, dominant, woody stem, or trunk, that typically branches well above the ground level. Trees are generally taller, but height alone is not the deciding factor.
In contrast, a shrub is defined by its multi-stemmed structure, where numerous woody shoots arise directly from the ground or near the soil line. Shrubs often have a dense, bushy appearance because they lack a single, clear trunk and usually reach a maximum height of 10 to 20 feet.
The Lilac’s Natural Classification
The common lilac (Syringa vulgaris) is classified as a deciduous shrub, an identity rooted in its inherent growth pattern. In its natural state, the plant produces multiple stems that emerge from the base, often forming a dense, bushy thicket.
This multi-stemmed nature fits the structural definition of a shrub. The plant has a strong genetic disposition for suckering, producing secondary shoots from the roots or the base.
If left unmanaged, these suckers spread outward, creating a clonal colony over time. The typical height range for a mature common lilac is between 8 and 15 feet, which is within the size parameters commonly associated with shrubs.
Training and Cultivation of Lilacs into Tree Forms
The confusion over the lilac’s identity is understandable because the plant is frequently manipulated into a “standard” or “tree form.” This transformation is achieved through selective pruning and training techniques that counteract the lilac’s natural multi-stemmed habit.
To create the appearance of a small tree, growers select one strong, upright stem to serve as the main trunk. All other secondary stems and suckers emerging from the base are removed.
Low-growing branches are also pruned away, forcing the foliage and flowers to develop higher up and create a distinct canopy. This process requires consistent annual maintenance, as the lilac’s genetics continually drive it to produce new suckers that must be removed to preserve the single-trunked shape. While visually resembling a small tree, this cultivated form remains a shrub coerced into a different structure.