The lilac, belonging to the genus Syringa, is a highly popular, deciduous shrub or small tree celebrated for its showy, fragrant flowers that bloom in spring. These woody plants are native to southeastern Europe and eastern Asia, but they have been widely cultivated across temperate regions for centuries. Understanding the ultimate size and growth habits of a lilac is important for successful landscape planning. Choosing the correct variety ensures the plant thrives in its designated space without requiring constant, aggressive pruning to control its dimensions.
The Size Spectrum
The mature size of a lilac depends significantly on its species and cultivar, ranging from tall, tree-like specimens to compact, mounding shrubs. The common lilac, Syringa vulgaris, which is the species most people visualize, can typically reach a height of 10 to 15 feet, with some older specimens or specific cultivars growing over 20 feet tall. This large size necessitates careful placement, as the plant also develops a wide, rounded spread that can be 8 to 12 feet across at maturity.
Dwarf varieties are better suited for smaller gardens or foundation plantings. The Dwarf Korean Lilac, Syringa meyeri ‘Palibin’, is an excellent example, often maturing to a height of only 4 to 5 feet and a width of 5 to 7 feet. Another popular smaller choice, Syringa patula ‘Miss Kim’, typically remains in the 4- to 7-foot height and width range. The difference between height and spread is also a consideration, as some varieties are more upright while others, like the Dwarf Korean Lilac, have a naturally dense, low-spreading habit.
Beyond shrubs, some species, such as the Japanese Tree Lilac (Syringa reticulata), grow into small trees that can reach 30 feet in height and are often trained to a single trunk. Selecting a lilac involves matching the plant’s genetic potential for size with the available space, ensuring a healthy and manageable plant for decades to come.
Growth Characteristics
Lilacs are moderate-to-fast growing, adding about 1 to 2 feet of new growth per year once they are well-established. This rate can vary based on the specific species, with smaller cultivars sometimes exhibiting a slower development. Even a moderate growth rate over many years results in a substantial mature size.
A defining characteristic of many lilac varieties, especially Syringa vulgaris, is their tendency to sucker, which affects their overall spread and density. Suckers are new shoots that sprout directly from the base or roots of the plant, often leading to a dense, multi-stemmed thicket. If not managed, this suckering habit allows the plant to slowly increase its diameter, creating a colonial spread over time. This production of new shoots is the plant’s natural mechanism for rejuvenation and helps ensure a long lifespan. However, it requires monitoring, especially on grafted plants, where rootstock suckers may emerge and should be promptly removed.
Managing Size and Shape
Controlling the size and maintaining the health of a lilac relies on proper pruning techniques and timing. Since lilacs bloom on the previous year’s growth, all pruning should be done immediately after the flowers have faded in late spring or early summer, before the plant sets buds for the next season. Pruning later in the year will result in the removal of those developing flower buds, leading to a reduction in blooms the following spring.
Deadheading is the process of removing spent flower panicles before they develop seeds, which is often done on smaller shrubs to encourage the plant to divert energy into creating new flower buds and foliage. For shaping and thinning, the goal is to improve air circulation and maintain a desired size by selectively cutting back individual branches.
Thinning involves removing the oldest, thickest stems down to the ground to encourage new, vigorous growth from the base. This method keeps the shrub productive and prevents it from becoming a dense, overgrown mass where flowering occurs only at the top.
Rejuvenation Pruning
Rejuvenation pruning can be accomplished in one of two ways. One method involves cutting the entire shrub down to 6 to 12 inches from the ground while the plant is dormant in late winter, sacrificing one to two years of blooms. A less severe, multi-year approach is to remove only one-third of the largest, oldest stems each year for three consecutive years, allowing the shrub to be renewed incrementally while still permitting some flowering.