The common lilac (Syringa vulgaris) is a long-lived perennial plant. A perennial is defined as any plant that lives for more than two years, returning each growing season. Lilacs are substantial, woody shrubs or small multi-stemmed trees that can survive for decades, often exceeding a hundred years. This longevity and annual return of foliage and fragrant flowers firmly establish the lilac as a perennial. Its life cycle includes a seasonal period of dormancy, allowing it to withstand cold temperatures and reliably bloom each spring.
Understanding Perennials and Woody Plants
The term perennial covers a wide range of plants, distinguished from annuals, which complete their life cycle in a single season, and biennials, which require two seasons. Perennials are further divided into two major types: herbaceous and woody. Herbaceous perennials, like hostas or daylilies, die completely back to the ground each winter, with only the roots surviving underground.
The lilac is classified as a woody perennial, meaning its above-ground structure—stems, branches, and trunk—does not die back. This woody framework persists through the winter, gaining size and strength over many years. This permanent structure allows the plant to reach impressive sizes, often growing between 8 and 15 feet tall. The common lilac is deciduous, meaning it sheds its leaves in the fall but retains its enduring branches.
The Lilac Life Cycle: Dormancy and Return
As a woody perennial adapted to cooler climates, the lilac requires vernalization, or winter chilling, to trigger its spring bloom cycle. During the colder months, the plant enters a period of dormancy, conserving energy and protecting its flower buds. The leaf and flower buds need to accumulate a specific number of “chilling hours,” which is the cumulative time spent at temperatures typically between 32°F and 45°F.
For the common lilac, this requirement can be significant, often demanding over two thousand hours of chilling to ensure successful flowering. Once this cold requirement is met, the plant breaks dormancy in spring, leading to a flush of new growth and the appearance of its fragrant blossoms.
Essential Care for Established Lilacs
Caring for an established lilac involves specific practices to manage its woody structure and promote flowering. Lilacs bloom on “old wood,” meaning the flower buds for the next spring develop on the growth produced during the previous summer. Therefore, pruning should occur immediately after the current year’s flowers fade, allowing the shrub sufficient time to form new buds for the following season.
To prevent older shrubs from becoming overgrown, renewal pruning is recommended. This involves removing one-third of the oldest, thickest stems right down to the ground over a three-year period. Lilacs need full sun—at least six hours of direct sunlight daily—to produce the best blooms. They prefer well-drained soil that is neutral to slightly alkaline in pH and generally require minimal fertilization, as too much nitrogen encourages leafy growth at the expense of flowers.