More than 4,000 cultivars of lilac (Syringa spp.) are available, according to the University of Nebraska-Lincoln Extension; size varies significantly in the cultivars. Some lilac varieties can become very tall and wide, but compact varieties have also been developed. Routine pruning keeps a lilac blooming vigorously and also slows its growth. Well-maintained lilacs may reach maturity without reaching their maximum size, leading people to underestimate the potential size of the lilac species.
One of the tallest lilacs is the tree lilac (Syringa reticulata), growing to a maximum of 25 feet, though it's typically 15 to 20 feet tall and about 15 feet wide. Dwarf or compact species, such as "Miss Kim" (S. patula), are shorter, from 5 to 8 feet. Common lilac (S. vulgaris) grows 8 to 15 feet tall and 6 to 15 feet wide. Littleleaf lilac (S. microphylla) is wider than it is tall -- it's usually about 6 feet tall and 9 to 10 feet wide.
Key considerations for planting the lilac are sun exposure and spacing. Lilacs need at least six hours of sunlight a day to set flower buds for the following spring -- lilac blooms on the previous year's growth. Too much shade will inhibit flowering, and if lilacs are crowded, the plant will get tall and leggy, with flowers growing only at the top. The planting site should be sized for the height and spread of the mature shrub. Otherwise, the lilac will spread out and crowd other plants.
Choosing the appropriate size of plant for your site is usually less complicated and labor intensive than relying on pruning to control the size, and some plants are harmed when too much of their mass is reduced. Lilacs respond well to pruning -- routine pruning keeps branches from becoming crowded and promotes vigorous blooming. For height control, the lilac can be pruned 1 foot lower than the desired height to allow for growth.
Lilacs in good condition have blooms all over the shrub, with the most profuse blooming coming from stems that are 1 to 2 inches thick. Stems 3 inches thick bloom less profusely. Pruning to select the younger, slender branches also keeps the shrub smaller; one quarter to one third of the largest stems can be removed annually, leaving six to 12 major stems. Older, overgrown lilacs are pruned for rejuvenation; up to one third of the lilac can be pruned at a time, removing the old growth and retaining the young stock.
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