Lilacs at the Arnold Arboretum
How To Plant And Care For Lilacs
Lilacs grow best in full sun and well-drained soil, where they take two to three years to establish themselves in a new site. Once established they can live for centuries. Soil pH (alkalinity or acidity of the soil) may affect the plant's growth. Lilacs do well in a slightly acid to alkaline soil. New England soils are often very acidic and require modification for optimal lilac growth. Contact your county agricultural extension service for soil-testing information and instructions for altering soil pH.
To ensure abundant flowering, cut off all spent blossoms each year and prune the flowering stem back to a set of leaves in order to prevent seeds forming, thereby directing the energy usually spent on seeds to next year's flower production. If this is not done, good flowering years may be followed by bad.
When the plant becomes leggy, renewal pruning is required. Remove about one-third of the oldest stems at ground level each year for three years. This encourages the growth of vigorous new stems from the base. By the end of the three years the plant should be fully rejuvenated with its blossoms once more at nose level.
The plants should be fertilized in early spring and again directly after flowering with an all-purpose fertilizer such as a 10-10-10, watered in well. And tough as lilacs are, they need supplemental water during periods of drought.
In our area the most serious insect pests are the lilac borer (Podosesia syringae var. syringa) and the oyster-shell scale (Lepidosaphes ulmi). Borers signal their presence by leaving eighth-inch-sized holes in stems and larger branches, often one to two feet above ground level. Oyster-shell scale, which looks like small, elongated, brown or gray warts on the stems, can be controlled by pruning out the most heavily infested branches, followed by an application of dormant oil spray. Contact your county agricultural agent for specific advice on pest identification and control methods.
Lilacs can fall victim to leaf diseases in late summer and early fall. These include powdery mildew fungus (Microsphaera alni) and leafroll necrosis. Powdery mildew produces unsightly whitish patches on the leaves, but the problem tends to be more aesthetic than physiological. Leafroll necrosis seems to be caused by air pollution. Some lilacs have proven less susceptible than others. The Arboretum conducts ongoing research to record annual levels of disease injury and to select resistant individuals.
Adapted from a brochure prepared by John H. Alexander III and Nan Blake Sinton, "Lilacs and the Arnold Arboretum," 1990.