A springtime favorite of many Iowans is the lilac. Though they offer mainly one season of interest, their spring flower displays are greatly appreciated after a long, hard winter. If you have lilacs or if you are interested in planting one, there are several factors to consider.
Lilacs are adapted to USDA Hardiness Map zones 3, 4, 5 and milder areas of zone 2. They thrive in sunny sites with good air circulation. Lilacs on their own roots are far more hardy than grafted lilacs. Lilacs need at least 4 to 6 hours of sunlight a day for best flower production. Plants should be spaced between 10 and 15 feet apart for specimen displays and 5 to 8 feet apart for a hedge effect. Since many lilacs have bloomed, now is the best time to remove the spent flower heads. This will help the plant to produce more flowers for next season's display. While pruning a lilac in late winter or early spring is to be done to remove the oldest stems, flower numbers will be reduced. In the long run one will come out ahead by continually removing one-third of the oldest branches in the late winter or early spring.
Since lilacs are chiefly grown for their attractive, fragrant flowers, many gardeners are disappointed when plants don't bloom quickly. Lack of flowering may be due to several factors. Many lilacs won't bloom for 4, 5, or more years after planting. Lilacs and most woody plants must grow and mature before they are capable of blooming. Exposure could also be a factor. Lilacs need at least 6 hours of sun to bloom well. Improper pruning is another possibility. Many lilacs bloom on the previous season's growth. The flower buds form during the summer months. Pruning lilacs in fall or late winter could remove much of the blooming wood. There is little that individuals can do to encourage lilacs to bloom. Fertilizing the shrubs encourages vegetative growth, but may actually delay flower formation.
Powdery Mildew is a disease that affects lilacs, infesting the leaves and leaving a gray film on the leaf surface. Because the disease normally appears at the end of the growing season, it seldom does permanent damage to the plants. Lilacs should be planted in full sun and in areas with good air movement to discourage this disease. Humid, overcast weather when days are warm and nights are cool favor powdery mildew development. Planting varieties resistant to powdery mildew is the easiest, least expensive, and preferred method of disease management. Unfortunately, variety descriptions are often ambiguous and don't identify resistance to specific diseases. Selections of various lilacs differ in their mildew susceptibility, so ask about disease resistance to powdery mildew when buying them.
This article originally appeared in the June 7, 2002 issue, p. 77.