By James Romer

Iowa Master Gardener Coordinator

Iowa State University Extension

A springtime favorite of many Iowans is the lilac. Though they offer mainly one season of interest, their spring flower displays and fragrance are greatly appreciated after a long, hard winter. If you have lilacs or if you are interested in purchasing one, consider several factors prior to planting.

Lilacs are adapted to USDA Hardiness Map zones 3 to 7 and milder areas of zone 2. They thrive in sunny sites with good air circulation. Lilacs need at least four to six hours of sunlight a day for best flower production. Plants should be spaced between 10 and 15 feet apart for specimen displays and five to eight 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. More severe or renewal pruning to control or shape growth can be done in late winter or early spring. However, pruning during the dormant period will reduce flower production the following 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 four, five or more years after planting. Lilacs and most woody plants must grow and mature before they are capable of blooming. Exposure to light could also be a factor. Lilacs need at least six hours of direct sun to bloom well. Improper pruning techniques, such as topping the lilac, are another reason they fail to bloom. Many lilacs bloom on the previous season’s growth. Since the flower buds form during the summer months, pruning lilacs in fall or late winter could remove much of the blooming wood. Heavy fertilization should be avoided as this encourages vegetative growth, often at the expense of flowers.

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 summer or early fall, 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. Dwarf Korean lilac, Manchurian lilac and Preston lilac cultivars are resistant to powdery mildew.

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