During a seven- to 10-day period in late spring, impressive blossoms cover lilac plants and fill the air with a delicate scent. The low-maintenance lilac shrub remains a non-descript garden plant during the rest of the year. But for most gardeners, the week of fragrant flowers is a strong reason to plant lilacs in the garden. The blossoms of French or common lilacs range from white, pink and lavender to purple and magenta with both single and double flowers. Many varieties of French lilacs bloom successfully after a winter chill. Often this requires several months where temperatures drop below 45 degrees. For climates without the winter chill, newer varieties of Descanso Hybrids blossom without a winter chill requirement.
French lilacs reach 8 to 15 feet at maturity. They prefer abundant space, sunshine and water. The usual spacing is 10 to 15 feet between plants. But when planted 6 feet apart, they make an attractive hedge.
French lilacs prefer a well-drained soil with a pH of 6.5 to 7.0 or slightly higher. They thrive in the alkaline soil found in the Western United States. If the soil is very sandy or heavy clay, add soil amendments. Otherwise French Lilacs thrive in a variety of soil types.
Lilac shrubs grow slowly from seed. Three to four years are necessary for the shrubs to produce the first blossoms. To hasten growth, gardeners purchase container-grown lilac plants in the early spring or fall.
Lilac plants are readily propagated by cuttings. Take 4 to 6 inch cuttings off new green terminal shoots. Dip the root end of the cuttings in rooting hormone and plant in damp vermiculite, perlite or peat. The cuttings develop roots in three to six weeks and are ready to transplant to pots or a garden area. Other methods of propagation are grafting, layering and tissue culture.
Plant the French lilac plants in the coolest part of the garden. In climates with hot summers, plant in an area with some afternoon shade. Find a site with at least six hours of sunlight daily.
Water the French lilac plants regularly during the first year. Once established, lilacs only require occasional watering. Apply mulch around the plants to conserve moisture and control weeds.
Approximately one month after planting, apply an 18-6-12 fertilizer at the rate of 2 ounces per 4 square feet. An organic fertilizer such as blood and bone meal is an adequate substitute. Fertilize each year in the early spring or in October.
To improve the growth and appearance of French lilac shrubs, prune them immediately after flowering every one to three years. Thin out the top growth to allow sunlight to reach the center of the shrub. Remove up to one-third of all branches larger than 1.5 inches.
A fungal disease called powdery mildew causes leaves to fall off and the shrub to look unattractive. Allow plenty of space between each French lilac plant and select disease resistant varieties. Watch for bacterial blight. The disease causes brown spots on leaves, twig dieback and distorted leaves during wet, cool spring weather.
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