One of the garden's most fragrant flowering shrubs and small trees, lilacs (Syringa spp.) number about 20 species and dozens of cultivars. The precise timing of lilac blossoming varies by latitude and elevation across the United States. It ranges anywhere from early to late spring across U.S. Department of Agriculture Plant Hardiness Zones 3 through 8. Lilacs need a cold winter dormancy to bloom later and are not grown in the extreme southern U.S.
Among lilacs' many species, hybrids and cultivars, horticulturists discuss their flowering times relative to each other, such as being early, mid or late season blooming. S. x hyacinthoides and its cultivars are among the first to bloom. The common lilac (S. vulgaris) blooms midseason and is the standard to compare other plants' flowering time. The Japanese tree lilac (S. reticulata) is among the last to bloom in very late spring.
Across the southern half of the United States in USDA zones 7b and 8, the earliest lilacs begin flowering around April 1. In zones 5b through 7a, these same lilacs don't starting blooming until mid- to late April. In the northernmost states in zones 3b through 5a, the first lilacs bloom in early to mid-May, sometimes not until nearly Memorial Day near the Canadian border. Midseason and late-blooming lilacs follow in sequence across the country. Japanese tree lilacs begin blooming in the South around late April, but not until early June in states such as North Dakota.
A lilac plant in bloom lasts for seven to 10 days, two weeks at most. The shrubs or trees look attractive as the flower buds swell and are visible, making the flowering season seem to last a bit longer. Unusually warm, windy weather at flowering time shortens the flowering season. Conversely, cool and cloudy weather elongates the blossoms in much the same way placing cut flowers in a refrigerator prolongs their shelf life. Wind and rainfall strips lilac flower clusters of individual blossoms, making the display look peaked and haggard.
The commercial cut-flower trade today is worldwide, allowing anyone to request and receive lilacs in bloom any time of year. In the U.S., lilac plants are forced into bloom after an appropriate exposure to chilly temperatures and then warmed in greenhouses simulating spring. From February to November, facilities across North America produce cut lilac flower stems. However, from November to April, lilacs q43 imported from growers and suppliers in Holland.