When most people think of lilacs, they think of the fragrant, old-fashioned common lilac (Syringa vulgaris), which blooms for a few weeks each spring, but the genus is quite diverse. By selecting carefully, it is possible to have two months of spring bloom (particularly if the weather is cool), plus some repeat flowering in early fall, and even fall foliage color. For the earliest blooms, choose hybrid S. x hyacinthiflora, followed by common lilacs, which offer the longest blooming, largest flowers with the best fragrance. Extend the lilac season to summer with species such as S. patula, which also has good fall color, and tree lilacs such as S. x chinensis 'Saugeana' or the Japanese tree lilac (S. reticulata).
Most lilacs grow happily in Zones 4-7, but some are adaptable farther south. Common lilacs, natives of somewhat chilly mountainous zones, require a cold period each winter for their flowers buds to mature and bloom the following spring. Lilacs don't take kindly to the Deep South or the desert, but some cultivars, such as 'Blue Skies,' 'Exel,' and 'Sister Justina' have been bred for warmer regions (Zones 8 and 9). The cutleaf lilac (S. laciniata) also handles some heat, and its fine-textured foliage is particularly attractive.
Lilacs require a sunny spot with rich, well-drained, fairly neutral soil, ideally 6.5-7 pH. If necessary, dig in some dolomitic lime to raise the soil pH. In addition to selecting a spot that is large enough to accommodate the mature size of your plant, choose one that has good air circulation to reduce the likelihood of fungal diseases such as powdery mildew. Early spring and fall are the best times to plant -- many growers prefer to plant lilacs in fall, especially in milder climates.
Water young plants regularly. Once established, lilacs need infrequent watering, except during droughts. Feed plants with an all-purpose fertilizer after pruning, and side-dress them with some well-rotted manure or compost each spring.
Lilacs are relatively low maintenance. Powdery mildew, one of the most common lilac problems, is a fungal disease that covers the foliage with a gray-white powder, but it is usually not life threatening. To reduce powdery mildew, spray plants with horticultural oil, following label directions, in summer after they have bloomed.
Mature lilacs won't bloom if they do not receive enough sun (at least 6 hours daily) or if they have not been pruned correctly. Young lilacs can take up to 3 years to reach maturity and bear flowers, but once established, they need pruning only to promote flowering, to reshape, and to remove unwanted suckers.
Unless they have been long neglected, lilacs rarely need more than regular deadheading and some reshaping in early summer after they bloom.
Because lilacs form their flower buds in summer for blooms the following spring, it's best to deadhead just after flowers fade and to prune before July 4. Remove the large, unattractive seedpods that form after flowers fade. Prune out any dead, damaged, or diseased branches as you see them, cutting just above a bud. Suckers (shoots that emerge from the base of the plant) should be thinned regularly or removed as necessary to maintain the plant's shape. Larger suckers can be dug up with their roots intact and grown in pots until they are large enough to be transplanted into the garden.
Long-neglected lilacs can easily become gangly monstrosities that flower little but overshadow their environs. Overgrown lilacs need complete rejuvenation, a 3-year process that entails cutting out one-third of the wood to the ground each year in late winter. The first year, choose the oldest, least productive third of the branches, and cut them all the way to the ground. Follow the same process in the second and third years for shapely, free-flowering plants.
Lilacs bloom in seven official colors: white, violet, blue, lavender, pink, magenta, and purple, with many shades of each, and in many sizes, ranging from 4-foot-tall dwarf bushy types to rangy, 20-foot-tall common lilacs to 30-foot trees. Florets, the tiny flowers that make up the larger flower heads, may be single (with one row of petals) or double, and vary widely in size and shape as do the clusters.
'Adelaide Dunbar,' a disease-resistant common lilac, bears spikes of sweet-scented, double, purple flowers and grows 10-12 feet tall.
'Angel White,' which reaches 10-12 feet tall, bears an abundance of fragrant, pure-white blooms, and thrives as far south as Zone 8.
'Annabel,' a hybrid S. x hyacinthiflora, reaches 10 feet tall. Its double, pink flowers bloom earlier than common lilacs.
'Firmament,' introduced in 1932, has long been favored for its showy single, pale-blue flowers, and grows 8-15 feet tall.
Japanese tree lilac (S. reticulata) has striated, shiny bark resembling that of a cherry tree. Its creamy flowers bloom very late in the season on 20- to 30-foot branches.
'Krasavitsa Moskvy,' also known as 'Beauty of Moscow,' is a distinctive, double-flowered 8-10-foot-tall shrub whose pink-lilac buds open to white tinged with the palest lavender.
'Little Boy Blue,' a dwarf common lilac that remains shorter than 5 feet, bears sweet-scented, single, sky-blue flowers.
'Marie Francis,' a small shrub at 5-6 feet tall, has lovely, clear-pink flowers with a strong fragrance.
'Miss Kim' Korean lilac (S. patula) is a late-blooming lilac reaching 6-10 feet tall, with spicy-scented, blue-lavender flowers and reddish fall foliage.
'Primrose,' introduced in 1949, grows to 12 feet tall and remains rare among common lilacs for its pale-yellow flowers.
'Sensation,' first known in 1938, is unique for its bicolor deep-purple petals edged in white on 8-12-foot-tall shrubs.
'Wedgwood Blue,' as its name implies, has fragrant flowers in hues resembling blue Wedgwood on spreading shrubs that reach 6 feet tall.