Common lilac

Identification and origin

Syringa vulgaris belongs to the olive family, Oleaceae. In the wild it grows on the Balkan Peninsula: Romania, Bulgaria, Greece, and the western Balkans. Spread through cultivation, it has naturalized across much of Europe and North America.

It is a multi-stemmed deciduous shrub with an upright habit that sends up suckers from its roots. In gardens it usually reaches 8–16 ft tall and 6–12 ft wide; the wild species can form a thicket about 23 ft in each direction. Its opposite, heart-shaped, pointed leaves, 2–5 in long, are smooth and gray-green to blue-green, with no fall color. The bark is light gray-brown with lenticels.

Bloom comes in mid- to late spring and lasts about two weeks. The small, tubular, four-lobed flowers are borne in conical panicles 6–8 in long, often in pairs, with a strong, sweet fragrance. Single flowers have four petals; doubles have several sets of four. They attract bees and butterflies and make excellent cut flowers. Flat brown capsules follow, with no ornamental value.

There are hundreds of cultivars: ‘Sensation’, purple edged in white; ‘Madame Lemoine’, double white; ‘Charles Joly’, magenta; ‘Ludwig Spaeth’, dark purple; ‘Beauty of Moscow’, with pink buds opening to double pinkish-white flowers; and ‘Katherine Havemeyer’, double dark pink. Some are grafted onto standards to form small “tree lilacs.”

Light

Lilac blooms best with at least six hours of direct sun a day. It tolerates light shade, but less sun means fewer flowers and more powdery mildew. If a neighboring tree has grown to shade it, thin out that tree. A sheltered spot limits late-frost damage to the flower buds.

Watering

Water thoroughly at planting, then keep the soil moderately moist while the shrub establishes over its first two or three years: check the soil and water when it is dry a few centimeters down. Once established, lilac handles dry spells better than most garden shrubs.

It does not tolerate standing water, though: soil that stays soggy leads to stunted growth, fewer flowers, and root rot. Also avoid overhead watering that wets the foliage, which encourages bacterial disease.

Potting mix

Lilac likes soil rich in organic matter, moist but well drained, with a neutral to slightly alkaline pH; it thrives on chalk and tolerates clay, but not highly acidic or poorly drained ground. Plant it at the same depth it grew in the nursery, away from competition with trees and other shrubs.

Once established, two to three years after planting, an all-purpose shrub fertilizer every few years is enough. Avoid lawn fertilizers, which are high in nitrogen and push leaves at the expense of flowers: plant lilacs in garden beds rather than in the lawn.

Temperature

Lilac needs a long, cold winter for its flower buds to mature; it blooms poorly in mild climates, although a few “low-chill” cultivars such as ‘Angel White’ have been developed. It is hardy throughout northern Europe, down to -4–5 °F, and in North America it is grown in USDA zones 3a to 7a. Temperature during bloom also affects fragrance and flower color.

Repotting

Lilac is grown in the ground. Spring is the usual planting season, but container-grown plants can go in at any time during the growing season. Choose plants 2–4 ft tall and space them 6–7 ft apart, depending on the cultivar’s size: crowded plants bloom less, need more pruning, and suffer more from powdery mildew. A young plant can take a year or two to bloom.

Pruning

Lilac blooms on the previous year’s wood and sets its buds as soon as flowering ends, so prune immediately after bloom, never later, or you will cut off next year’s flowers. Cut spent clusters back to a pair of leaves, shorten overlong branches to an outward-facing bud, and aim for a rounded shape, never a flat top. Remove unwanted suckers at ground level, especially rootstock suckers on a grafted lilac, and always cut above the graft union.

A young plant needs no pruning for two to three years. After that, to rejuvenate it, remove a third of the thickest stems at the base each year for three years. A badly overgrown old lilac can be cut back entirely to 2–6 in above the ground; it will bloom again after two to three years.

Propagation

• Suckers: the easiest method. In early spring, before the buds open, dig up a sucker that already has roots and replant it; it will make a good plant within a few years.
• Layering: make a 2 in slanting cut on a flexible branch, 12 in from its tip, pin it to the ground, bend the tip upward, and cover the wound with 2–4 in of firmed soil. Keep it moist; allow about a year before severing the new plant.
• Cuttings: in early summer, take stems 8–10 in long, strip the lower leaves, dip the base in rooting hormone, and set them in sand or perlite under high humidity.
• Grafting and seed: best left to professionals and breeders; lilacs generally do better on their own roots.

Common problems

• No flowers: plant not yet established, too little sun, too much nitrogen, pruning too late, or buds killed by a late frost.
• Grayish-white coating on leaves: powdery mildew, favored by shade and warm, humid weather; mostly cosmetic late in the season.
• Brown spots turning black, shoots dying in spring: bacterial blight, in cool, wet weather; cut affected branches 10–12 in below the symptoms, in dry weather.
• Wilting leaves, stems that snap: wood-boring larvae (lilac borer).
• Other pests: scale insects, lilac leaf miner, thrips, and caterpillars.
• General decline: honey fungus or Phytophthora, often linked to poorly drained soil.

A single symptom can have several causes: check sun exposure, drainage, fertilizing, and when the shrub was last pruned.

Toxicity

No toxicity is reported for people or for dogs, cats, and horses; the flowers are even edible, raw or fried as fritters. Do not confuse it with “Persian lilac” or chinaberry (Melia azedarach), an unrelated tree that is toxic to animals. Its strong fragrance can, however, be overpowering for some people.