Several species, with hundreds of cultivars and hybrids, are in cultivation. The common lilac is the most popular of all, and spreads vegetatively to form large thickets at old homesites and along roadsides. Spread by seed seems less common. Lilacs have doubtless spread from cultivation in many more counties than those mapped. Syringa ×persica L. was collected persisting on an old homesite in Washtenaw Co. in 1973; it has generally smaller leaves (mostly less than ca. 5 cm long and 2.5 cm wide) that are more tapering to the base than S. vulgaris. Other lilacs are also grown and may escape.

1. Corolla tube exserted only ca. 1 mm beyond calyx, about as long as the corolla lobes; anthers exserted, flowers creamy white; plants usually single trunked small to medium-sized trees.

S. reticulata

1. Corolla tube exserted ca. 6-12 mm beyond calyx, much longer than the corolla lobes; anthers included within the tube; flowers lavender or pink (occasionally pure white); plants bushy, suckering shrubs or small trees.

2. Panicles produced from terminal buds, as well as laterals; undersides of leaves (especially on the veins) pubescent; leaves elliptic; tapering to the base.

S. × henryi

2. Panicles produced from lateral buds, terminal buds usually lacking; undersides of leaves glabrous; leaves broadly ovate, truncate to cordate at base.

S. vulgaris