Nouv. Arch. Mus. Hist. Nat., sér. 2, 10: 65. 1887–1888
Shrubs, 5–30 dm. Stems branched, tomentose. Leaves often with auriculate stipules; petiole 5 mm; blade ovate to ovate-lanceolate, 10–15 × 2–4 cm, base attenuate or cuneate, margins serrate or subentire, apex acute or acuminate, abaxial surface tomentose, adaxial glabrous or glabrescent. Inflorescences terminal, tapered-spicate or narrowly paniculate thyrses, 10–20 × 2–4 cm, cymes almost encircling inflorescence axis, 10–25 pairs, 3–30-flowered. Pedicels essentially absent; bracteoles present. Flowers fragrant; calyx sparsely hairy externally, tube 1.5–2 mm, lobes 0.5–1.5 mm; corolla straight, violet or pink, rarely white, often with yellow-orange eye, salverform, tube 8–10 mm, lobes suborbiculate, 2–3 × 2–3 mm; stamens inserted medially in corolla tube, included in tube; ovary ovoid, 2 mm, glabrous or puberulent; stigma clavate, 1 mm. Fruits capsules, brown, narrowly ellipsoid, 5–9 × 1.5–2 mm, glabrous or puberulent, dehiscence primarily septicidal. Seeds threadlike, 3–4 × 0.5 mm, wings long. 2n = 76.
Phenology: Flowering May–Oct; fruiting Jul–Nov.
Habitat: Roadsides, railroad embankments, quarries, streambeds, landslide scars, sandy lakeshores, disturbed sites.
Elevation: 0–1300 m.
Introduced; B.C., Ont., Ala., Calif., Conn., Del., D.C., Ga., Ill., Ky., Md., Mass., Mich., Mo., N.J., N.Y., N.C., Ohio, Oreg., Pa., R.I., S.C., Tenn., Va., Wash., W.Va., Asia (China), introduced also in Central America, South America, Europe, Africa, Pacific Islands (Hawaii, New Caledonia, New Zealand), Australia.
Buddleja davidii has been designated as a noxious weed in Oregon and Washington; it is a serious invader also in England, New Zealand, and Australia. It can often form dense thickets and produce abundant seeds (N. G. Tallent-Halsell and M. S. Watt 2009).