Publication: Horae Phys. Berol. 73, plate 15.
selected vouchers: WTU
Notes: FNA3: "Widely planted in North America and elsewhere as an ornamental, roadside, and reclamation plant, with many color forms in the horticultural trade, it often escapes but usually does not persist.
This species is highly variable (more than 90 infraspecific taxa have been described), not only among different plants and locations but also within individual plants over the course of the growing season, especially in petal size and color (see W. L. Jepson 1909-1943, vol. 1, part 7, pp. 564-569)."