Chaenorhinum minus subsp. minus
Stems much-branched from or near base, sometimes unbranched, often zig-zag, (4–)8–28(–40) cm. Leaves: blade 5–15(–30) × 1–4(–5) mm, base tapered, apex obtuse to acute, surfaces glandular-pubescent. Pedicels ascending. Flowers: calyx lobes accrescent, abaxial 1.5–2.5 × 0.3–0.4 mm, adaxial 2.8–3.5 × 0.4–0.8 mm, margins entire, apex obtuse; corolla 8–11 mm (including 1–2.3 mm spur), sparsely glandular-pubescent externally, glandular-pubescent internally on abaxial surface, especially along ridges of palate and into throat, throat 2–3 mm diam., abaxial lobes spreading, adaxial projecting; stamens included, anthers opposite, navicular, marginally coherent, pollen sacs of longer pair of stamens 0.3–0.4 mm, of shorter pair of stamens 0.1–0.2 mm, glabrous; ovary glandular-pubescent; style 1.5–2 mm, glandular-pubescent proximally. Capsules obovoid to ellipsoid, 4–5.4 × 2–3.6 mm, glandular-pubescent distally. Seeds 0.6–0.9 mm, prominently ribbed longitudinally. 2n = 14, 28 (both Europe).
Phenology: Flowering May–Oct.
Habitat: Gravelly railroad rights-of-way, road shoulders, urban areas, stream beds.
Elevation: 0–1600 m.
Introduced; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Ala., Ark., Colo., Conn., Del., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., Tenn., Tex., Vt., Va., Wash., W.Va., Wis., Europe, sw Asia, introduced also in e Asia (Russian Far East).
Subspecies minus is the only member of Chaenorhinum minus to have become widely established as a weed; the other three subspecies occur in Europe, one each in Corsica, Crete, and Turkey.
First collected in Camden, New Jersey (1874), and St. John, New Brunswick (1881), Chaenorhinum minus likely came to North America as a contaminant in ship ballast, with subsequent dispersal along railroad lines (M. P. Widrlechner 1983). R. M. Arnold (1981, 1982) noted that self-compatibility, a short generation time, drought tolerance, and seed dispersal enhanced by passing trains allowed C. minus to be a successful colonizer of railroad rights-of-way, where it was once common but is now scarce because of herbicide use.