Flixweed, tansy mustard

Sisymbrium sophia Linnaeus, Sp. Pl. 2: 659. 1753; Hesperis sophia (Linnaeus) Kuntze; S. parviflorum Lamarck; Sophia parviflora (Lamarck) Standley

Annuals; eglandular; sparsely to densely pubescent, sometimes glabrous distally, trichomes dendritic. Stems erect, un-branched or branched distally, (1-)2-7(-10) dm. Basal leaves: petiole 0.1-2(-3) cm; blade 2- or 3-pinnate, ovate or oblong to obovate in outline, to 15 cm, lateral lobes linear or oblong, (to 10 × 2 mm), margins entire. Cauline leaves sessile or shortly petiolate; blade smaller distally, distal lobes often narrower, surfaces often glabrous. Racemes considerably elongated in fruit. Fruiting pedicels divaricate to ascending, straight, (5-) 8-15(-20) mm. Flowers: sepals erect to ascending, yellowish, oblong, 1.8-2.8 mm, glabrate to sparsely pubescent; petals narrowly oblanceolate, 2-3 × 0.4-0.6 mm; median filaments 2-3 mm; anthers 0.3-0.4 mm. Fruits divaricate-ascending to erect, narrowly linear, torulose, (12-)15-27(-30) × 0.5-0.8(-1) mm, (straight or curved upward); valves each with distinct midvein; septum with a broad central longitudinal band appearing as 2 or 3 veins; ovules 20-48 per ovary; style obsolete, 0.05-0.2 mm, glabrous. Seeds uniseriate, reddish brown, oblong, 0.7-1.3 × 0.3-0.6 mm. 2n = 28.

Flowering Mar-Jul. Roadsides, waste places, disturbed sites, railroad embankments, hillsides, mountain slopes, canyon bottoms, stream banks, fields, lawns, pastures, deserts, sagebrush and pinyon-juniper communities; 0-3000 m; introduced; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.Mex., N.Y., N.C., N.Dak., Okla., Oreg., Pa., R.I., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Eurasia; introduced also in Mexico, Central America, South America, South Africa, Australia.

Deviant chromosome counts (e.g., 2n = 12, 14, 20, 38; see R. C. Rollins 1993, N. H. Holmgren 2005b, S. I. Warwick and I. A. Al-Shehbaz 2006) are most certainly erroneous, and the species appears to be exclusively tetraploid based on x = 7.

SELECTED REFERENCE Best, K. F. 1977. The biology of Canadian weeds. 22. Descurainia sophia (L.) Webb. Canad. J. Pl. Sci. 57: 499-507.