17. Cardamine flexuosa Withering, Arr. Brit. Pl. ed. 3. 3: 578. 1796.
Cardamine flexuosa subsp. debilis O. E. Schulz; C. flexuosa var. debilis (O. E. Schulz) T. Y. Cheo & R. C. Fang; C. hirsuta Linnaeus subsp. flexuosa (Withering) F. B. Forbes & Hemsley; C. scutata Thunberg subsp. flexuosa (Withering) H. Hara
Annuals or biennials; sparse-ly to densely hirsute basally or throughout, or glabrous. Rhizomes absent. Stems erect, ascending, or decumbent, branched or unbranched, (0.6-)1-5 dm, (slightly flexuous). Basal l eaves (often withered by anthesis), not rosulate, 5-15-foliolate, (2.7-)4-14 (-19) cm, leaflets petiolulate; petiole 0.7-5 cm, (ciliate or not); lateral leaflet blade oblong, ovate, or elliptic, smaller than terminal, margins entire, repand, crenate, or 3 (or 5)-lobed; terminal leaflet (petiolule 0.3-1.7 cm), blade reniform, broadly ovate, or suborbicular, 0.5-2.5 cm × 4-30 mm, margins repand, crenate, or 3 or 5-lobed. Cauline leaves 3-15, 5-15-foliolate [leaves (2-)3.5-5.5(-7) cm, including petiole], petiolate, leaflets petiolulate; petiole base not auriculate; lateral leaflets similar to basal, (0.4-2.5 mm wide). Racemes ebracteate. Fruiting pedicels divaricate or ascending, (5-)6-14(-17) mm. Flowers: sepals oblong, 1.5-2.5 × 0.7-1 mm, lateral pair not saccate basally; petals white, spatulate, 2.5-4(-5) × 1-1.7 mm; (stamens rarely 4, lateral pair absent); filaments 2-3 mm; anthers ovate, 0.3-0.5 mm. Fruits linear, (torulose), (0.8-)1.2-2.8 cm × 1-1.5 mm; ovules 18-40 per ovary; style 0.3-1(-1.5) mm. Seeds brown, oblong or subquadrate, 0.9-1.5 × 0.6-1 mm, (narrowly margined or not). 2n = 32.
Flowering Apr-Jul. Disturbed areas, fields, nurseries, plantations, gardens, flower beds, lawns, roadsides; 0-1100 m; introduced; B.C., Nfld. and Labr. (Nfld.), Ont.; Ala., Calif., Fla., Ga., Ill., Ind., La., Md., Mich., N.Y., N.C., Ohio, Oreg., R.I., Tex., Va., Wash.; Europe; e Asia; introduced also in Mexico, Central America, South America, Australia.
According to J. Lihová et al. (2006), the populations referred to Cardamine flexuosa in North America comprise two taxa of different polyploid origins and evolutionary histories: tetraploid C. flexuosa (2n = 32), native to Europe, and the octoploid taxon informally called "Asian C. flexuosa" (2n = 64), native to eastern Asia. For the latter, the name C. flexuosa subsp. debilis can be used. Nevertheless, these two taxa should be recognized at species level and the correct name for the Asian species should be sought. Based on available data, both taxa occupy the same habitats in North America, but the Asian taxon is much more widespread. The occurrence of European C. flexuosa was, until now, confirmed only for Washington, where both taxa have been recorded. More detailed studies of the North American distributions of both these weeds are needed.