Cl. Crucif. Emend., 105. 1769.
Annuals [perennials]; not scapose; glabrous or pubescent. Stems erect, unbranched or branched basally. Leaves basal and cauline; petiolate or subsessile; basal not rosulate, petiolate, blade margins usually pinnate to lyrately pinnatifid, rarely undivided, dentate; cauline subsessile or shortly petiolate, blade (base not auriculate), margins lobed, subentire, or dentate. Racemes (corymbose, several-flowered), greatly elongated in fruit. Fruiting pedicels erect [ascending], (usually appressed to rachis), slender to stout. Flowers: sepals ascending, oblong, lateral pair not saccate basally, (hispid [glabrous or with subapical tuft of hairs]); petals yellow, obovate, claw differentiated from blade, (apex ± truncate); stamens tetradynamous; filaments not dilated basally; anthers ovate to suboblong, (apex obtuse); nectar glands confluent, median glands present. Fruits silicles, indehiscent, sessile, segments 2, elliptic to oblong, torulose, (slightly to strongly constricted at transverse joint), terete or angular; (valvular segment persistent, dehiscent [indehiscent], 1(–3)-seeded, longitudinally striate or smooth, occasionally seedless and nearly as wide as pedicel; terminal segment indehiscent, caducous at maturity, usually 1-seeded, rarely seedless); valves glabrous or pubescent; septum complete; ovules 2–4; (style persistent, filiform); stigma capitate, (flattened), 2-lobed. Seeds uniseriate, slightly compressed, not winged, ovoid [oblong]; seed coat (smooth), not mucilaginous when wetted; cotyledons conduplicate. x = 8.
Introduced; s Europe (Mediterranean region), introduced also nearly worldwide.
Species 2 (1 in the flora).
Both species of Rapistrum have been introduced into North America; only R. rugosum has persisted with naturalized populations. Rapistrum perenne (Linnaeus) Allioni was first collected in 1922 from southeastern Saskatchewan but has not been seen or collected from there since 1932. It can be distinguished by being a perennial with a conical style shorter than the strongly 8-ribbed terminal segment, whereas R. rugosum is an annual with a slender, filiform style longer than the slightly ribbed terminal segment.