London rocket

Sisymbrium irio is an erect annual herb of the Brassicaceae family growing 20-80 cm tall from a slender taproot. Stems are slender, simple at the base and much-branched above, glabrous to sparsely hairy in the lower part and otherwise smooth. Lower leaves form a basal rosette and are deeply pinnatifid to lyrate-pinnatifid with two to four pairs of small triangular lateral lobes and a much larger triangular terminal lobe, the whole 4-15 cm long; upper stem leaves are smaller, hastate to lanceolate with narrow lobes or entire. The inflorescence is an elongating raceme of small pale yellow flowers borne above the developing fruits, the open flowers being characteristically overtopped by the upper fruits. Sepals are four, oblong, greenish; petals are four, spathulate, 3-4 mm long. Stamens are six in two whorls. The fruit is a slender, terete, erect to ascending siliqua 3-5 cm long and about 1 mm wide on a short stout pedicel, sharply contrasting with the smaller flowers; valves are three-nerved. Seeds are numerous, oblong, brown and become mucilaginous when wet. Native to Eurasia and North Africa and widely naturalised, especially in arid and semi-arid regions of the Americas.