Rorippa austriaca

Rorippa austriaca, the Austrian fieldcress or Austrian yellowcress, is a vigorous perennial herb of the Brassicaceae native to central, eastern and southern Europe and western Asia, and widely naturalised in temperate North America where it is regarded as a noxious agricultural and roadside weed. It spreads aggressively from creeping rhizomes and deep, branching lateral roots, producing erect, glabrous, often reddish stems 30-100 cm tall. The leaves are alternate, simple, oblong-lanceolate to elliptic, 4-12 cm long, with finely toothed, slightly clasping bases and an entire or shallowly lobed blade; basal leaves wither early. From late spring through summer the stems produce branched, terminal racemes of small, four-petalled bright yellow flowers 3-5 mm across, the petals about twice the length of the green sepals. The fruits are small, globose to ellipsoid siliques 2-4 mm long on slender, spreading pedicels and frequently fail to produce viable seed in introduced populations, which spread mainly by vegetative root fragments along moist ditches, river banks, cultivated fields and disturbed ground.