アブラナ科アブラナ属の越年草で、学名は Brassica juncea (syn. Brassica juncea var. cernua)。英名は Leaf mustard。
The Leaf mustard (Brassica juncea) belongs to Brassiaceae (the Mustard family). It is a biennial herb that is probably native to Central Asia. It was introduced into Japan in in Yayoi Era (2300-1700 years ago). This herb is cultivated, also runs in wild and can reach 1-1.5 m in height. The lower leaves are simular to Japanese radish and divided into irregular lobes pinnately. The yellow flat cruciate flowers come in April to May. The leaves and young stems are used for pickle, boiled green and stir-fry.
This website assumes that "Brown mustard" is a wild species that originated from "Leaf mustard" and became a naturalized plant after the Meiji era (1868-1912).