mustard greens primarily Brassica juncea, come from a wide range of wild and cultivated mustard plants, almost all belonging to the genus Brassica, that of the cabbage. They are of minor importance in western countries but are eaten on a large scale in China.
Two wild European mustards, ancestors of the cultivated species, have edible, although bitter, leaves. The first is field mustard, Brassica rapa, whose cultivated varieties include Indian colza and Indian rape. In its wild form this is called kalewort or summer rape in England. The second is charlock, Sinapis arvensis. This plant, also known as wild or corn mustard, often used to be eaten, especially in Ireland, the Hebrides, and Sweden.