Wikipedia Article About Mustard greens on Wikipedia The mustards are several plant species in the genus Brassica whose proverbially tiny mustard seeds are used as a spice and, by grinding and mixing them with water, vinegar or other liquids, are turned into a condiment also known as mustard. The seeds are also pressed to make mustard oil, and the edible leaves can be eaten as mustard greens. In addition to the mustards, the genus Brassica also includes cabbages, cauliflower, rapeseed and turnips.