Collard greens (Brassica oleracea, Acephala group) are the broad, flat, smooth dark blue-green leaves of a loose-leaf, non-heading cabbage — a close kin of kale that never forms a head and grows instead as an open rosette of large paddle-shaped leaves on thick pale stems. Earthy, mineral and frankly bitter when raw, with the sulfurous cabbage edge shared by all the cole crops, the sturdy leaves are slow-simmered until silky and sweet. They are a cornerstone of the cooking of the American South and of West and Central Africa, most famously stewed for an hour or more with smoked or salted pork until the leaves go tender and the cooking liquid becomes the prized, savoury pot likker.