Headless form of cabbage ( Brassica oleracea , Acephala group), in the mustard family .
It bears the same botanical name as kale , differing only in that collard leaves are much broader, are not frilled, and resemble the rosette leaves of head cabbage. The main stem has a rosette of leaves at the top. Lower leaves commonly are harvested progressively; sometimes the entire young rosette is harvested. The leaves are highly nutritious, rich in minerals and in vitamins A and C.