Kale, collard, cabbage, cauliflower, broccoli, kohlrabi, and brussels sprouts all seem quite different in appearance and flavor. Each is used in a different way in cuisine, and one is rarely substituted for another.
All of these vegetables are members of the family Brassicaceae. This is commonly referred to as the cabbage family or the mustard family. Plants in this family are largely biennial (require two years to complete a life cycle) and all have four-petaled flowers:
But not only are kale, collard, cabbage, cauliflower, broccoli, kohlrabi, and brussels sprouts are all members of the cabbage family, they’re all members of the same genus, Brassica.
But what’s really fascinating, however, is that all of these different vegetables are the same species. Brassica oleracea is known as wild cabbage, but thousands of years of human cultivation and selective breeding have created all of these different plants. Humans have bred individual plants with desirable traits in order to create offspring with those traits. Over time, these traits have been magnified, and now we have individuals very different from one another. Despite the differences in appearance they can all hypothetically interbreed, so they’re still the same species.