Driving the Deep South’s back roads in late fall or winter offers glimpses of a shade of green bluer and darker than most of the vegetation you’ll see, arranged in garden rows with hints of purple and yellow. The untrained viewer—just trying to keep eyes on the road, for goodness’ sake—may not realize that these verdant patches are in fact a unique marker of American history: the beloved, if at times belittled, collard green.

Collards are unknown in most of the …