cornflower, common herb ( Centaurea cyanus ) of the family Asteraceae (aster family). It is a garden flower in the United States but a weed in the grainfields of Europe. It is called bluebottle or bluet in England and bluebonnet in Scotland; in North America the cornflower shares with other plants the names ragged robin, bachelor's button, or ragged sailor. The long-stemmed blue heads of the flowers, having radiating bottle- or vase-shaped florets, yield a juice which, mixed with alum, has been used as a dye. Cornflowers are classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.