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ageratum (ăj´ərā´təm, əjĕr´ə–) [Gr.,=unaging], any plant of the genus Ageratum, tropical American annuals of the family Asteraceae (aster family). The commonly cultivated species is the Mexican A. houstonianum, with thick terminal clusters of blue flowers. The similar mistflower, a Eupatorium (see boneset), is a perennial sometimes called hardy ageratum. Ageratum is classified in the division Magnoliophyta, class Magnoliopsida, order Asterales, family Asteraceae.