Description from Flora of China
Caelestina Cassini; Carelia Fabricius; Isocarpha Lessing (1830), not R. Brown (1817).
Subshrubs or annual to perennial herbs. Leaves opposite or sometimes alternate; blade elliptic or lanceolate to deltate or ovate, margin entire to dentate. Synflorescence cymose to subcymose, sometimes subumbellate. Phyllaries 30-40, distant, 2- or 3-seriate, equal or subequal, lanceolate, markedly hardened, often with scarious margin; receptacle conical, glabrous or paleaceous. Florets 20-125; corollas white, blue, or lavender, funnelform or with distinct basal tube; lobes 5, as long as wide, partially papillose and sometimes hispidulous on outer surface, papillose on inner surface; antheropodium cylindric; style base not enlarged, glabrous, style branches linear, usually strongly and densely papillose. Achenes prismatic, 4- or 5-ribbed, glabrous or ribs setuliferous; carpopodium distinct; pappus of 5 or 6 free, flattened, sometimes awnlike, scales or lacking or coroniform. x = 10, 20.
See Johnson (Ann. Missouri Bot. Gard. 58: 6-88. 1971) and Robinson (Phytologia 69: 93-104. 1990).
About 40 species: Central and South America, one species (Ageratum houstonianum) widely cultivated and another species (A. conyzoides), although sometimes cultivated, a widespread weed throughout the tropics in both the Old and New Worlds; two species (both introduced) in China.