1. Ageratum houstonianum Mill. (syn.: A. mexicanum Sims) (C-Am.) – A rather rare and always ephemeral escape from cultivation. Probably first recorded in Vorst (Brussel) in 1908. Subsequently collected in scattered locations (Embourg, Hoboken, Malonne, Turnhout, Wellen,…), most often in or near gardens or on dumps. Recently seen, for instance, on the gravelly banks of river Maas in Maaswinkel.
Ageratum houstonianum is still frequently cultivated as an annual for summer-bedding but it is rather rarely found in the wild.
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Selected references:
Giri G.S. & Majumdar N.C. (1984) Miscellaneous notes: 25. Identity and distribution of Ageratum houstonianum Miller (Compositae) in India. J. Bombay Nat. Hist. Soc. 80(2): 453-456.
Pradhan G., Gurung P.B. & Kumar Y. (1989) Ageratum houstonianum Mill. (Asteraceae): a new distributional record from north east India. J. Econ. Taxon. Bot. 13(2): 413-415.
Sahu T.R. (1982) Taxonomic studies of the genus Ageratum L. in India. Feddes Repert. 93(1-2): 61-65.
Sharma V.S. (1987) Comments on the identity of Ageratum conyzoides L., and A houstonianum Mill.: two naturalized weeds in India. Feddes Repert. 98(11-12): 557-560.