Acanthaceae is a large family with more than 3,000 species (Mabberley 2008). Most, however, are confined to the tropics and none is native in Belgium. Several genera are cultivated as ornamentals (see Cullen & al. 2000 for an extensive account for Europe) but only few in West or Central Europe (e.g. Jäger & al. 2008). Two species from two genera (Acanthus and Thunbergia) are also more or less frequently cultivated in Belgium and have been recorded as escapes. In addition to these genera a third genus of the Acanthaceae has recently been discovered as an escape from cultivation in western Europe, Hygrophila R. Br. One species, Hygrophila polysperma (Roxb.) T. Anders., occurs as an aquatic neophyte in the river Erft in Germany, relatively close to the Belgian frontiers. It is an escaped aquarium plant (of tropical origin) that survives in the heated wastewater of a mining factory (Hussner & al. 2007).
Useful information on the Acanthaceae family is provided by the ‘Acanthaceae Online Resource’ at http://www.rsabg.org/acanthaceae/.
1 Twining herb. Corolla usually yellow or orange (except sometimes in cultivars), borne singly in leaf axils. Bracts and leaves without spines === Thunbergia
Erect herb, not twining. Corolla white, aggregated in verticils. Bracts and leaves conspicuously spiny === Acanthus
Cullen J., Alexander J.C.M., Brickell C.D., Edmondson J.R., Green P.S., Heywood V.H., Jörgensen P.-M., Jury S.L., Knees S.G., Maxwell H.S., Miller D.M., Robson N.K.B., Walters S.M. & Yeo P.F. (eds.) (2000) Acanthaceae. In: The European Garden Flora, vol. 6. Cambridge University Press, Cambridge: 356-370.
Hussner A., Josephs M. & Schmitz U. (2007) Über Hygrophila polysperma (Roxb.) T. Anderson und Pontederia cordata L. in Nordrhein-Westfalen. Flor. Rundbr. 40: 25-30.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.