Acorus L.
Depending on species delimitation, Acorus is a genus of three to six species from temperate regions in the northern hemisphere and tropical Asia.
In addition to the widespread Acorus calamus (the only species present in Belgium), a second species is grown as an ornamental, A. gramineus Sol. (Jäger & al. 2008). It is much smaller in stature with narrow, grass-like leaves less than 10 mm wide. It is unlikely to be confused with A. calamus and might pop up in the wild if it would become more widely offered in the horticultural trade. Another species (of debated taxonomic rank) is A. americanus (Raf.) Raf. (syn.: A. calamus var. americanus Raf.). This is exceedingly similar to A. calamus and used for the same purposes (Thompson 2000). Contrary to the latter it is fertile and thus produces seeds. In addition to the distinct midvein up to five additional veins are more or less equally raised above leaf surface. A. calamus in general also has longer and wider leaves and longer spadices. It is not impossible that A. americanus has been overlooked in western Europe; it should be looked for.
Literature:
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.
Rost L.C.M. (1978) Biosystematic investigations with Acorus L. (Araceae). Proc. Kon. Ned. Akad. Wetensch. C. 81: 428-441.
Rost L.C.M. (1979) Biosystematic investigations with Acorus, 4. Communication. A synthetic approach to the classification of the genus. Pl. Med. 37: 289-307.
Rost L.C.M. & Bos R. (1979) Biosystematic investigations with Acorus L. Pl. Med. 36(4): 350-361.
Rudall P.J. & Furness C.A. (1997) Systematics of Acorus: ovule and anther. Int. J. Pl. Sci. 158(5): 640-651. [available online at: https://www.researchgate.net/publication/249157992_Systematics_of_Acorus...
Thompson S.A. (2000) Acoraceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 22. Oxford University Press, New York-Oxford: 124-127. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=20008]
Webb D.A. (1984) Acorus. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 2: 85-86. Cambridge University Press, Cambridge.
Wieffering J.H. (1972) Some notes on the diploid chromosome number of the genus Acorus L. (Araceae). Acta Bot. Neerl. 21(5): 555-559. [available online at: http://natuurtijdschriften.nl/search?identifier=539876]