Vallisneria americana Michaux (eastern U.S.A.) – A very rare escape from cultivation. A small population of ca. 60 m² was discovered in a canal in Geel in 2016. Early 2018 its presence was confirmed and population size reassessed. It was estimated to cover a surface of ca. 180 m².
Like Vallisneria spiralis the plants found in Geel have female spathes on coiled stalks. Its leaves, however, are much wider and longer. V. gigantea, at least in the sense of Nelson (1986), has similar leaves but females spathes are borne on straight stalks.
Assessing the exact identity of this population possibly requires further study. The recently described Vallisneria australis S.W.L. Jacobs & D.H. Les (Les & al. 2008) is much reminiscent of V. americana and only distinguished from it by the base of the androecium being glabrous (pubescent in V. americana) and by its narrower seeds. Genuine V. gigantea (a synonym of V. nana R. Brown, according to Les & al. 2008) is a tropical relative and less likely to be found in the wild in western Europe. V. australis is also grown as an aquarium ornamental and is considered an invasive species in Japan (Wasekura & al. 2016).
Another very similar species, also grown as an aquarium plant, is Vallisneria neotropicalis Marie Vict. It is not simply a larger growth form of V. americana, but a genetically distinct lineage (Les & al. 2008). Vegetatively, V. neotropicalis can be separated from V. americana by its foliage, which is suffused with reddish pigment striations.
There seem to be very rare additional records of broad-leaved species of Vallisneria elsewhere in Europe. Only from Hungary, as V. nana, such plants have been recorded so far (Hussner 2012).
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Selected literature:
Hussner A. (2012) Alien aquatic plants in European countries. Weed Research 52: 397-406. [available online at: https://www.researchgate.net/profile/Andreas_Hussner/publication/2576507...
Les D.H., Jacobs S.W.L., Tippery N.P., Chen L., Moody M.L. & Wilstermann-Hildebrand M. (2008) Systematics of Vallisneria (Hydrocharitaceae). Syst. Bot. 33(1): 49-65. [available online at: http://hydrodictyon.eeb.uconn.edu/people/les/Manuscript_Files/Syst_Bot_33(49)%5B2008%5D.pdf]
Nelson E.C. (1986) Vallisneria. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1. Cambridge University Press, Cambridge: 116.
Wasekura H., Horie S., Fujii S. & Maki M. (2016) Molecular identification of alien species of Vallisneria (Hydrocharitaceae) species in Japan with a special emphasis on the commercially traded accessions and the discovery of hybrid between nonindigenous V. spiralis and native V. denseserrulata. Aquatic Bot. 128: 1-6.