Vallisneria spiralis L. (Medit.) – A very rare alien. First seen around 1864 in a canal near Willebroek, apparently naturalized at that time. In the 1950s recorded repeatedly in river Meuse, at first south of Namur (Dave, Lustin), later also elsewhere (Flémalle-Haute, Ramet, Givet; cf. Castagne 1956). Probably gone by now (no records posterior to 1980). In the past years Vallisneria spiralis seems to extend northwards and new records will probably come to light, especially in southern Belgium. In 2009 and 2010 it was reported, for instance, from Ciney (Hally; see also http://waarnemingen.be/waarneming/view/44510156) and Ottignies (Bois de Lauzelle) respectively. In 2014 recorded in several riverlets in the Kempen, near Dessel and Lommel. Possibly overlooked so far in this area.
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Selected literature:
Castagne E. (1956) La Vallisneria spiralis L. en Belgique. Bull. Soc. Roy. Bot. Belg. 88: 33.
Hussner A. (2006) Die aquatischen Neophyten in Nordrhein-Westfalen. Decheniana 159: 39-50.
Van de Weyer K., Krautkrämer V., Kormann B. & Gohr F. (2013) Die Wasserschraube (Vallisneria spiralis L.) im Gremminer See – Erstfund für Sachsen-Anhalt. Mitt. florist. Kart. Sachsen-Anhalt 18: 11-14. [available online at: http://www.bv-st.de/Publ2013.htm]