Hydrocotyle ranunculoides L. f. (syn.: H. natans Cyr.) (N-Am.) – An increasing, noxious, fully naturalised alien, first recorded in 1992 north of Gent (Wachtebeke) (Verloove & Heyneman 1999) and by now very widely dispersed (compare the distribution maps in Verloove 2002 and Verloove 2006b). Hydrocotyle ranunculoides is most common in Flanders (especially in the surroundings of Gent and in the Kempen) but recently also occurred in Wallonia. It apparently easily survived the successive cold winters of 2008-2009 and 2009-2010.
Hydrocotyle ranunculoides is an aggressive invader in various aquatic habitats (including nature reserves). It was initially introduced on purpose in the Langelede in Wachtebeke (for experimental water purification) but almost immediately escaped. Hydrocotyle ranunculoides was offered for sale as a fast-growing aquatic ornamental and subsequently occurred as an escape or throw-out at various locations. By now, it is of major concern and widely controlled or even eradicated in Belgium. Hydrocotyle ranunculoides severely affects ecosystems (for instance in out competing vulnerable native aquatic vegetation) and hinders human use of water bodies (for instance in hindering fishing or water recreation). Additional information on the invasive behaviour of Hydrocotyle ranunculoides in Belgium and elsewhere in western Europe is available at: http://ias.biodiversity.be/species/show/63 and http://www.q-bank.eu/Plants/Factsheets/Hydrocotyle%20ranunculoides%20NL.pdf.
Elsewhere in western and central Europe, Hydrocotyle ranunculoides behaves in a similar way and it currently surely is one of the most undesirable aquatic weeds (see also literature references below).
Watson (1997) erroneously refers to Hydrocotyle ranunculoides as H. americana L. The latter is a more slender, terrestrial species and is probably not in cultivation in Europe. Hydrocotyle ranunculoides is known since the beginning of the 19th century from parts of southern Italy (primarily Sicily) and often cl aimed as a native. However, this is rather unlikely.
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Selected literature:
Additional references and general information on Hydrocotyle ranunculoides and its invasiveness in Germany is available at www.aquatischeneophyten.de.
Baas W.J. & Duistermaat L. (1998) De opmars van Grote waternavel (Hydrocotyle ranunculoides L.f.) in Nederland, 1996-1998. Gorteria 24: 77-82.
Baas W.J. & Holverda W.J. (1996) Hydrocotyle ranunculoides L. f.: infiltrant in waterland? Gorteria 21: 193-198.
Delay J. & Petit D. (2008) Les espèces aquatiques invasives du Nord de la France et leur biologie. Bull. Soc. Bot. N. Fr. 61(1-4): 5-22.
Dortel F. (2012) Un plan de lutte contre Hydrocotyle ranunculoides L.f. dans les Pays de la Loire. E.R.I.C.A. 25: 185-194.
Hussner A. (2007) Zur Biologie des aquatischen Neophyten Hydrocotyle ranunculoides L. f. (Apiaceae) in Nordrhein-Westfalen. Flor. Rundbr. 40: 19-24.
Hussner A. & Lösch R. (2007) Growth and photosynthesis of Hydrocotyle ranunculoides L. fil. in Central Europe. Flora 202: 653-660.
Hussner A. & van de Weyer K. (2004) Hydrocotyle ranunculoides L.f. (Apiaceae) ein neuer aquatischer Neophyt im Rheinland. Flor. Rundbr. 38: 1-6.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006a) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p.
Verloove F. (2006b) Hydrocotyle ranunculoides. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 480-481.
Verloove F. & Heyneman G. (1999) Hydrocotyle ranunculoides L.f. (Grote waternavel), snel uitbreidend in de omgeving van Gent (Oost-Vlaanderen, België). Dumortiera 74: 16-20.
Watson M.F. (1997) Hydrocotyle. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 392.