Hydrocotyle verticillata Thunb. (Trop.) – An exceptional escape from cultivation or, more likely, a throw-out. Known since 2009 from a pond in Bredene (nature reserve d’Heye; comm. M. Leten 2013) but initially erroneously taken for a vigorous form of Hydrocotyle vulgaris. Possibly overlooked elsewhere.
This species is a pantropical weed and deserves special attention. It is exceedingly similar to native Hydrocotyle vulgaris and should be looked for. It is sold as “Hydrocotyle vulgaris” in nurseries (at least in the Netherlands; see van Valkenburg & Pot 2008) and might be more widespread as an escape or even as a deliberate introduction. Hydrocotyle verticillata usually grows in more damp areas, has leaves with 9-13 veins (vs. 7-9 in H. vulgaris), glabrous petioles (vs. hairy in H. vulgaris), inflorescences as long as to longer than the subtending leaf (vs. shorter in H. vulgaris) and darker fruits (see for instance Carretero 1997, Medina 2003). It also is much more vigorous (larger leaves, thicker stems, etc.). Records of “Hydrocotyle vulgaris” in areas where it is not originally native (e.g. newly created ponds) are suspect and should be critically assessed.
The genuine presence of Hydrocotyle verticillata in Europe long remained uncertain (Cannon 1967, 1968) and it may still be overlooked in large parts of Europe. Most claims of Hydrocotyle vulgaris from the surroundings of Valencia in Spain, for instance, turned out to be referable to H. verticillata (Carretero 1997, Medina 2003).
Despite being of (sub-) tropical origin Hydrocotyle verticillata has easily survived the successive cold winters of 2009-2013 in its single Belgian locality. Given its reputation as an undesirable environmental weed the species will be eradicated in 2014 (comm. M. Leten).
Hydrocotyle verticillata is surprisingly omitted in recent European garden floras (e.g. Watson 1997, Jäger & al. 2008).
Additional useful information on Hydrocotyle verticillata and its incipient invasive behavior in Europe is available at: http://www.q-bank.eu/Plants/BioloMICS.aspx?Table=Plants%20-%20Species&Rec=1067&Fields=All.
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Selected literature:
Cannon J.F.M. (1967) Short Notulae: Umbelliferae: Hydrocotyle vulgaris and H. verticillata. Feddes Repert. 74: 34-35.
Cannon J.F.M. (1968) Hydrocotyle. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 2. Cambridge University Press, Cambridge: 319.
Carretero J.L. (1997) Hydrocotyle verticillata Thunb. (Umbelliferae) en la flora iberica. Fl. Montiber. 5: 63.
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.
Kasselmann C. (1980) Hydrocotyle vulgaris und Hydrocotyle verticillata. Aqua Planta 5(2): 5-8.
Khatun B.M.R., Rahman O. & Sultana S.S. (2010) Hydrocotyle verticillata Thunb. (Apiaceae) - A new angiospermic record for Bangladesh. Bangladesh J. Plant Taxon. 17(1): 105-108. [available online at: http://www.banglajol.info/index.php/BJPT/article/viewFile/5398/4229]
Medina L. (2003) Hydrocotyle. In: Nieto Feliner G. & al. (eds.), Flora iberica, vol. 10. Real Jardín Botánico, CSIC, Madrid: 24-28.
Paffrath K. (1982) Das Pflanzenportrait: Hydrocotyle verticillata Thunberg, 1870. Aqua Planta 7(4): 13-14.
Van der Vlugt P. (1993) Hydrocotyle vulgaris L. und H. verticillata Thunb. Aqua Planta 18 (2): 46-53.
Van Valkenburg J. & Pot R. (2008) Landoltia punctata (G. Mey.) D.H. Les & D.J. Crawford (Smal kroos), nieuw voor Nederland. Gorteria 33: 41-49.
Watson M.F. (1997) Hydrocotyle. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 392.