1. Myriophyllum aquaticum (Velloso) Verdc. (syn.: M. brasiliense Camb.) (S-Am.) – A rather rare but much increasing, locally naturalised escape from or relic of cultivation. Collected in Louvain-la-Neuve in 1983 (Scavée du Biéreau) but obviously introduced intentionally. Perhaps first recorded in the wild in 1984 in the nature reserve Osbroek in Aalst (Verloove 2002) and in 1987 in Kasterlee (Oude Hofloop) (as Myriophyllum spec.). However, the spread and naturalisation of Myriophyllum aquaticum only started around 1995 when several new localities came to light, mainly in Flanders: Genk, Lanaye and Uikhoven, soon afterwards (1999) also in Ieper, Jonkershove, Kerkhove, Kortrijk and Willebroek (Verloove & Vanhecke 2000). From then on reported from numerous new localities (Verloove 2002) and now present in large parts of Flanders (but usually rather scattered; see Verloove 2006). Myriophyllum aquaticum now increasingly grows in nature reserves (for instance in Langdonken in Aarschot) and in natural, vulnerable water courses (for instance Kleine Nete).
It is probably intentionally introduced (as an ornamental or from discarded garden debris) in most of its initial localities. Myriophyllum aquaticum is very competitive and invasive, especially in moderately disturbed (poluated) water bodies. Despite its South American origin it seems to be well-adapted to frostbite: it apparently easily survived the cold winters of medio 1980’s in the nature reserve Osbroek in Aalst (Verloove 2002) and the same holds true for the cold winters of 2008-2009 and 2009-2010. Mechanical removal of a large population of Myriophyllum aquaticum in a canal in Ieper initially led to a massive proliferation (Verloove 2002). However, more recently this population was succesfully destroyed after that the canal was drained. It accordingly seems that Myriophyllum aquaticum easily survives cold winters only when submerged.
Along with Hydrocotyle ranunculoides and Ludwigia grandiflora, Myriophyllum aquaticum is among the most noxious aquatic weeds in Belgium (and western Europe). Its availability in plant nurseries should be regulated (or forbidden). More information (and additional useful references) on the invasive behaviour of Myriophyllum aquaticum in Belgium and elsewhere in western Europe is available at http://ias.biodiversity.be/species/show/76 and http://www.q-bank.eu/Plants/Factsheets/Myriophyllum%20aquaticum%20NL.pdf.
With its emergent (even in the non-flowering phase), glaucous foliage Myriophyllum aquaticum cannot be confused with other species of this genus. In the herbarium (when colours and habit are unverifiable) the presence of numerous sessile glands on the leaves is very stricking.
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Selected literature:
Ebner L. (2007) Myriophyllum aquaticum in der Oberpfalz. Hoppea 68: 111-114.
Hussner A. (2016) Zur Biologie Invasiver Aquatischer Neophyten: Myriophyllum aquaticum. Florist. Rundbr. 50: 84-97. [available online at: https://www.researchgate.net/publication/313653456_ZUR_BIOLOGIE_INVASIVE...
Salanon R. (2000) Trois xénophytes envahissantes des cours d'eau, récemment observées dans les Alpes-Maritimes (France): Myriophyllum aquaticum (Velloso) Verdcourt, Ludwigia grandiflora (Michaux) Greuter et Murdet et Sagittaria latifolia Willd. Biocosme Mésogéen 16(3): 125-145.
Teles A.N. & Pinto da Silva A.R. (1975) A “pinheirinha” (Myriophyllum aquaticum (Vell.) Verdc.), uma agressiva infestante aquatica. Agron. Trop. 36(3): 307-323.
Van der Ham R.W.J. (1984) Myriophyllum aquaticum (Vell.) Verdc. verwilderd gevonden bij Moordrecht. Gorteria 12(1): 14-15.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Myriophyllum aquaticum. 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: 605.
Verloove F. & Vanhecke L. (2000) Myriophyllum aquaticum: expansieve waterneofiet in Vlaanderen. Dumortiera 76: 28-29.