Egeria densa Planch. (syn.: Elodea densa (Planch.) Caspary) (S-Am.) – A rare escape from cultivation, reported for the first time in 1999 in a small pond in Ezemaal (Robijns & al. 2002). The number of records has increased subsequently and Egeria densa is now fully naturalised in a few locations in Flanders (Denys & al. 2004). The species penetrates in semi-natural habitats, for instance in the valley of the Kleine Nete and Dommelvallei (Peer) and could locally become invasive. Additional information on the incipient invasion of Egeria densa in Belgium and elsewhere in western Europe is available at: http://ias.biodiversity.be/species/show/54 and http://www.q-bank.eu/Plants/Factsheets/Egeria%20densa%20NL.pdf.
Although very closely related to Elodea, this species is usually readily distinguished in the field, even in the vegetative state. Plants are often much more robust with thicker and still longer stems and leaves are longer (somehow reminiscent of those of Elodea nuttallii but much wider and never strongly recurved). It might still pass unrecorded due to confusion with the latter.
A close relative of Egeria densa is also cultivated in western Europe, E. naias Planch. It might have been overlooked. Information on their distinction is available at: http://www.q-bank.eu/Plants/Factsheets/Egeria%20naias%20NL.pdf.
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Selected literature:
Denys L., Packet J. & Van Landuyt W. (2004) Neofyten in het Vlaamse water: signalement van vaste waarden en rijzende sterren. Natuur.focus 3(4): 120-128.
Hussner A. (2006) Die aquatischen Neophyten in Nordrhein-Westfalen. Decheniana 159: 39-50.
Robijns J., Vanhecke L. & Asperges M. (2002) Egeria densa, een valse waterpest met grote bloemen, nieuw voor de Belgische flora. Dumortiera 79: 17-19.
Þórðarson T. (2010) Forekomst av Egeria densa på Island. Blyttia 68(4): 231-244.