Ludwigia peploides (Kunth) P.H. Raven (S-Am. and southern N-Am.) – A very rare escape from cultivation. Discovered around 1994 in Canal de Roubaix near Leers-Nord (Duvigneaud & al. 1996), along with Lagarosiphon major. Still present and slowly but definitely expanding: by now it has reached the Flemish part of this canal (Spierre-kanaal) near Spierre-Helkijn. Ludwigia peploides has been claimed from several other localities but all proved to be in error for L. hexapetala. However, the contrary proved to be true as well: several different records of L. hexapetala from Assebroek (Brugge: Gemene Weiden) proved to belong to L. peploides. In this area as well, this species seems firmly established. Both species are, indeed, rather similar but easily separated (see key and below).
The Belgian populations of Ludwigia peploides have been ascribed to subsp. montevidensis (Spreng.) P.H. Raven, mainly on behalf of their stem indumentum (flowering stems densely covered with long spreading hairs) (Raven 1963).
This species and Ludwigia hexapetala are often much alike, especially in the vegetative state. Even in flower they have many characters in common and features like pilosity and floral dimensions often seem to be unreliable. According to Dandelot & al. (2005) they are best distinguished on leaf shape (of flowering branches only!) and sepal length. In areas where they occur sympatrically hybrids have not been recorded.
In parts of France, especially in the Mediterranean area, the more recently introduced Ludwigia peploides now increasingly replaces L. hexapetala (Dandelot & al. 2005). Additional information on its (potential) invasive behaviour in western Europe is available here: http://www.q-bank.eu/Plants/BioloMICS.aspx?Table=Plants%20-%20Species&Rec=171&Fields=All.
Ludwigia peploides, Assebroek (Brugge), Gemene Weidebeek, ditch, August 2013, J. Steuperaert
Selected literature:
Barbier A. & Contré E. (1973) Une plante en extension: Ludwigia peploides (Kunth) P.H. Raven: sa présence dans la Vienne. Bull. Soc. Bot. Centre-Ouest n.s. 4: 30-32.
Bou Manobens J. & Font García J. (2016) Situació de Ludwigia peploides (Onagraceae) a Catalunya. Butlletí de la Institució Catalana d’Història Natural 80: 57-58. [available online at : https://www.researchgate.net/publication/305618262_Situacio_de_Ludwigia_...
Dandelot S., Verlaque R., Dutartre A. & Cazaubon A. (2005) Ecological, dynamic and taxonomic problems due to Ludwigia (Onagraceae) in France. Hydrobiologia 551: 1-6.
De Graaf A. (1990) Aquarienpflanzen aus den USA: Ludwigia peploides (Kunth) Raven. Das Aquarium 255(9): 22-24.
Duvigneaud J., Saintenoy-Simon J. & Delmarche C. (1996) Un complément à apporter à la Nouvelle Flore de la Belgique et des Régions voisines: le traitement du genre Ludwigia. Adoxa 10: 9-10.
Raven P.H. (1963) The Old World species of Ludwigia (including Jussiaea), with a synopsis of the genus (Onagraceae). Reinwardtia 6(4): 327-427.