The two genera in this account, both non-natives, are treated in their traditional circumscription. Molecular data have shown that Eichhornia currently is paraphyletic, Pontederia L. and Monochoria C. Presl being embedded in it (Graham et al. 1998; see also Ness et al. 2011). Pellegrini & al. (2018) recently proposed a new circumscription of these genera; in their concept Pontederia includes Monochoria and Eichhornia and counts 26 species.
1 Leaves ovate, up to 8 cm long, leaf stalk inflated (markedly swollen). Plant very rarely (never?) flowering outdoors in western Europe === Eichhornia
Leaves triangular to narrowly ovate, up to 25 cm long, leaf stalks long, never swollen. Plant normally flowering in western Europe === Pontederia
Eckenwalder J.E. & Barrett S.C.H. (1986) Phylogenetic systematics of Pontederiaceae. Syst. Bot. 11: 373-391. [available online at: http://labs.eeb.utoronto.ca/barrett/pdf/schb_49.pdf]
Graham S.W., Kohn J.R., Morton B.R., Eckenwalder J.E. & Barrett S.C.H. (1998) Phylogenetic congruence and discordance among one morphological and three molecular data sets from Pontederiaceae. Syst. Biol. 47: 545-567.
Nelson E.C. (1986) Pontederiaceae. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1. Cambridge University Press, Cambridge: 331-332.
Ness R.W., Graham S.W. & Barrett S.C.H. (2011) Reconciling Gene and Genome Duplication Events: Using Multiple Nuclear Gene Families to Infer the Phylogeny of the Aquatic Plant Family Pontederiaceae. Mol. Biol. Evol. 28(11): 3009-3018. [available online at: http://mbe.oxfordjournals.org/content/28/11/3009.full.pdf+html]
Pellegrini M.O., Horn C.N. & Almeida R.F. (2018) Total evidence phylogeny of Pontederiaceae (Commelinales) sheds light on the necessity of its recircumscription and synopsis of Pontederia L. PhytoKeys 108: 25-83. [available online at: https://phytokeys.pensoft.net/article/27652/list/9/]