Azolla is a small, taxonomically difficult genus from warm-temperate and subtropical zones. About seven species are usually recognised, including three New World species, but some are probably better lumped. Evrard & Van Hove (2004) recently merged Azolla caroliniana Willd. and A. filiculoides. The exact origin of the Belgian populations remains uncertain. It was probably initially introduced as an ornamental.
In addition to Azolla filiculoides a second species might be present in Belgium, A. cristata. Its historical presence was confirmed from Italy (Lastrucci & al. 2019; see also Brusa & Bona 2019).
Literature
Brusa G. & Bona E. (2019) Occurrence of a species in the group Azolla caroliniana/A. cristata (Salviniaceae, Pteridophyta) recently discovered to be spontaneous in Italy. Natura Bresciana. Ann. Mus. Civ. Sc. Nat. Brescia 42: 57-62. [available online at: https://www.researchgate.net/publication/339123358_Occurrence_of_a_speci...
Evrard C. & Van Hove C. (2004) Taxonomy of the American Azolla species (Azollaceae): a critical review. Syst. Geogr. Pl. 74: 301-318.
Lastrucci L., Fiorini G., Lunardi L. & Viciani D. (2019) Herbarium survey on the genus Azolla (Salviniaceae) in Italy: distributive and taxonomic implications. Pl. Biosyst. DOI: https://doi.org/10.1080/11263504.2018.1549601
Lumpkin T.A. (1993) Azollaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 2. Oxford University Press, New York-Oxford: 338-342.
Sotiaux A. (1979) Contribution à une meilleure connaissance du genre Azolla dans le territoire de la Nouvelle Flore. Dumortiera 13: 1-5.
Zimmerman W.J., Lumpkin T.A. & Watanabe I. (1989) Classification of Azolla spp., section Azolla. Euphytica 43: 223-232.