The systematics of the Linaceae family are still uncertain. Recent molecular phylogenetic research places the native genus Radiola in a broadly circumscribed genus Linum. Linum catharticum as well as the yellow-flowered flaxes have been shown to be closer to Radiola than they are to the remainder of Linum (McDill & al. 2009 a,b). Pending further studies Radiola and Linum are here treated in their traditional sense.
Two genera are known from Belgium, with native as well as non-native representatives (Lambinon & Verloove 2012).
1 Flowers 5-merous. Petals distinctly exceeding the calyx (at least twice as long). Sepals entire. Leaves alternate or opposite === Linum
Flowers 4-merous. Petals not exceeding the calyx. Sepals with 2-4 distinct teeth at apex. Leaves opposite (native) === Radiola
Lambinon J. & Verloove F. (avec coll. Delvosalle L., Toussaint B., Geerinck D., Hoste I., Van Rossum F., Cornier B., Schumacker R., Vanderpoorten A. & Vannerom H.) (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
McDill J.R. (2009a) Molecular phylogenetic studies in the Linaceae and Linum, with implications for their systematics and historical biogeography. PhD Dissertation Texas University. Available online at: http://repositories.lib.utexas.edu/handle/2152/ETD-UT-2009-12-657.
McDill J.R. (2009b) The Phylogeny of Linum and Linaceae Subfamily Linoideae, with Implications for Their Systematics, Biogeography, and Evolution of Heterostyly Syst. Bot. 34(2): 386-405. [available online at: https://www.researchgate.net/publication/235443463_The_Phylogeny_of_Linum_and_Linaceae_Subfamily_Linoideae_with_Implications_for_Their_Systematics_Biogeography_and_Evolution_of_Heterostyly]