Rhamnus L.
Rhamnus counts ca. 70 species that are very widely spread (Mabberley 2008). A single species, R. cathartica L., is native to Belgium (Lambinon & Verloove 2012). Some species are grown as ornamentals and one of these has been observed as an escape from cultivation.
1. Leaves deciduous, at least in part opposite. Shrub usually spiny (native) === Rhamnus cathartica
1. Leaves persistent, all alternate. Shrub without spines ==== R. alaternus
Selected references
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.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.