Ziziphus Mill.
Ziziphus counts ca. 170 species in tropical and warm-temperate regions of the world (Nesom 2016). A few are cultivated as ornamentals in Europe (McKean 1997), mostly in southern Europe, since all species are frost-sensitive. One of these, Z. lotus (L.) Lam., has been recorded as an escape from cultivation in Belgium.
A second species, Ziziphus jujuba Mill. (a native of China), is much grown in the Mediterranean and is naturalizing in southern Europe (France, Spain; Herrero 2013, Tison & de Foucault 2014). It is a much taller shrub with larger leaves.
Ziziphus is rather heterogeneous and possibly not monophyletic. Old World species are more closely related to Paliurus than to New World species (Mabberley 2008). Several species are economically important (for timber, edible fruits or as ornamentals). Three species are cultivated in Europe (McKean 1997).
Literature:
Herrero, A. (2013) Ziziphus L. In: Castroviejo S. & al. (Eds.), Flora iberica, vol. 9. Real Jardín Botánico, CSIC, Madrid. [available online at: http://www.floraiberica.es/floraiberica/texto/imprenta/tomoIX/09_109_01_...
Islam M.B. & Simmons M.P. (2006) A thorny dilemma: Testing alternative intrageneric classifications within Ziziphus (Rhamnaceae). Syst. Bot. 31: 826-842.
Johnston M.C. (1963) The species of Ziziphus indigenous to United States and Mexico. Amer. J. Bot. 50: 1020-1027.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
McKean D.R. (1997) Ziziphus. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 5. Cambridge University Press, Cambridge: 189-190.
Nesom G. (2016) Ziziphus. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 12. Oxford University Press, New York-Oxford. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=135333]
Tison J.-M. & de Foucault B. (coord.) 2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.