Morus nigra  

Morus nigra L. (As.) – A regularly planted ornamental tree, although possibly less frequently so nowadays. Rarely escaping but probably more often than M. alba. In the past decades seedlings have been recorded in several localities, for instance in Balen, Gent, Hoboken, etc. These, however, rarely grow tall. M. nigra has also been recorded as an unintentional introduction in olive tree containers in plant nurseries.

According to Tison & de Foucault (2014) M. nigra is not able to reproduce autonomously (contrary to M. alba). In several localities in Belgium, however, seedlings have been recorded. This may point at misidentifications. Asian M. nigra and American M. rubra are morphologically very similar and both are grown as ornamental trees in Europe. Perhaps some (or most?) of the recent claims of M. nigra in the wild in Belgium in fact refer to M. rubra. The latter tends to be a larger tree with more densely hairy leaves and slightly longer male catkins (Nelson 1997) but is otherwise poorly seperated.  
Pending further studies the Belgian plants with rough upper- and persistently hairy lower leaf surfaces are here referred to as M. nigra. Some claims, however, especially of seedlings, may refer to M. rubra.

Selected references

Andreas A. (2013) Zum Vorkommen von Maulbeeren (Morus sp., Moraceae) in Nordwest-Sachsen. Mitteilungen der Deutschen Dendrologischen Gesellschaft 98: 205-213.

Browicz K. (2001) Where is the place of origin of Morus nigra (Moraceae)? Fragm. Flor. Geobot. 45(1-2): 273-280.

Nelson E.C. (1997) Morus. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 3. Cambridge University Press, Cambridge: 86-87.

Roloff A. & Bärtels A. (2006) Flora der Gehölze (2e Auflage). Ulmer, Stuttgart: 844 p.

Tison J.-M. & de Foucault B. (coord.) (2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.