Humulus japonicus
Humulus japonicus Sieb. et Zucc. (syn.: ?H. scandens (Lour.) Merrill?) (As.) – An exceptional escape from cultivation. Reported as being naturalized in an old hedge in Schaarbeek in 1954 (but most probably gone by now). In 1954 and 1955 also recorded on a dump in Mechelen (Lambinon 1957).
Humulus japonicus much resembles native H. lupulus. It is not unlikely that the former is overlooked, mainly near gardens or on dumps.
Small (1997) defends the use of the binomial Humulus japonicus against H. scandens. According to him the latter binomial is better not accepted as a synonym.
Grudzinskaya (1988) segregated Humulus scandens from Humulus, creating a new monotypic genus, Humulopsis, with the single species Humulopsis scandens (Loureiro) Grudzinskaya.
Humulus japonicus, although a recent newcomer (not mentioned yet, for instance, by Tutin 1964), behaves like an invasive environmental weed in parts of Europe (e.g. in Italy, Hungary,…) (EPPO 2006, Balogh & Dancza 2008, etc.).
Selected literature:
Balogh L. & Dancza I. (2008) Humulus japonicus, an emerging invader in Hungary. In: B. Tokarska-Guzik, J.H. Brock, G. Brundu, L. Child, C.C. Daehler & P. Pyšek (eds.), Plant Invasions: Human perception, ecological impacts and management. Backhuys Publishers, Leiden, The Netherlands: 73-91. [available online at: https://www.researchgate.net/profile/Lajos_Balogh4/publication/296704569...
EPPO (2006) Humulus japonicus: an emerging invader. In: European and Mediterranean Plant Protection Organization (EPPO) Reporting Service 8(174): 20. [available online at: http://archives.eppo.org/EPPOReporting/2006/Rse0608.pdf]
Grudzinskaya I.A. (1988) On the taxonomy of Cannabaceae (in Russian). Bot. Zhurn. 73: 589-593.
Lambinon J. (1957) Contribution à l’étude de la flore adventice de Belgique. I. adventices rares ou nouvelles pour la Belgique. Bull. Soc. Roy. Bot. Belg. 89: 85-100.
Small E. (1997) Cannabaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 3. Oxford University Press, New York-Oxford: 381-387. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=105522]
Tutin T.G. (1964) Humulus. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 1. Cambridge University Press, Cambridge: 67.