3. Petasites japonicus (Sieb. et Zucc.) Maxim. (Japan) – A rare but increasing, locally naturalised alien. First collected in 1989 in Neder-over-Heembeek (marshy part of an old parc). Subsequently recorded since 1994 from a pond margin in woodland (in the vicinity of a castle) in the Veldbos in Ardooie and in similar conditions since 1999 in Heks (Borgloon) (Verloove 2002b). Recently also reported from Sivry-Rance (present at least since 2004; Lecron 2010). Furthermore recorded, for instance, in Hertsberge (few plants in a wood margin, since 2009), in Sint-Denijs-Westrem (a large, well-established population on the verge of marshy woodland, since 2010) and in the Osbroek nature reserve in Aalst since 2012. New locations surely will come to light in a near future and a wider naturalisation is feasible.
In most cases, Petasites japonicus was initially introduced on purpose (as an ornamental) but readily escapes. Some populations obviously originated from garden waste. The usual plant in cultivation (and as an escape) is subsp. giganteus Kitam. (syn.: P. japonicus var. giganteus (F. Schmidt) Nichols, P. amplus Kitam.) with very large leaves (see also Jäger & al. 2008, Lecron 2010).
A hybrid with Petasites fragrans was recently described from Great Britain (Desjardins & al. 2016).
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Selected literature:
Desjardins S.D., Hoare A.G. & Stace C.A. (2016) A new natural hybrid in the genus Petasites: P. japonicus × P. pyrenaicus (Asteraceae). New Journal of Botany 6(2-3): 64-70. [available online at: http://www.tandfonline.com/doi/full/10.1080/20423489.2016.1271383]
Grenfell A.L. (1984) Adventive News 29. BSBI News 38: 13-18.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Kloos A.W. Jr. (1950) Aanwinsten voor de Nederlandse flora in 1945, 1946 en 1947. Nederl. Kruidk. Arch. 57: 199-243.
Lecron J.-M. (2010) Petasites japonicus en voie de naturalisation dans le sud de la province de Hainaut (Belgique). Dumortiera 98: 13-22.
MacPherson P. (1973) Petasites japonicus. Glasgow Naturalist 19: 67.
Verloove F. (2002b) Petasites japonica (sic), ook in België. Dumortiera 79: 21.