Brassica rapa L. (origin obscure) – A rather rare, always ephemeral alien. Known as an escape from cultivation since the 19th century but poorly documented and largely neglected and mis-understood by Belgian botanists. B. rapa is still regularly seen, especially as birdseed alien, for instance in port areas or on dumps.
Several races of Brassica rapa are cultivated and occur as escapes but they are often hardly distinguished (see for instance Stace 2010, Lambinon & Verloove 2012, Tison & de Foucault 2014).
Brassica rapa is known to have naturalized, self-sustaining feral populations in eastern Canada (Gulden & al. 2008).
Selected literature:
Gladis T. & Hammer K. (1992) Die Gaterslebener Brassica Kollektion: Brassica juncea, B. napus, B. nigra und B. rapa. Feddes Repert. 103(7-8): 469-507.
Gulden R.H., Warwick S.I. & Thomas A.G. (2008) The Biology of Canadian Weeds. 137. Brassica napus L. and B. rapa. Can. J. Plan Sci. 88: 951-996. [available online at: http://www.nrcresearchpress.com/doi/pdf/10.4141/CJPS07203]
Lambinon J. & Verloove F. (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.
Oost E.H. (1987) Lectotypification of Brassica rapa L., B. campestris L. and neotypifications of B. chinensis L. (Cruciferae). Taxon 36: 625-634.
Oost E.H. & Toxopeus H. (1986) Scope and problems of cultivar group formation as exemplified in Brassica rapa L. Acta Hort. 182: 117-123. [available online at: https://library.wur.nl/WebQuery/wurpubs/fulltext/29712]
Rich T. (1987) Cabbage patch. B.S.B.I. News 45: 6-8. [available online at: https://archive.bsbi.org.uk/BSBINews45.pdf]
Rich T.C.G. (1988) Brassica rapa L./B. napus L./B. oleracea L. Plant Crib 1988: 23-24.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.
Tison J.-M. & de Foucault B. (coord.) (2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.
Wojtczak A. (2003) Brassica rapa. B.S.B.I. News 92: 51-52. [available online at: https://archive.bsbi.org.uk/BSBINews92.pdf]