Brassica napus L. (Cult.) – A common but usually ephemeral alien. Widely cultivated as an oilseed crop since long in Belgium. Reported as an escape since the 19th century. Nowadays increasingly cultivated on a large scale. Occurring in abundance along road verges (especially of motorways) from spilled seeds, often in very large, apparently established populations. As such, it is a very conspicuous and very widely dispersed roadside alien that sometimes dominates motorway sidings in April, especially in the vicinity of exit and entry slip roads. Sometimes it is also seen as birdseed alien (on dumps, in urban areas,…).
The exact degree of naturalization (strictly ephemeral or locally more or less naturalized) of Brassica napus in Belgium is uncertain. In several places it occurs each year in quantity as if it were naturalized. However, in the absence of soil disturbance, rapid secondary succession (principally the growth of perennial grasses) tends to lead to local extinction within three years. Given sufficient soil disturbance rape population density appears to be seed limited, and seed spillage can cause a two- to fivefold increase in mean population density (Crawley & Brown 1995).
The usual escape belongs to subsp. napus. Brassica napus is believed to be a hybrid of B. oleracea and B. rapa.
Selected literature:
Cheung W.Y., Champagne G., Hubert N. & Landry B.S. (1997) Comparison of the genetic maps of Brassica napus and Brassica oleracea. Theor. Appl. Genet. 94(5): 569-582. [available online at: http://www.ask-force.org/web/Brassica/Cheung-ComparisonGeneitc-Maps-1997...
Crawley M.J. & Brown S.L. (1995) Seed limitation and dynamics of Feral Oilseed Rape on the M25 motorway. Proc. Roy. Soc. London, Ser. B, Biol. Sci. 259: 49-54.
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]
Hasterok R. & Maluszynska J. (2000) Cytogenetic markers of Brassica napus L. chromosomes. J. Appl. Genet. 41(1): 1-9. [available online at: http://www.wbios.us.edu.pl/hasterok/papers/2000/Hasterok_2000b.pdf]
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.