Cannabis sativa L. (C-As.) – A common but strictly ephemeral alien. Known at least since the middle of the 19th century in Belgium (probably much longer but early collections are merely of cultivated plants). Cannabis sativa now frequently occurs on dumps, in port areas (especially near grain mills, unloading quays), along road verges (from spilled seed), in urban areas (for instance where birds are fed), etc.
Belgian collections of Cannabis sativa are fairly uniform although more than 700 cultivars have been described (Mabberley 2008). Up to present, only cultivated hemp (subsp. sativa) has occurred. A weedy race with unresolved taxonomy (var. spontanea Vavilov?, C. ruderalis Janisch.?) is of increasing concern in parts of Central and eastern Europe (see for instance Benécs-Bárdi 2002, Benécs-Bárdi 2003), and possibly elsewhere. It has smaller seeds (2,5-3,5 mm, versus 3,5-5 mm) that are brownish and mottled (uniformly greyish in cultivated hemp). The taxonomic value of this race is probably restricted. However, given its reputation as a troublesome weed of agricultural fields it is worth looking for.
Selected literature:
(see also references under Cannabis)
Benécs-Bárdi G. (2002) Taxonomy and morphology of uncultivated hemp (Cannabis sativa L.) as weed in Hungary. Acta Bot. Hung. 44(1-2): 31-47.
Benécs-Bárdi G. (2003) Distribution, morphology, biology and weed control possibilities of uncultivated hemp (Cannabis sativa L.) as weed in Hungary. Doctoral thesis. [available online at: http://phd.szie.hu/JaDoX_Portlets/documents/document_3162_section_3317.pdf]
Klimko M. (1980) Morphological variability of Cannabis sativa L. Bull. Soc. Amis Sci. Lett. Poznan, D 20: 127-134.
Lee I.J., Iida O., Sekita S., Uchiyama H., Koyama T. & Satake M. (2003) Morphological and chromosomal variations of Cannabis sativa L. analytical studies on C. sativa introduced into Japan: part 1. Makinoa, n.s.2: 45-58.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Merzouki A., Casares Porcel M. & Molero Mesa J. (1996) A morphological achene study of some "species" of Cannabis: a taxonomic approach. Arch. Geobot. 2(1): 65-71.
Small E. & Cronquist A. (1976) A practical and natural taxonomy for Cannabis. Taxon 25: 405-435.