14. Amaranthus retroflexus L. (N-Am.) – A common and probably widely naturalised alien. Apparently first documented from the railway station in Gent in 1857. Introduced through a wide variety of vectors. Formerly seen as a wool alien in the valley of river Vesdre but nowadays usually associated with grain importation. Most often seen in port areas (near grain mills, unloading quays,…), on wasteland and dumps, along road verges and railway tracks or in chicken runs. Still frequently introduced and perhaps commoner as an ephemeral introduction than as a fully naturalised alien. Genuinely naturalised populations are probably mostly confined to agricultural fields, although Amaranthus retroflexus is perhaps less frequent as a weed than A. powellii subsp. bouchonii. Dense populations often occur, for instance, in beet fields.
In general habit Amaranthus retroflexus mostly resembles A. powellii subsp. powellii but they are easily distinguished on floral characters (see key). Moreover, Amaranthus retroflexus always has a densely pubescent stem, especially in the upper half.
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Selected literature:
Mandák B. (2011) Population genetic structure of the noxious weed Amaranthus retroflexus in Central Europe. Flora 206: 697-703.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Amaranthus retroflexus. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 132.
Weaver S.E. & McWilliams E.L. (1980) The biology of Canadian weeds: 44. Amaranthus retroflexus L., A. powellii S. Wats. and A. hybridus L. Canad. J. Pl. Sci. 60: 1215-1234.