3. Bidens pilosa L. (incl. B. alba (L.) DC., B. odorata Cav.) (S-Am.) – A rare and always ephemeral alien. Apparently first recorded in 1903 (without further details). More or less regularly observed as a wool alien in the Vesdre valley, at least between 1920 and 1967 (its achenes with barbed awns evidently easily adhere in sheep wool). More recently always associated with grain and oilseed importation (probably chiefly soybeans): in 1992 recorded in Deinze in a plantation near a grain mill and since 1993 regularly seen at the Ghent Grain Terminal in the port of Gent (most recently probably in 2007). In 2015 also seen by a roadside near a grain mill in Izegem.
Bidens pilosa is a rather variable species and possibly rather represents a complex of several closely related micro-species (see Ballard 1986). However, recent authors tend to treat it in a broader way (Strother & Weedon 2006). In Belgium Bidens pilosa very rarely reaches maturity and the application of names for infraspecific taxa is difficult. Most mature plants seen only have tubular florets and belong to var. pilosa. Plants with outer florets ligulate (ligules white) belong with var. radiata Schultz-Bip.
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Selected literature:
Ballard R. (1986) Bidens pilosa complex (Asteraceae) in North and Central America. Amer. J. Bot. 73: 1452-1465.
Strother J.L. & Weedon R.R. (2006) Bidens. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 21. Oxford University Press, New York-Oxford: 205-218.
Verloove F. & Vandenberghe C. (1993) Nieuwe en interessante graanadventieven voor de Noordvlaamse en Noordfranse flora, hoofdzakelijk in 1992. Dumortiera 53-54: 35-57.