6. Bidens triplinervia Humb., Bonpl. et Kunth var. macrantha (Wedd.) Sherff (syn.: B. ferulifolia auct. hort. p.p., non (Jacq.) DC.) (C and S-Am.) – A rare and probably always ephemeral alien (but sometimes persisting for some years). This taxon has become a very popular ornamental plant in recent times, especially for the decoration of hanging baskets. It easily self-seeds and off-spring is regularly found in cracks of pavement, foot of walls,… or as a weed in nurseries. It was probably first collected on a dump in Roeselare-Rumbeke in 2000 and subsequently recorded in several cities. In 2010 also seen on a gravelly river bank of river Maas near Lanaken.
Some plants do not mature and probably belong to a sterile hybrid of horticultural origin (possibly also involving genuine Bidens ferulifolia). The latter is an annual with more or less angular stems and glabrous inner phyllaries that are 2-3 times as wide as the outer phyllaries. In Bidens triplinervia inner phyllaries are sparsely hairy and but slightly wider than the outer (McVaugh 1984; see also Hart 1979, Jäger & al. 2008). It is surely also cultivated. It has been claimed from the British Isles (Clement 2001) and was seen also as an urban weed in Dunkerque in France (pers. obs.). However, Bidens triplinervia (or hybrids or cultivars of it) appears to be the usual garden plant in Belgium. Plants in cultivation have deeply pinnately divided leaves and are distinguished as var. macrantha.
Bidens triplinervia and B. ferulifolia are widely confused with B. aurea (Ait.) Sherff in horticulture but, surprisingly, also by Strother & Weedon (2006). The latter is an erect perennial with entire leaves and is not even remotely related.
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Selected literature:
Clement E.J. (2001) Bidens ferulifolia is in town. BSBI News 86: 48-49 (+ frontcover).
Hart C.R. (1979) The systematics of the Bidens ferulaefolia complex (Compositae). Syst. Bot. 4: 130-147.
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
McVaugh R. (1984) Compositae. In: Anderson W.R. (ed.), Flora Novo-Galiciana, vol. 12. The University of Michigan Press, Ann Arbor: 1157 p.
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.