Bidens L.
Bidens is a rather large genus of 150-250 species (ca. 200 according to Mabberley 2008). It is nearly cosmopolitan but by far most species are confined to (sub-) tropical and warm-temperate regions of the New World (with its highest species diversity in Mexico). Two species are native in Belgium: B. tripartita L. and B. cernua L. The generic limits of Bidens are still somewhat unclear. Many morphological peculiarities are shared with the genera Coreopsis, Cosmos and others (see also Tadesse & al. 1995). Based on molecular studies these genera were recently merged, all species being transferred to Bidens, the genus that has nomenclatural priority (Banfi & al. 2018).
An accurate identification usually requires mature achenes. Most other diagnostic features (for instance leaf shape) are exceedingly variable.
1 Leaves pinnatisect. Achenes linear, fusiform === 2
1 Leaves entire to pinnate. Achenes narrowly oblong and angled to strongly flattened, rarely fusiform (in B. pilosa) === 4
2 Ray-flowers always present, conspicuous, ca. 15-30 mm long. Trailing perennial (but cultivated as an annual) === Bidens triplinervia var. macrantha
2 Ray flowers absent or inconspicuous, (0-) 2-8 mm long. Erect annual === 3
3 Achenes few (less than 20, often much less). Leaves with ultimate lobes obovate or lanceolate === B. bipinnata
3 Achenes more numerous, usually at least 20. Leaves with ultimate lobes narrower, lanceolate-acuminate === B. subalternans
4 Achenes linear, fusiform. Ray flowers, if present, white === B. pilosa
4 Achenes never linear-fusiform (narrowly oblong and angled to strongly flattened). Ray-flowers usually absent (if present: yellowish) === 5
5 At least lower leaves pinnate === 6
5 Lower leaves entire to lobed, never pinnate === 7
6 Outer involucral bracts (calyculi) 5-10 (usually ca. 8) === B. frondosa
6 Outer involucral bracts 10-16 === B. vulgata
7 Inner achenes at maturity with 4 +/- equal faces (angled), scarcely flattened, faces warty === B. connata
7 Inner achenes at maturity flattened, much wider than thick, faces usually smooth === 8
8 Heads usually nodding at maturity. Ray-flowers often present, conspicuous. Leaves not lobed (native) === B. cernua
8 Heads always erect at maturity. Ray flowers absent or inconspicuous. Leaves usually lobed, sometimes entire === 9
9 Achenes ca. 3-5 mm, always with 2 bristles. Heads much wider than long at maturity. Tubular florets very numerous (100-240). Outer involucral bracts usually ca. 9-12 === B. radiata
9 Achenes ca. 5-8,5 (-11) mm, with 2-4 bristles. Heads about as wide as long at maturity. Tubular florets less numerous (ca. 20-60). Outer involucral bracts usually ca. 5-8 (native) === B. tripartita
Literature:
Banfi E., Galasso G. & Bartolucci F. (2018) Nomenclatural novelties for the Euro+Med flora. Nat. Hist. Sc. 5(1): 53-57. [available online at: http://sisn.pagepress.org/index.php/nhs/article/view/365]
Burton R.M. (1979a) Bidens in Britain. London Naturalist 58: 9-14.
Debray M. (1963) Les espèces du genre Bidens introduites en France. Morphologie et géographie. Cah. Nat., Bull. N. P. 19: 33-50.
Debray M. (1964) Clé de détermination des espèces du genre Bidens connue en Belgique et dans les régions voisines. Nat. Mosana 17 : 51-52.
Debray M. (1968) Les espèces du genre Bidens introduites en France. Rectifications et nouvelles données. Cah. Nat., Bull. N. P. 24: 53-58.
Dress W.J. (1965) Notes on the cultivated Compositae, 8. Thelesperma, Bidens, Coreopsis. Baileya 13: 20-42.
Hipken C. (1990) Bidens (Bur-marigolds) in Glamorgan. BSBI News 56: 12-15.
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.
Jovet P. (1983) A propos d'une étude de Marcel Debray sur les Bidens naturalisés en France. Compt. Rend. Séances Soc. Biogéogr. 59: 71-75.
Lambinon J., Delvosalle L., Duvigneaud J. (avec coll. Geerinck D., Lebeau J., Schumacker R. & Vannerom H. (2004) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Cinquième édition. Jardin botanique national de Belgique, Meise: CXXX + 1167 p.
Lhotská M. (1968) Die Gattung Bidens L. in der Tschechoslowakei. Folia Geobot. Phytotax. 3: 65-98.
Mabberley D.J. (2008) Mabberley’s plant-book (3th ed.). Cambridge University Press, Cambridge: XVIII + 1021 p.
Schumacher A. (1941) Die fremden Bidens-Arten in Mitteleuropa. Feddes Repert., Beiheft 131: 42-93.
Sherff E.E. (1937) The genus Bidens. Publ. Field Mus. Nat. Hist., Bot. Ser. 16: 709 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.
Tadesse M. (1993) An account of Bidens (Compositae: Heliantheae) for Africa. Kew Bull. 48: 437-516.
Tadesse M., Crawford D.J. & Smith E.B. (1995) Comparative capitular morphology and anatomy of Coreopsis L. and Bidens L. (Compositae), including a review of generic boundaries. Brittonia 47: 61-91.