2. Bidens frondosa L. (syn.: B. melanocarpa Wiegand) (N-Am.) – A commonly naturalised and much increasing alien. Probably first recorded on wasteland in Tournai in 1886 but doubtlessly ephemeral. The genuine (local) naturalisation of Bidens frondosa apparently started in the 1940’s. Initially it was nearly restricted to the Kempen and some of the larger river valleys, especially those of rivers Dijle, Meuse and Schelde and totally absent in large parts of Wallonia (see map in van Rompaey & Delvosalle 1979). In the past decades much extending and by now common in large parts of Flanders and Wallonia (see map in Verloove 2006b).
Bidens frondosa grows in a wide range of (at least temporarily) damp, usually nitrophilous habitats: canal banks, ditches, pond margins or marshes. It is also found in man-made environments like urban waste ground, dumps, unloading quays in ports (often near grain mills), demolition sites, etc.
Bidens frondosa has much extended its Belgian distribution range in recent times and has displaced native B. tripartita in many localities. It is now often more common than the latter.
Two more or less distinct varieties have been recorded, var. frondosa and var. anomala Porter ex Fernald. Loos & al. (2003) suggested specific rank for the later taxon (Bidens anomala (Porter ex Fernald) Loos, Keil et Hentsch) while Strother & Weedon (2006) simply include it in the variability of Bidens frondosa. Both are distinguished in the following couplet:
- Achenes (incl. awns) with all barbes antrorse === var. anomala
- Achenes with barbes antrorse on the margins of the body of the achene but retrorse on the awns of the achene === var. frondosa
In Belgium var. frondosa is by far the commonest variety but var. anomala is possibly overlooked. The latter was probably first collected in Oisquercq (Tubize) in 1959 (canal bank). Van Damme (1980a,b) drew the attention to its massive presence alongside the canal Brussel-Charleroi. Elsewhere in Belgium var. anomala seems to be rather rare and unevenly distributed (although perhaps overlooked). Records are available from river Maas-Meuse (Dilsen-Stokkem, Lives-sur-Meuse), river Sambre (Landelies) and the valley of river Dender (Denderleeuw, Herdersem). Furthermore also recorded in Merksem (Albertkanaal) in 1994.
Bidens frondosa is still frequently confused with B. tripartita although both are easily distinguished on leaf and achene characters. Confusion with Bidens vulgata is more likely (see below). The very similar North American Bidens discoidea (Torr. et A. Gray) Britton might have been overlooked as well. It is characterized by fewer outer involucral bracts (often only 4) that are not ciliate, smaller heads (ca. 4-7 x 4-9 mm) and achene margins that are entirely antrorsely barbed (like in Bidens frondosa var. anomala). Previous European claims for Bidens discoidea (see Debray 1968) may have been erroneous as a result of confusion with the latter. All Belgian collections seen of var. anomala have distinctly ciliate involucral bracts and their identity is unequivocal.
Flowering heads of Bidens frondosa are nearly always discoid but a form with small but distinct ligules has been recorded, for instance in Balen in 2010 (see: http://waarnemingen.be/waarneming/view/49721121).
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Selected literature:
Brändel M. (2004) Dormancy and germination of heteromorphic achenes of Bidens frondosa. Flora 199: 228-233.
Debray M. (1968) Les espèces du genre Bidens introduites en France. Rectifications et nouvelles données. Cah. Nat., Bull. N. P. 24: 53-58.
Hejný S. & Lhotská M. (1964) Zu der Art der Ausbreitung von Bidens frondosa L. in die Teichgebiete der Tschechoslowakei. Preslia 36: 416-421.
Keil P. (1995) Häufigkeit und Verbreitung von Bidens frondosa var. anomala Porter ex Fernald 1903 am Beispiel des unteren Ruhrtales im westlichen Ruhrgebiet. Flor. Rundbr. 29(2): 129-134.
Keil P. (1999) Ökologie der gewässerbegleitenden Agriophyten Angelica archangelica ssp. Litoralis, Bidens frondosa und Rorippa austriaca im Ruhrgebiet. Diss. Bot. 321: 186 p.
Klotz J. (2009) Die Verbreitung von Bidens frondosa bei Regensburg. Hoppea 70: 105-118.
Köck U.-V. (1988) Ökologische Aspekte der Ausbreitung von Bidens frondosa L. in Mitteleuropa. Verdrängt er Bidens tripartita L.? Flora 180: 177-190.
Lhotská M. (1966) Der älteste Fund der Art Bidens frondosa L. in Europa. Folia Geobot. Phytotax. 1: 186-189.
Loos G.H., Keil P. & Hentsch M. (2003) Bidens frondosa anomala – Varietät, Form oder Art? Flor. Rundbr. 37: 95-101.
Ludwig W. (1994) Bidens frondosa var. anomala in Hessen. Hess. Flor. Briefe 43(2): 17-19.
Nehon J. (1956) Remarques sur deux Bidens (Composées) d'origine américaine: B. frondosa L. et B. connata Mühl. Bull. Soc. Sci. Bretagne 30: 49-57.
Sluschny H. (2005) Bidens frondosa var. anomala und Bidens connata var. anomala – zwei bisher in Mecklenburg-Vorpommern unbeachtete Zweizahn-Sippen. Bot. Rundbr. Mecklenburg-Vorpommern 40: 137-140.
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.
Sumberova K., Tzonev R. & Vladimirov V. (2004) Bidens frondosa (Asteraceae) - a new alien species for the Bulgarian flora. Phytologia Balcanica 10: 179-181.
Van Damme Ch. (1980a) Bidens frondosa L. var. anomala Port. ex Fern. (Asteraceae) en Belgique. Dumortiera 16: 13-15.
Van Damme Ch. (1980b) Données concernant la répartition de Bidens frondosa L. var. anomala Port. ex Fern. (Asteracea) en Belgique. Dumortiera 17: 17-18.
Van Rompaey E. & Delvosalle L. (1979) Atlas de la flore belge et luxembourgeoise. Ptéridophytes et Spermatophytes (2ième éd.). Jardin Botanique National de Belgique, Meise: 1542 cartes.
Verloove F. (2006b) Bidens frondosa. 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: 185.