Bidens bipinnata L. (E As.) - A very rare and ephemeral alien, most often associated with cereals. Observed at the end of the 19th and the beginning of the 20th century, first in Tournai (1885), then in Antwerp (1915). Both specimens were initially identified as Cosmos sulfureus (Duvigneaud 1975). In 2020 a single individual was observed in Charleroi, without obvious vector of introduction.
This species is much reminiscent of B. subalternans and both are often confused. The latter has more numerous achenes and wider ultimate leaf lobes. Another very similar species is B. parviflora Willd., also from Asia and likely to occur as a weed elsewhere in the world. It has leaves like B. subalternans but 4-lobed disk corollas (vs. 5-lobed in the other two species). Its achenes have two awns whereas in B. bipinnata and B. subalternans achenes usually have three or four awns. However, sometimes there are only two, like in the plant recently seen in Charleroi.
Selected references
Duvigneaud J. (1975) Une espèce nouvelle pour la flore belge: Bidens subalternans. Bull. Soc. Roy. Bot. Belg. 108: 203-207.
Sîrbu C. & Oprea A. (2008) New Alien Species for the Flora of Romania: Bidens bipinnata L. (Asteraceae). Turk. J. Bot. 32: 255-258. [available online at: https://journals.tubitak.gov.tr/botany/issues/bot-08-32-3/bot-32-3-7-071...
Van Ooststroom S.J. & Reichgelt Th.J. (1965) Aanwinsten voor de Nederlandse adventief-flora. Gorteria 2: 137-143. [available online at: http://natuurtijdschriften.nl/search?identifier=535899]