Ambrosia trifida
3. Ambrosia trifida L. (incl. A. integrifolia Muhlenb. ex Willd., A. aptera DC.) (N-Am.) – A rather rare and always ephemeral alien. Already collected by Lejeune in 1829 but this specimen probably refers to a cultivated individual (in Hortus Leodiensis). From 1896 (Heverlee, Stavelot) onwards recorded from several, widely scattered locations. Exceptionally seen as a wool alien in the Vesdre valley (only in 1919-1920). Most records are associated with cereals and are from mills and granaries, dumps or gardens (from spilt seeds). Sometimes persistent for some years (or perhaps rather repeatedly introduced?) in some locations. Ambrosia trifida is still a rather characteristic grain- and soybean alien.
Ambrosia trifida is a variable species, especially as regards leaf shape. Plants with entire leaves are reminiscent of Cyclachaena xanthiifolia. However, in Ambrosia male and female flowers are in separate heads and leaves are rough to the touch (male and female flowers together in the same heads and leaves more or less velvety in Cyclachaena).
Selected literature:
Bassett I.J. & Crompton C.W. (1982) The biology of Canadian weeds: 55. Ambrosia trifida L. Canad. J. Pl. Sci. 62(4): 1003-1010.
Chauvel B., Rodriguez A., Moreau C., Martinez Q., Billon R. & Fried G. (2016) Développement d’Ambrosia trifida L. en France : connaissances historiques et écologiques en vue d’une éradication de l’espèce. Journal de Botanique 71: 25-38.
Follak S., Dullinger S., Kleinbauer I., Moser D. & Essl F. (2013) Invasion dynamics of three allergenic invasive Asteraceae (Ambrosia trifida, Artemisia annua, Iva xanthiifolia) in central and eastern Europe. Preslia 85: 41-61. [available online at: http://www.preslia.cz/P131Follak.pdf]
Vasic O. (1990) Ambrosia trifida L. 1753 (Asteraceae): new adventive plant in Slovenia (Yugoslavia). Razpr. Slov. Akad. Znan. Utmet. 31: 391-396.