1. Geranium carolinianum L. (N-Am.) – A very rare and ephemeral alien. Recorded in 1998, 2011 and 2013 on the unloading quay for cereals at the Amerikadok in the port of Antwerpen (scattered specimens; see Verloove & Vandenberghe 1999). Possibly overlooked.
Much reminiscent of native Geranium pusillum in general appearance (both with pale lilac petals). However, Geranium carolinianum has 10 fertile stamens (versus 5), sepals with a distinct mucro 1,5-2 mm (mucro at most 0,1 mm in G. pusillum), longer fruits (21-25 versus 9-11 mm), a rostrum with a long narrowed apex, blackish mericarps with longer hairs, etc. Confusion is even more likely with native Geranium dissectum, especially in the absence of flowers. Both have deeply divided leaf blades, hairy mericarps, and sepals with a distinct mucro. Geranium carolinianum, however, is larger in every fruit detail (mericarp, rostrum; see key).
Geranium carolinianum is very similar to G. bicknellii. Aedo (2000) distinguished both basically on fruit characters: fruit rostrum with a narrowed apical part 3,5-4,5 mm long in Geranium bicknellii and 1-2 mm long in G. carolinianum. However, in practice, this distinction is often less obvious and the single Belgian collection somehow reminds of G. bicknellii as well. The latter usually is less weedy and has a more restricted geographical distribution. Moreover, in Geranium carolinianum (especially its var. confertiflorum Fernald, which much looks like our collection) the inflorescence is much more tightly clustered. Nonetheless, adventitious records of Geranium carolinianum (for instance from Great-Britain, the Netherlands and Scandinavia; see Verloove & Vandenberghe 1999) should be carefully checked and might represent G. bicknellii as well.
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Selected literature:
Aedo C. (2000) The genus Geranium L. (Geraniaceae) in North America. I. Annual species. Anales Jard. Bot. Madrid 58(1): 39-82.
Fernald M.L. (1935) Geranium carolinianum and allies of Northeastern North America. Rhodora 37: 295-301.
Verloove F. & Vandenberghe C. (1999) Nieuwe en interessante voederadventieven voor de Belgische flora, hoofdzakelijk in 1998. Dumortiera 74: 23-32.