Aconogonon xfennicum Reiersen (syn.: Persicaria xfennica (Reiersen) Stace, Koenigia xfennica (Reiersen) T.M. Schuster & Reveal; = A. alpinum (All.) Schur x weyrichii (F. Schmidt) H. Hara) (Cult.) – A very rare, possibly locally naturalized escape from cultivation or garden throw-out. Discovered in 2014 on two occasions: at first on rough ground in Tielt and soon afterwards also in Overpelt.
This artificial hybrid of Aconogonon alpinum x weyrichii parentage closely resembles A. molle (D. Don) H. Hara, a species with which it was initially confused before being described as new to science (Jonsell 1999). The latter is readily distinguished in fruit, its perianth becoming blackish and succulent in fruit. At flowering time their separation is much less straightforward. Contrary to Stace (2010) tepal length does not seem to be a useful feature since in A. xfennicum they can be as less as 2 mm long.
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Selected literature:
Conolly A. P. (1983) What is this Polygonum? Watsonia 14: 450.
Jonsell B. (1999) Additional nomenclatural notes to Flora Nordica (Lycopodiaceae to Polygonaceae). Nordic Journal of Botany 19: 385-387.
Porter H. & Bailey J. (1991) An alien Polygonum hybrid new to the British Isles. B.S.B.I. News 57: 54. [available online at: http://archive.bsbi.org.uk/BSBINews57.pdf ]
Reiersen J. (2000) In: Jonsell B. (ed.), Flora Nordica, vol. 1. The Bergius Foundation, Stockholm: 236-240.
Stace C.A. (2002) A new name in the British Flora: Persicaria x fennica (Reiersen) Stace. Watsonia 24: 109-110. [available online at: http://archive.bsbi.org.uk/Wats24p107.pdf]
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.