Petrorhagia nanteuilii (Burnat) P.W. Ball et Heywood (W-Eur., NW-Afr.) – An exceptional but possibly overlooked alien. Recorded once with certainty in a dry, bare road verge in the port of Antwerp in 1992 (initially taken for Petrorhagia prolifera), not re-located afterwards. Populations of Petrorhagia in this area should be carefully examined.
Petrorhagia nanteuilii is intermediate between P. prolifera and P. dubia (Rafin.) G. López et Romo (syn.: P. velutina (Guss.) P.W. Ball et Heywood) and probably represents an allotetraploid derived through hybridization between these two species. Petrorhagia dubia itself should also be looked for. It looks like Petrorhagia nanteuilii but has still longer leaf sheaths (up to 9 mm long) and is often more or less glandular between the nodes (see Clement 2005 for further details).
See also here: http://alienplantsbelgium.be/content/petrorhagia-prolifera-and-its-look-alikes-belgium-look-out-p-nanteuilii-and-others.
Selected literature:
Beckett G.A. (1992) Childing Pink, a Norfolk plant, Petrorhagia nanteulii (Burnat) P.W. Ball & V. Heywood. B.S.B.I. News 62: 20-21.
Clement E.J. (2005) Petrorhagia dubia established in S. Hants (V.C. 11). BSBI News 100: 46.
Dawson H.J. (1986) Petrorhagia nanteulii (Burnat) P.W.Ball & Heywood in Mid-Glamorgan. Watsonia 16(2): 174-175.
Desmots D. (2007) Découverte de Petrorhagia nanteuilii (Burnat) P.W. Ball & Heywood en Vendée. E.R.I.C.A. 20: 29-32.