8. Geranium purpureum Vill. (syn.: G. robertianum subsp. purpureum (Vill.) Nyman) (SW and S-Eur., SW-As., N-Afr., Macaronesia) – A rather rare but much increasing, naturalized alien. Apparently first collected in 1967 in Jamioulx (on rubble) but its genuine naturalization started in the 1990's when it was recorded in 1994 on a disused railway yard near Lanaken. Soon afterwards, in 1997, also seen on a dry slope along river Meuse in Anseremme (Duvigneaud & Saintenoy-Simon 1998). These early records in the eastern part of Belgium suggest an introduction via Germany. However, subsequently also observed in various localities in western and southern Belgium (possibly as a result of a range extension from France) (Verloove 2000). By now known from several localities but surely still overlooked (under-recorded in the map of Flanders provided by Van Landuyt 2006). In Wallonia fast spreading by railway tracks and now a commonly naturalized xenophyte in the vicinity of the larger cities (for instance Charleroi, Liège, …). Geranium purpureum mostly grows on gravelly, well-drained, sunny substrates. However, it has also been recorded in urban habitats (pavement weed), on waste land (Vannerom 2001) or in more natural ones (forest margins, coastal dunes).
Geranium purpureum is still imperfectly known in Belgium. It is usually readily distinguished from Geranium robertianum by its smaller petals and yellow anthers. In autumn, flowers of the latter are often smaller and then both should be distinguished on fruit characters (see key).
This species and G. robertianum rarely hybridize and then form sterile, triploid hybrids. In western Germany (and doubtlessly elsewhere), however, introgression between both species seems to occur recently: two more or less intermediate, fertile diploid (micro-) species were described: G. urbanum Bomble and G. alboroseum Bomble (Bomble 2016). See there for identification keys (incl. sterile hybrids) and numerous illustrations.
Selected literature:
Bomble F.W. (2016) Zwischen Geranium purpureum und Geranium robertianum vermittelnde Sippen in Aachen. Veröff. Bochumer Bot. Ver. 8(8): 76-99. [available online at: http://www.botanik-bochum.de/publ/OVBBV8_8_Bomble_Geranium_robertianum_G...
Duvigneaud J. & Saintenoy-Simon J. (1998) Présence en Belgique de Geranium purpureum Vill. (Syn.: G. robertianum subsp. purpureum (Vill.) Nyman). Adoxa 18: 10.
Eliáš jun. P. (2011) Geranium purpureum Vill. – new alien species to the Slovak flora. Thaiszia – J. Bot. 21: 21-28. [available online at: http://www.upjs.sk/public/media/6567/021-028-elias-jun-upr.pdf]
Hügin G., Mazomeit J. & Wolff P. (1995) Geranium purpureum ein weit verbreiter Neophyt auf Eisenbahnschotter in Südwestdeutschland. Flor. Rundbr. 29: 37-41.
Lang W. (2003) Zur Verbreitung und Vergesellschaftung des Purpur-Storchschnabels (Geranium purpureum Vill.) in der Pfalz und angrenzenden Gebieten. Ber. Bot. Arbeitsgem. Südwestdeutschland 2: 79-86. [available online at: http://www.botanik-sw.de/BAS/media/texte/Bericht_2.pdf
Vannerom H. (2001) Geranium purpureum ook in het Kempens district. Dumortiera 78: 28.
Verloove F. (2000) Geranium purpureum Vill., ook in Vlaanderen. Dumortiera 76: 18-20.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Van Landuyt W. (2006) Geranium purpureum. In: Van Landuyt W., Hoste I., Vanhecke L., Van den Bremt P., Vercruysse W. & De Beer D., Atlas van de flora van Vlaanderen en het Brussels gewest. Instituut voor Natuur- en Bosonderzoek, Nationale Plantentuin van België en Flo.Wer: 437-438.