Polycarpon tetraphyllum (L.) L. (W-Eur., Medit., SW-As.) – A rare but much increasing, locally naturalized alien. First collected in 1844 in Theux (without further details). Between 1894 and 1949 regularly observed as a wool alien in the valley of river Vesdre. More or less naturalized as a pavement weed in front of a brewery in Linkebeek, at least between 1944 and 1952. Much increasing since the 1990’s as a weed of urban habitats. Discovered in 1994 in Gent (Brugse Poort) (Verloove 2002) and subsequently in Brussel around 1999 (Lawalrée 2000) and in Brugge since 2006. In 2007 found for the first time in the city of Antwerpen (Kiel). An up-to-date overview of its current distribution in Belgium is available here: http://waarnemingen.be/soort/view/7216. It is possibly still overlooked.
Polycarpon tetraphyllum persists well and has now become fully naturalized. However, it took ca. 10 years before it started spreading in Gent. By 2009 it had become very common in several quarters of the city. A further spread within the aforementioned cities and towards new cities in a very near future is inevitable. Polycarpon tetraphyllum is occasionnally seen in other, non-urban habitats but then probably rather ephemeral (for instance in Lombardzijde, Knokke, etc.).
The exact mode of introduction of Polycarpon tetraphyllum in Belgian urban habitats (and in many other western European cities) remains speculative. It is very regularly introduced in containers with imported olive trees in garden centers (Hoste & al. 2009) and the nursery trade may be an important vector for the introduction and spread of this species in western Europe). It might as well have been introduced by immigrants and tourists.
Polycarpon tetraphyllum grows well in any kind of thermophilous and nitrophilous habitat: foot of walls, cracks in pavement, gravel, etc. It mostly grows along with very banal indigenous species like Capsella bursa-pastoris, Stellaria media, etc.
Polycarpon tetraphyllum is a rather variable species (see Chater & Akeroyd 1993). All Belgian populations are uniform and belong to subsp. tetraphyllum (plant entirely greenish, not purplish near base).
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Selected literature:
Chater A.O. & Akeroyd J.R. (1993) Polycarpon. In: Tutin T.G. & al. (eds.), Flora Europaea, vol. 1 (2nd ed.). Cambridge University Press, Cambridge: 184-185.
De Graaf J.M. (1996) Polycarpon tetraphyllum L. (Kransmuur). Gorteria 22: 147-148.
Höfler J., Meier M. & Reif A. (2016) Vorkommen von Polycarpon tetraphyllum in Baden-Württemberg mit besonderer Berücksichtigung von Freiburg. Mitt. Badischen Landesvereins Naturkunde Naturschutz e.V. 22(1): 59-84.
Hoste I., Verloove F., Nagels C., Andriessen L. & Lambinon J. (2009) De adventievenflora van in België ingevoerde mediterrane containerplanten. Dumortiera 97: 1-16.
Iamonico D. & Domina G. (2015) Nomenclatural notes on the Polycarpon tetraphyllum aggregate (Caryophyllaceae). Plant Biosystems 149(4): 720-727.
Lawalrée A. (2000) Une station de Polycarpon tetraphyllum (L.) L. (Caryophyllaceae) dans l’agglomération bruxelloise. Adoxa 26-27: 1.
MacPherson P. (1991) Persistence for 20 years of Guernsey stowaways in a Glasgow garden. Glasgow Naturalist 22: 90.
Mazomeit J. (2002) Zum Status und zur Ausbreitung von Polycarpon tetraphyllum L. (L.) in Mitteleuropa. Flor. Rundbr. 36: 15-24.
Savelsbergh E. (1997) Polycarpon tetraphyllum (L.) L. (Vierblättriges Nagelkraut) - ein Neuling der Aachener Stadtflora (TK 25 5202/21). Flor. Rundbr. 31(2): 157-158.
Spitaler R. (2005) Erstnachweis von Polycarpon tetraphyllum in Nordtirol (Caryophyllaceae). Berichte des naturwissenschaftlich-medizinischen Vereins in Innsbruck 92: 41-43.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Polycarpon tetraphyllum. 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: 679.
Weddeling K. (2007) Eingebürgert im Bonner Raum - Bromus carinatus Hook. & Arnott, Polycarpon tetraphyllum (L.) L. und Juncus ensifolius Wikstr. Flor. Rundbr. 40: 1-6.