Cymbalaria muralis P. Gaertn., B. Mey. et Scherb. (syn.: Linaria muralis (L.) Miller) (SC Eur.) – A rather commonly naturalized escape from cultivation. Already known in the second half of the 16th century (Van Landuyt 2006). Its expansion, however, is fairly recent (van Rompaey & Delvosalle 1978, Verloove 2002). At present it has become, along with Asplenium ruta-muraria, one of the most widespread and characteristic wall plants in large parts of Belgium. From its secondary distribution range, the Cymbalarietum muralis association has been described, a pioneer community of vertical wall surfaces (e.g. Szczęśniak & Świerkosz 2003).
Although mostly restricted to highly artificial habitats (i.e., walls in urban areas) Cymbalaria muralis is sometimes considered an invasive species or undesirable weed, for instance in Japan (http://www.nies.go.jp/biodiversity/invasive/DB/detail/81470e.html). Its seeds are transported by ants and easily germinate in cracks and fissures. Once established plants are hard to eradicate and become weedy. These days, Cymbalaria muralis is more of a weed rather than a garden ornamental in Belgium.
Cymbalaria muralis is a variable species. The naturalized taxon is glabrous at maturity and belongs to subsp. muralis. A persistently pubescent variant with long pedicels was recorded once in 1900 (subsp. pubescens (C. Presl) D.A. Webb; see Verloove 2006). According to Carnicero & al. (2017) this is best accepted as a separate species, C. pubescens C. Presl. Another hairy variant (subsp. visianii (Kümmerle ex Jav.) D.A. Webb) was thoroughly studied recently by Junghans (2017). Based on various differences concerning flower length, spur shape and length and seed size and number it was raised to species rank, C. visianii (D.A. Webb) T. Junghans. Such plants are known from Amsterdam in the Netherlands (https://waarneming.nl/soort/view/135104) and the British Isles (Wilcox & Ruhsam 2020) and should be looked for in Belgium as well. A peculiar variant with deeply incised leaves is sometimes distinguished as f. toutonii (A. Chev.) Cuf. (syn.: C. toutonii A. Chev.) and was recorded in 2011 in gravel in Queue-du-Bois (see also McClintock 1982, Coombe 1992). Finally, a white-flowered form has also been recorded (cv ‘Alba’). Some of these variants have been given species rank (see for instance Lattanzi & Leporatti 1980, Bigazzi & Raffaelli 2000).
Selected literature:
Bigazzi M. & Raffaelli M. (2000) Taxonomy of two Cymbalaria species (Scrophulariaceae) endemic to Italy: C. glutinosa sp. nov. and C. pubescens (J. and C. Presl) Cuf. Webbia 54(2): 193-205.
Carnicero P., Sáez L., Garcia-Jacas N. & Galbany-Casals M. (2017) Different speciation types meet in a Mediterranean genus: The biogeographic history of Cymbalaria (Plantaginaceae). Taxon 66(2): 393-407. [available online at: http://digital.csic.es/bitstream/10261/150499/3/Different_speciation_Gar...
Coombe D.E. (1992) Cymbalaria muralis forma toutonii. B.S.B.I. News 60: 62. [available online at: http://archive.bsbi.org.uk/BSBINews60.pdf]
Dane F. & Olgun G. (1989) A new record for the flora of European Turkey: Cymbalaria muralis subsp. muralis. Istanbul Univ. Ecz. Fak. Mecm. 25: 37-39.
Feder J. (2002) Das Mauer-Zimbelkraut (Cymbalaria muralis P. GAERTN., B. MEY & SCHREB.) in Ostfriesland. Beiträge zur Vogel- und Insektenwelt Ostfrieslands 186: 10.
Junghans T. (2003) Untersuchungen zur Besiedlung von Mauern durch Cymbalaria muralis. Braunschweig. [available online at: http://www.ruderal-vegetation.de/epub/cymbalar.pdf]
Junghans T. (2004) Zur Diasporenausbreitung von Cymbalaria muralis (Scrophulariaceae). Braunschweig: 19 p. [available online at: http://digisrv-1.biblio.etc.tu-bs.de:8080/docportal/receive/DocPortal_document_00001637]
Junghans T. (2008) Aspects of dispersal in Cymbalaria muralis (Scrophulariaceae). Botanische Jahrbücher für Systematik, Pflanzengeschichte und Pflanzengeographie 127(3): 289-298. [available online at: http://www.schweizerbart.de/papers/pde/detail/127/52904/Aspects_of_dispersal_in_Cymbalaria_muralis_Scrophulariaceae]
Junghans T. (2017) Zur Taxonomie von Cymbalaria muralis unter besonderer Berücksichtigung Blüten-, Kapsel- und Samenmorphologischer Merkmale - mit einer Neukombination von Cymbalaria visianii. Flor. Rundbr. 51: 22-33.
Lattanzi E. & Leporatti G.M.L. (1980) Sulla distribuzione nel Lazio di Cymbalaria pilosa (Jacq.) L. Bailey e di Cymbalaria muralis Gaertner, Meyer & Scherb ssp. visianii D.A. Webb. Ann. Bot. (Italy) 38(1-2): 103-105.
Leporatti M.L., Del Caldo L. & Pavesi A. (1994) Confronto tra Cymbalaria muralis Gaertn., Mey., et Scherb. subsp. muralis e C. muralis subsp. visiani D.A. Webb osservazioni sulla anatomia, morfologia e distribuzione in Italia. Ann. Bot. (Rome) 52 suppl. 2: 479-488.
McClintock D.A. (1982) The story of Cymbalaria toutonii A. Chev. Watsonia 14: 182-183. [available online at: http://archive.bsbi.org.uk/Wats14p177.pdf]
Oberdorfer E. (1969) Zur Soziologie der Cymbalaria-Parietarietea, am Beispiel der Mauerteppich-Gesellschaften Italiens. Vegetatio 17: 208-213.
Szczęśniak E. & Świerkosz K. (2003) Cymbalaria muralis P. Gaertn., B. Mey. & Schreb.and Cymbalarietum muralis Görs 1966 in Lower Silesia - expansion or regression? In: Phytogeographical problems of synanthropic plants: 185-193. [available online at: http://www.academia.edu/2281509/Cymbalaria_muralis_L._and_Cymbalarietum_muralis_Gors_66_in_Lower_Silesia_exctincion_or_regresion]
Truchan M. (2006) Distribution of Cymbalaria muralis P. Gaertn., B. Mey. & Scherz. in the central part of Polish Pomerania. Biodiversity: research and conservation 1-2: 98-101.
Van Landuyt W. (2006) Cymbalaria muralis. 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: 322-323.
Van Rompaey E. & Delvosalle L. (1978) Atlas de la flore belge et luxembourgeoise. Ptéridophytes et Spermatophytes. Commentaires. Jardin Botanique National de Belgique, Meise: 116 p.
Verloove F. (2002) Ingeburgerde plantensoorten in Vlaanderen. Mededeling van het Instituut voor Natuurbehoud n° 20: 227 p.
Verloove F. (2006) Catalogue of neophytes in Belgium (1800-2005). Scripta Botanica Belgica 39: 89 p.
Watt H.B. (1956) Notes on the introduction and distribution of Cymbalaria muralis Gaertn. Mey. & Scherb. in Scotland. Bot. Soc. Brit. Isles Proc. 2: 123-125.
Wilcox M. & Ruhsam M. (2020) The genus Cymbalaria Hill (Toadflaxes) in Britain and the discovery of C. muralis × C. pallida. B.S.B.I. News 143: 13-21. [available online at: https://www.researchgate.net/publication/339141502_The_genus_Cymbalaria_...