Heuchera L.
Heuchera is a New World genus and counts ca. 45 species (Folk 2013, Folk & Alexander 2015). All are confined to North America and Mexico, with many species being endemic to the U.S.A. The genus is notoriously complex taxonomically, mostly as a result of morphological intergradation and possibly hybridization. Moreover, character states, especially in flower and fruit, vary during development (Fortson Wells & Greene Shipes l.c.). A fortiori, this also applies to plants grown for ornament: in addition to ca. 10 species numerous cultivars and hybrids are offered for sale in the horticultural trade (e.g. Miller 1995, Jäger & al. 2008) and their separation is sometimes problematic. Two of the more popular ones have been recorded as escapes from cultivation in Belgium, mostly in cracks of concrete, foot of walls, pavements, etc. in urban areas, usually in the vicinity of planted individuals. Their names may be merely tentative; assessing their correct identity requires further study.
1 Stamens and style included. Hypanthium dark pink to red === Heuchera sanguinea
Stamens and style exserted. Hypanthium greenish white (but often tinged with red or pink in cultivars!) === H. micrantha
Literature
Folk R. (2013) Heuchera lakelae (Saxifragaceae), a new species from the Sierra La Marta and Sierra Coahuilón, Coahuila and Nuevo León, Mexico. Phytotaxa 124(1): 37-42. [available online at: http://www.mapress.com/phytotaxa/content/2013/f/pt00124p042.pdf]
Folk R. & Alexander P.J. (2015) Two New Species, Heuchera soltisii and H. inconstans, with Further Taxonomic Notes for the Western Group of Heuchera Section Heuchera (Saxifragaceae). Systematic Botany 40(2): 489-500. [available online at: https://www.researchgate.net/publication/275019501_Two_New_Species_Heuch...
Fortson Wells E. & Greene Shipes B. (2009) Heuchera. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 8. Oxford University Press, New York-Oxford: 84-104. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=115382]
Jäger E.J., Ebel F., Hanelt P. & Müller G. (eds.) (2008) Rothmaler Band 5. Exkursionsflora von Deutschland. Krautige Zier- und Nutzpflanzen. Springer Verlag, Berlin: 880 p.
Miller D.M. (1995) Heuchera. In: Cullen J. & al. (eds.), The European Garden Flora, vol. 4. Cambridge University Press, Cambridge: 282-283.
Rosendahl C.O., Butters F.K. & Lakela O. (1936) A monograph on the genus Heuchera. Minnesota Stud. Pl. Sci. 2: 1-180.
Soltis D.E. (1980) Karyotypic relationships among species of Boykinia, Heuchera, Mitella, Sullivantia, Tiarella, and Tolmiea (Saxifragaceae). Syst. Bot. 5(1): 17-29.
Soltis D.E. (1984) Karyotypic relationships among Elmera, Heuchera and Tellima (Saxifragaceae). Syst. Bot. 9(1): 6-11.
Wells E.F. (1984) A revision of the genus Heuchera (Saxifragaceae) in eastern North America. Syst. Bot. Monogr. 3: 45-121.