Helleborus L.
Helleborus is a relatively small genus with ca. 15-25 species (depending on species delimitation) and is confined to the Old World. More than half of the species occur in Europe, the others in the Mediterranean area and Asia. The genus is particularly diverse in the Balkans and taxonomically complex, especially with respect to the acaulescent species. Two taxa are native in Belgium: H. foetidus L. and H. viridis L. subsp. occidentalis (Reut.) Schiffn. (Lambinon & Verloove 2012). The genus is very popular in the horticultural trade. At least 14 species (and even more cultivars and hybrids) are grown as ornamentals in Europe (e.g. Mathew 1997, Jäger & al. 2008). An increasing number of these has been recorded in the wild in Belgium. The two native species have a relatively restricted distribution in Belgium. Both, however, are also cultivated as ornamentals and are now naturalized outside their natural range.
Molecular studies have shown the genus to be monophyletic (Sun & al. 2001). The same holds true for the six sections that have been recognized based on morphological features (sections Syncarpus, Griphopus, Chenopus, Helleborus, Helleborastrum and Dicarpon; Zonneveld 2001). The traditional split of the genus in caulescent and acaulescent species, however, is not confirmed by molecular data (Sun & al. l.c.).
Additional useful information on the genus is presented online by Hellebores.org at https://web.archive.org/web/20060615054459/http://www.hellebores.org/. A well-illustrated and detailed overview of wild and cultivated species in Germany is available here: http://www.blumeninschwaben.de/Zweikeimblaettrige/Hahnenfuss/nieswurz.ht...
1 All leaves basal (sometimes some leaf-like bracts present). Rhizomatous with short-lasting herbaceous stems. Bracts deeply divided. Inflorescences 2-4 flowered === 2
Stem leafy, without basal leaves. Rhizomes absent with overwintering, more or less woody stems. Bracts nearly entire. Inflorescences many-flowered === 3
2 Follicles free to base, shortly stalked. Flowers usually purplish, not fragrant. Leaves usually overwintering, leathery === Helleborus orientalis
Follicles connate at base for ca. ¼ of their length, sessile. Flowers pale green, fragrant. Leaves usually not overwintering (native) === H. viridis (subsp. occidentalis)
3 Leaves ternate, not stinking === H. lividus
Leaves pedate with 7-11 segments, stinking when crushed (native) === H. foetidus
Additional alien: Helleborus niger L. (Alps, garden escape). Locally more or less naturalized in the 19th century and earlier (see Durand 1899 for an overview). Recent claims require confirmation.
References
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Burrell C. & Tyler R. (2006) Hellebores: A Comprehensive Guide. Timber Press: Portland, Oregon: 296 p.
Demoly J.P. (2000) Annexe IV. Nouveautes nomenclaturales dans le genre Helleborus L. (Ranunculaceae). In: Lemonnier M. (ed.), Les hellebores aux Jardins de Bellevue. Paris: Editions Franklin Picard: 94 p.
Jagel A. & Höggemeier A. (2016) Helleborus niger – Christrose (Ranunculaceae) sowie Schneerosen und Lenzrosen. Bochumer Botanischer Verein – www.botanik-bochum.de – Pflanzenporträts 2016. Available online at: https://www.botanik-bochum.de/jahrbuch/Pflanzenportraet_Helleborus_niger...
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Zonneveld B.J.M. (2001) Nuclear DNA contents of all species of Helleborus (Ranunculaceae) discriminate between species and sectional divisions. Pl. Syst. Evol. 229(1-2): 125-130. [available online at: https://www.researchgate.net/publication/227037366_Nuclear_DNA_contents_...