Cardamine L.
(incl. Dentaria L.)
In its current circumscription Cardamine is native to the temperate regions of the world. The number of species varies considerably among authors, illustrating the notorious taxonomic complexity of the genus. Cardamine probably counts at least 200 species, most of which are occurring in Eurasia. Six species are native in Belgium: Cardamine amara L., C. bulbifera (L.) Crantz (syn.: Dentaria bulbifera L.), C. flexuosa With., C. hirsuta L., C. impatiens L. and C. pratensis L. (Lambinon & Verloove 2012). Two additional species are naturalized weeds.
Several species of the former segregate genus Dentaria are cultivated as ornamentals (Jäger & al. 2008), including native Cardamine bulbifera. It is locally naturalized in Belgium outside of its native area, for instance in Steengelaag in Stekene. C. impatiens at one time was restricted to parts of southern Belgium and now is expanding locally in Flanders.
Species of bittercress are among the most prolific and costly weeds of the container nursery industry (Post & al. 2011).
1 Bulbs present in upper leaf axils. Petals pink or purple. Rhizomatous perennial (native) === Cardamine bulbifera
Bulbs absent. Petals white or pink. Annual, biennial or perennial === 2
2 Stem decumbent to ascending, often rooting at nodes. Stem leaves absent or few. Inflorescence very few-flowered and often with very short axis, almost corymbose (sometimes flowers solitary and/or with only 1-2 petals) === Cardamine corymbosa
Stem erect, not rooting at nodes. Stem leafy. Inflorescence axis usually distinct, at least in fruit === 3
3 Petals 6-20 mm long. Fruit 20-80 mm long. Rhizomatous perennial (native) === C. amara and C. pratensis
Petals smaller, sometimes absent. Fruit 15-30 mm long. Annual or biennial, sometimes short-lived perennial (then not rhizomatous) === 4
4 Stem leaves distinctly auriculate, with 9-23 leaflets with acute apices. Petals early deciduous (native) === C. impatiens
Stem leaves not auriculate, with usually less numerous leaflets with rounded apices. Petals persisting === 5
5 Basal leaves present at flowering, in a compact rosette. Stamens usually 4, rarely 6. Fruit suberect (inclined at less than 45° to rachis) (native) === C. hirsuta
Basal leaves usually absent at flowering, or in a non-compact rosette. Stamens usually 6. Fruit erecto-patent to spreading (inclined at 45°-90° to rachis) === 6
6 Upper leaf surface of middle stem leaves distinctly hairy, with ultimate leaflet lobed or not (if so, with deep and shallow sinuses). Basal half of stem markedly hairy (native) === C. flexuosa
Upper leaf surface of middle stem leaves glabrous, with ultimate leaflets usually distinctly 3-5-lobed, with deep and sharp sinuses. Basal half of stem glabrous to slightly hairy === C. occulta
Additional alien: Cardamine parviflora L. (Euras., vector unknown)
Literature:
Al-Shehbaz I.A., Arai K. & Ohba H. (2006) Cardamine. In: Iwatsuki K., Boufford D.E. & Ohba H. (Eds.), Flora of Japan, vol. IIa, Angiospermae, Dicotyledoneae, Archichlamydeae(a). Kodansha Ltd., Tokyo: 482–490.
Al-Shehbaz I.A., Marhold K. & Lihová J. (2010) Cardamine. In: Flora of North America Editorial Committee. (Ed.), Flora of North America North of Mexico, vol. 7. Oxford University Press, New York & Oxford: 464–484. [available online at: http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=105607]
Carlsen T., Bleeker W., Hurka H., Elven R. & Brochmann C. (2009) Biogeography and Phylogeny of Cardamine (Brassicaceae). Annals of the Missouri Botanical Garden 96(2): 215-236. [available online at: http://www.bioone.org/doi/abs/10.3417/2007047]
Franzke A., Pollmann K., Bleeker W., Kohrt R. & Hurka H. (1998) Molecular systematics of Cardamine and allied genera (Brassicaceae): ITS and non-coding chloroplast DNA. Folia Geobot. 33(3): 225-240.
Heenan P.B. (2017) A taxonomic revision of Cardamine L. (Brassicaceae) in New Zealand. Phytotaxa 330(1): 1-154.
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.
Lambinon J. & Verloove F. (2012) Nouvelle Flore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des Régions voisines (Ptéridophytes et Spermatophytes). Sixième édition. Jardin botanique national de Belgique, Meise: CXXXIX + 1195 p.
Post A.R. (2008) A Systematic Study of Cardamine Weed Species in United States Container Nurseries. [available online at: https://repository.lib.ncsu.edu/handle/1840.16/2122]
Post A.R., Ali R., Krings A., Xiang J., Sosinski B.R. & Neal J.C. (2011) On the Identity of the Weedy Bittercresses (Cardamine : Brassicaceae) in United States Nurseries: Evidence from Molecules and Morphology. Weed Science 59(1): 123-135.
Pritchard G.G. (1957) Experimental taxonomic studies on species of Cardamine Linn. in New Zealand. Trans. Roy. Soc. New Zealand 84: 75-89. [available online at: https://paperspast.natlib.govt.nz/periodicals/TPRSNZ1957-85.2.5.5?query=...
Rich T.C.G. (1988) Cardamine L., small flowered species. Plant Crib 1988: 29-30.
Schulz O.E. (1903) Monographie der Gattung Cardamine. Botanische Jahrbücher für Systematik, Pflanzengeschichte und Pflanzengeographie 32: 280‒623.
Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.