Cardamine occulta

Cardamine occulta Hornem. (syn.: C. hamiltonii G. Don, C. flexuosa subsp. debilis O.E. Schulz, ‘Asian’ C. flexuosa) (SE As.) – A frequent but still widely overlooked, naturalized alien. Based on herbarium study, it was shown that C. occulta is present in Belgium since at least 1968; however, it was only relatively recently recognized. It passed unnoticed for decades as a result of confusion with native C. flexuosa. The latter is usually found in more natural habitats (mostly damp woodlands) but a variant (?) also occurs in highly anthropogenic habitats: as such it occurs as weed in plant nurseries, cities, etc. (see also Tison & de Foucault 2014).

The presence of this Far Eastern weed in Europe was first formally detected from the banks of Lake Constance in southern Germany, as late as in 2004, based on molecular evidence (Bleeker & al. 2008). Since then, C. occulta has been reported from numerous other European countries (see references), first as ‘Asian’ Cardamine flexuosa, subsequently as C. hamiltonii. However, Marhold & al. (2016) showed that the correct name for this weed is in fact C. occulta.

At present Cardamine occulta occurs in numerous, widely scattered localities in Belgium although it seems to be rare (or merely overlooked?) in the southern half of the country. It is usually found in man-made habitats, as plantation or urban weed, in cemeteries, etc. In other European countries, however, it has also been recorded in more natural habitats, for instance in riparian communities (exposed river or pond margins).

Cardamine occulta is not easily told apart from C. flexuosa (and, to a lesser extent, C. hirsuta). In a way it is more or less intermediate and at one time it was thought to be their hybrid, C. xzahlbruckneriana O.E. Schulz. The latter, however, is sterile and probably only exceptionally produced (Stace 2010). A detailed morphometric analyses of this and related species was recently carried out by Šlenker & al. (2018). It was shown that the species is best differentiated based on its glabrous upper leaf surfaces, the more prominently lobed terminal leaflets, the absence of a compact basal leaf rosette, the less hairy stem, etc.

Interestingly, octoploid C. occulta probably originated through hybridization between the tetraploids C. scutata and C. kokaiensis (Mandáková & al. 2019).

Selected references

Bleeker W., Klausmeyer S., Peintinger M. & Dienst M. (2008) DNA sequences identify invasive alien Cardamine at Lake Constance. Biological Conservation 141: 692–698. [available online at:]

Bomble F.W. (2014) Japanisches Reisfeld-Schaumkraut (Cardamine hamiltonii) in Aachen. Veröffentlichungen des Bochumer Botanischen Vereins 6: 1–5. [available online at:

Bomble F.W. & Joussen N. (2019) Größere Bestände des Japanischen Reisfeld-Schaumkrauts (Cardamine occulta) an der Rurtalsperre in der Eifel. Veröff. Bochumer Bot. Ver. 11(2): 25-29. [available online at:

Dirkse G.M., Zonneveld B.J.M. & Duistermaat L.H. (2015) Cardamine hamiltonii G. Don - Aziatische veldkers (Brassicaceae) in Nederland. Gorteria 47: 64–69. [available online at:]

Hepenstrick D. & Hoffer-Massard F. (2014) Un xénophyte asiatique du groupe Cardamine flexuosa: identification, nomenclature et génétique. Bulletin du Cercle vaudois de botanique 43: 69–76. [available online at:]

Honjo M.N., Marhold K. & Kudoh H. (2020) Experimental Evaluation of Differences in Plastic Phenotypes between Cardamine fallax and C. occulta: Effects of Seasonality on Phenology and Gross Morphology. Acta Phytotaxonomica et Geobotanica 71(1): 23-32. [available online at:]

Lihová J., Marhold K., Kudoh H. & Koch M.A. (2006) Worldwide phylogeny and biogeography of Cardamine flexuosa (Brassicaceae) and its relatives. American Journal of Botany 93: 1206–1221. [available online at:

Mandáková T., Marhold K. & Lysak M.A. (2014) The widespread crucifer species Cardamine flexuosa is an allotetraploid with a conserved subgenomic structure. New Phytologist 201: 982–992. [available online at:]

Mandáková T., Zozomová-Lihová J., Kudoh H., Zhao Y., Lysak M.A. & Marhold K. (2019) The story of promiscuous crucifers: origin and genome evolution of an invasive species, Cardamine occulta (Brassicaceae), and its relatives. Ann. Bot

Mansanet-Salvador C.J., Ferrer-Gallego P.P., Ferrando I. & Laguna E. (2015) Notas sobre el complejo taxonómico Cardamine flexuosa With. (Cruciferae) y su presencia en la Comunidad Valenciana. Flora Montiberica 59: 72–82. [available online at:]

Marhold K., Šlenker M., Kudoh H. & Zozomová-Lihová J. (2016). Cardamine occulta, the correct species name for invasive Asian plants previously classified as C. flexuosa, and its occurrence in Europe. PhytoKeys 62: 57-72. [available online at:]

Pliszko A. (2020) First record of Asian Cardamine occulta Hornem. (Brassicaceae) in Poland. BioInvasions Records 9(3): 655-659. [available online at:]

Rosenbauer A. (2011) Ausgewählte Cardamine-Arten in Baden-Württemberg. Zentralstelle für die floristische Kartierung von Baden-Württemberg, Rosenstein, 1 p]

Šlenker M., Zozomová-Lihová J., Mandáková T., Kudoh H., Zhao Y., Soejima A., Yahara T., Skokanová K., Španiel S. & Marhold K. (2018) Morphology and genome size of the widespread weed Cardamine occulta: how it differs from cleistogamic C. kokaiensis and other closely related taxa in Europe and Asia. Botanical Journal of the Linnean Society 187(3): 456–482.

Šlenker M., Zozomova-Lihova J. & Marhold K. (2019) Cardamine occulta - inconspicuous neophyte in Slovakia. Bull. Slov. Bot. Spoločn. 41(1): 13-23. [available online at:

Stace C. (2010) New flora of the British Isles, 3th ed.: XXXII + 1232 p. Cambridge University Press.

Tison J.-M. & de Foucault B. (coord.) (2014) Flora Gallica. Flore de France. Editions Biotope, Mèze : xx + 1196 p.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith