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Cardamine occulta, recently reported also as 'Asian C. flexuosa', is originally a native East Asian weed of paddy fields and other open habitats. It has been recorded throughout the world and is now also spreading widely in Europe. However, how this species differs morphologically from European C. flexuosa and its closest relatives in Asia (C. kokaiensis and C. scutata) is not fully known, particularly because it became widely recognized as a new taxon only recently. We used chromosome counting and flow cytometry to determine ploidies and genome size variation in these four species and morphometric analyses to ascertain their morphological differentiation. A uniformly octoploid level (2n = 64) is confirmed here for C. occulta and a tetraploid level for C. flexuosa and C. scutata (2n = 32). Here we formally describe C. kokaiensis, which is restricted to East Asia (Japan, Honshu; eastern China, Zhejiang Province; perhaps also the Russian Far East) and determine it to be a tetraploid. Considerable differences in monoploid genome size were revealed between these taxa, suggesting their mostly allopolyploid origins. Morphological differences between the species are demonstrated, and detailed morphological descriptions and an identification key are provided. [ABSTRACT FROM AUTHOR]