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The <i>Acorus calamus</i> group, or sweet flag, includes important medicinal plants and is classified into three species: <i>A. americanus</i> (diploid), <i>A. verus</i> (tetraploid), and <i>A. calamus</i> (sterile triploid of hybrid origin). Members of the group are famous as components of traditional Indian medicine, and early researchers suggested the origin of the sweet flag in tropical Asia. Subsequent research led to an idea of the origin of the triploid <i>A. calamus</i> in the Amur River basin in temperate Asia, because this was the only region where both diploids and tetraploids were known to co-occur and be capable of sexual reproduction. Contrary to this hypothesis, triploids are currently very rare in the Amur basin. Here, we provide the first evidence that all three species occur in Kazakhstan. The new records extend earlier data on the range of <i>A. verus</i> for c. 1800 km. Along the valley of the Irtysh River in Kazakhstan and the adjacent Omsk Oblast of Russia, <i>A. verus</i> is recorded in the south, <i>A. americanus</i> in the north, and <i>A. calamus</i> is common in between. We propose the Irtysh River valley as another candidate for a cradle of the triploid species <i>A. calamus</i>. It is possible that the range of at least one parent species (<i>A. americanus</i>) has contracted through competition with its triploid derivative species, for which the Irtysh River floods provide a tool for downstream range expansion. We refine our earlier data and show that the two parent species have non-overlapping ranges of variation in a quantitative metric of leaf aerenchyma structure.