The Stachys palustris L. complex in North America is composed of diploids, tetraploids, and hexaploids with the base number x = 17. Hexaploids are identical to S. palustris L. of Europe and are weedy plants introduced from that continent. Diploid and tetraploid plants of this complex are native to North America and should be referred to other species. The taxonomy of these native plants requires further study. Characteristics are presented that distinguish native diploids and tetraploids from the introduced weedy hexaploid S. palustris L.
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Mulligan et al. (1983) studied this question.