Wolffia columbiana is a member of the family Lemnaceae and native to the Americas. In 2013, the first two occurrences of Wolffia columbiana in Europe were found in Germany and the Netherlands. Differentiation between W. columbiana and the rare native W. arrhiza is difficult and a scanning electron microscope (SEM) was used to confirm plant identification. A brief description of both populations of W. columbiana and some identifying characteristics are given. Due to their small size, Wolffia species are easily overlooked in the field and/or misidentified, and thus it seems likely, that even more unknown occurrences of alien Wolffia species might occur in Europe. The remaining known occurrences of putative W. arrhiza should be inspected to check their identity. We hypothesize, that alien W. columbiana might be sometimes overlooked or misidentified as the native W. arrhiza, and thus it may have a more widespread distribution within Europe than thought.
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