Side by side, these two species are easier to distinguish than when apart. Spirodela polyrrhiza is the largest duckweed and has more roots than others in North America. Landoltia punctata usually has several roots, generally fewer than S. polyrrhiza, but, always more than Lemna species, which only have one. Both Spirodela and Landoltia have reddish purple coloration on the undersurface of the fronds, a result of anthocyanin production, which varies with environmental conditions. The undersurface of Landoltia punctata is covered with many sunken glands, reason for the epithet "punctata", that are seen as a heavier texture overall.

Spirodela polyrrhiza, ranges widely in its native North America, extending to all U.S. states and far north of the Canadian border (Landolt 1981). While the introduced Landoltia punctata appears more restricted to the Southeast, it might be expected in all regions with mild winters.