Spirodela polyrhiza is one of the largest-leaved duckweeds. Its leaves have a purplish color underneath. Leaves can be up to 10 mm long, according to Kasselmann. I have never seen the plant that big, but it is generally twice to three times bigger than the genus Lemna. Paul Krombholz, in central Mississippi, where we had light patchy frost on the 16th and 17th.