duckweed, any plant of the genus Lemna and sometimes of related genera. Duckweeds are tiny floating or submerged aquatic plants with reduced or obsolete roots. They flower only rarely, and their flowers are small and inconspicuous. Duckweeds grow in freshwater throughout most of the world. They are classified in the division Magnoliophyta, class Liliopsida, order Arales, family Lemnaceae.