Plants green or often blue-green to dark red, some red-fringed leaves usually present in nature, free-floating or forming a multilayer mat to 4 cm thick in early summer; plants frequently fertile. Stems prostrate, 1--1.5 cm. Largest hairs on upper leaf lobe near stem 2(--3)-celled; broad pedicel cell often 1/2 or more height of hair, apical cell curved, with tip nearly parallel to leaf surface. Megaspores not covered with raised angular bumps, pitted and sparsely covered with a few long filaments extending over surface.

Azolla mexicana is generally less cold tolerant and has a narrower environmental range than A . caroliniana . Both species are closely related and are similar vegetatively in culture. In the western United States, A . mexicana is often fertile. Distribution in the Great Plains area is tentative and needs further study. In the eastern United States, A . mexicana may have been occasionally introduced.