Individual azolla plants have slender, branched stems with minute, overlapping scalelike leaves only one millimeter long. Each plant resembles a little floating moss with slender, pendulous roots on its underside. The plants tend to clump together and often form compact mats on the water surface.
Azolla is sometimes called "duckweed fern". When growing in full sunlight, particularly in late summer and fall, azolla may produce reddish anthocyanin in the leaves, in contrast with the bright green carpets of duckweed and filamentous green algae.