Azolla caroliniana is small aquatic plant, the floating leaves growing from from thread-like, prostrate stems 0.5-1.0 cm long. These branch regularly above every third leaf. The general appearance of the plant is moss-like. The slender, inconspicuous roots are unbranched (simple). Plants can form mat to 4 cm thick.
Leaves alternate, tiny, often with red margins or sometimes entirely red if stressed; unequally two-lobed, the larger lobe submerged, and acting as a float, the smaller lobe sitting partially in the water. The upper surface of this smaller lobe is covered by tiny hairs. The interior is host to Anabaena azollae a blue-green algae (cyanobacterium) that fixes nitrogen (Flora of North America www.efloras.org).
Flowers none. Sporocarps are rarely found making positive identification of Azolla species difficult. Use of a microscope is necessary (Flora of North America www.efloras.org).
Wetland status: OBL
Frequency in NYC: Very infrequent.
Habitat: Stagnant or very slow-moving water in sunlight.
Notes: Azolla species have been used for rice cultivation for many centuries because of their ability to fix nitrogen, which is released when the plants decay. A. caroliniana has become invasive in many tropical countries.