Azolla cristata

Lore & Background

Azolla cristata is a species of the Azolla genus with scale-like fronds covered in tiny trichomes that give it a velvety appearance. It can fix nitrogen from the air through symbiotic cyanobacteria, and its fronds are green to reddish, often turning red in winter or strong light. Distinguishing it from the similar Azolla filiculoides requires examining the trichomes on the upper leaf surfaces: those of A. cristata are septate (two-celled), while those of A. filiculoides are unicellular.

This species has been erroneously known under the name Azolla caroliniana. Research by Evrard and Van Hove found that the type specimen of A. caroliniana actually consists of plants of Azolla filiculoides, so the name caroliniana has been improperly applied to this species. The fern is propagated by division and is often used as a floating plant in both coldwater and tropical aquaria, as well as in outdoor ponds.

Reader's Guide

Azolla cristata holds significance primarily as a biofertilizer in southern and eastern Asia, where it is commercially cultivated for its nitrogen-fixing ability. When grown under rice crops, it reduces the need for artificial fertilizer and also suppresses weed growth with its thick mat of fronds, which can reach up to 4 cm in thickness. Harvested fronds are additionally used as food for fish and poultry. In the aquarium hobby, it is a common floating plant suitable for both coldwater and tropical setups, as well as outdoor ponds, and is easily propagated by division. The species is native to a broad range in the Americas, from southern Ontario and the eastern United States through the Caribbean and into Central and South America as far south as northern Argentina and Uruguay. Its ability to survive winter water temperatures as low as 5 °C (41 °F) and thrive in summer temperatures between 25–30 °C (77–86 °F) makes it adaptable to various climates. The taxonomic confusion with Azolla caroliniana, resolved by Evrard and Van Hove, underscores the importance of careful botanical identification.

Did You Know?

• The trichomes on the upper leaf surfaces of Azolla cristata are septate (two-celled), distinguishing it from Azolla filiculoides, which has unicellular trichomes.
• This species was erroneously known as Azolla caroliniana, but the type specimen of that name was found to be Azolla filiculoides.
• Azolla cristata can fix nitrogen from the air via symbiotic cyanobacteria, making it valuable as a biofertilizer for rice crops.
• Its fronds are covered in tiny trichomes that give it the appearance of velvet.