Azolla caroliniana

Azolla caroliniana is a tiny floating water fern native to the Americas that carries within its leaves a symbiotic cyanobacterium (Anabaena azollae) capable of fixing atmospheric nitrogen — a unique trait that gives it natural fertilising properties. Under low light it remains green, but under bright light the fronds develop vivid red and orange tones, creating a spectacular fiery surface display. It spreads rapidly and is valued both as a shade provider and as a nitrogen-fixing biofertiliser in planted systems.

Light: Medium to high surface light; high light triggers the dramatic red-orange colouration. CO2: Not required; fixes atmospheric nitrogen via symbiotic Anabaena. Planting: Introduce a small handful to the water surface; spreads naturally within days. Maintenance: Thin the colony frequently as it can completely cover the surface; use net to remove excess. Note: Nitrogen fixation makes it a mild natural fertiliser — beneficial in low-nutrient systems but can promote algae if overdone.