Azolla — A Water Fern That Fertilizes a Rice Paddy for Free

Floating on the water of a rice paddy, the tiny fern Azolla does something unusual: with the help of a partner bacterium it pulls nitrogen straight out of the air, the same nutrient farmers pay for in a fertilizer sack. Asian rice farmers have used it for centuries as a living green manure and a free animal feed. Synthetic fertiliser pushed it aside; a changing climate and fertilizer prices are bringing it back.

At a glance Origin: a traditional practice across East and Southeast Asia · Maturity: Scaled · Effort: low cost, but real work to manage the fern and the water · Licence: open.

How it works. Grown on the standing water of a paddy, Azolla doubles its mass in a few days and builds a green mat. Worked into the mud or left to decompose, it releases its nitrogen to the rice; skimmed off, it feeds ducks, pigs, fish and poultry. It is a free nitrogen source that grows itself, as long as the conditions suit it.

What to expect — and what it does NOT do. In trials and on farms, rice yields have risen by a wide range — reported from roughly doubling up to about three times (around +216%) where a green-manure and intercrop approach are combined — when Azolla is used well; shown as a range because it depends on the paddy, the climate and how it’s managed, not a single headline figure. The limits are real and biological, not accidents: it needs standing water, warmth (it does best around 25–30 °C and struggles in cold or extreme heat), and phosphorus in the water to thrive. Where the water dries up, the climate is wrong, or phosphorus is lacking, it won’t perform — and that is a true constraint, not a documentation gap.

Who made it. No single inventor — a shared practice of generations of Asian rice farmers. In Vietnam (where it is called bèo hoa dâu) the agronomist Nguyễn Thị Trâm has worked to revive it, named here alongside, not above, the farmers who kept it alive.

Used it? Tell us how it went — how you grew it, whether it lifted your rice or fed your animals, how it handled your climate, what you’d change, and roughly where and when. We want the honest result, including what didn’t work. Your contact details stay private; we read and check every report before anything is published, and we’ll never publish a detail you’ve asked us to keep private.

Sources: the Azolla Foundation (theazollafoundation.org); FAO biofertiliser guidance (fao.org); Vietnamese-language growing guides (asiaorganic.org).