Propagating Azolla filiculoides (Water Fern)

How to multiply the floating water fern Azolla filiculoides by simple fragmentation, why it spreads so fast, and the responsibility that comes with this invasive species.

Overview

Azolla filiculoides is a tiny floating fern, each plant only 1 to 2 cm across, green and tinged pink, orange or red at the edges. It harbours the diazotrophic cyanobacterium Anabaena azollae, which fixes nitrogen from the air and contributes to the fern's metabolism. This partnership helps fuel its remarkable speed of reproduction.

Propagation Method

Propagation is by fragmentation. The plant branches freely and breaks into smaller sections as it grows, so each separated piece becomes an independent new plant. You do not cut or plant anything; you simply divide an existing floating mat into portions.

Step-by-Step

1. Scoop a healthy section of the floating mat from the surface.
2. Break or gently pull it into several smaller clumps.
3. Place each clump on the water surface of the target tank.
4. Leave them floating; they branch and break apart on their own.
5. Within days the portions spread into new mats.

Conditions for Healthy Growth

The fern floats at the surface and prefers calm water and good light. Under strong light it shifts from green toward reddish tones. It is capable of spreading over the surfaces of lakes to give complete coverage in only a few months, so growth is rarely a problem to encourage.

Maintenance

Because it doubles so quickly, the real work is removal, not encouragement. Skim off excess regularly to keep light reaching plants and livestock below. In cooler climates it largely dies back in winter, surviving by means of submerged buds.

Common Challenges

Dense mats block light penetration and can create an anaerobic environment, reducing water quality and aquatic biodiversity. Keeping it thinned is the main ongoing challenge in any tank.