Water fern
Azolla filiculoides · Salviniaceae
The gardener's calendar
Ideal time to plant water fern: set it out when the soil is neither frozen nor waterlogged, and water generously as it establishes.
ID card
- Family
- Salviniaceae
- Type
- Floating aquatic fern
- Origin
- South America (Mexico, Central and South America)
- Adult size
- 1 to 2 cm in height, coverage up to several meters
- Growth
- Very fast
- Hardiness
- Cold-sensitive (freezes below 5°C)
- Foliage
- Persistent, green changing to red-brown depending on conditions
- Environment
- Fresh water, ponds and water gardens
Care
Watering and aquatic environment
Azolla lives entirely floating on the water surface. Maintain a minimum water depth of 15 to 30 cm of fresh water, still or with weak circulation. The plant absorbs nutrients directly from the water. Stagnant or gently stirred water suits it better than strong currents.
Light exposure
Place Azolla in full light or partial shade for optimal growth. In bright light and cool conditions, the foliage takes on a reddish ornamental tint. Conversely, in dense shade, the plant becomes pale and grows less vigorously.
Substrate and water quality
No substrate is necessary since it floats. However, slightly acidic to neutral pH (6 to 7) and low-lime water promote its growth. Avoid water that is too cold (below 10°C) or treated excessively with chemicals.
Nutrition and fertilization
Azolla fixes atmospheric nitrogen itself through its symbiosis with the cyanobacterium Anabaena. Little fertilizer is needed; in an established pond, water is sufficient. For faster growth, occasionally add a highly diluted or balanced liquid fertilizer designed for aquatic plants.
Pruning and control
Azolla grows so quickly that it can colonize an entire pond. Regularly remove excess portions with a net or skimmer to prevent it from smothering other aquatic plants. This harvest can be composted or used as an amendment.
Pests and problems
Azolla is rarely susceptible to diseases. Decline can result from temperature that is too low, water that is too chalky, or total lack of light. Filamentous algae can compete with it in open ponds: maintain good balance through partial shading and nutrient control.
Did you know?
Azolla has been used for centuries in Southeast Asia, particularly in Vietnam and China, as an organic green manure in rice paddies. Thanks to its symbiosis with the cyanobacterium Anabaena, it naturally enriches water with nitrogen, fertilizing rice crops without chemical inputs. Today, it is being studied for its potential to feed livestock and even combat malnutrition.
Where to buy it
Plant in spring or early autumn, avoiding spells of intense heat. Pick a plant with healthy foliage and roots that are not exposed. Best time: March, April, May, September.
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Tips generated and checked by Viridia — adapt them to your local climate.