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Azolla pinnata - R.Br.

Common Name Water Fern, Ferny or Red Azolla
Family Azollaceae
USDA hardiness 11-12
Known Hazards Although we have found no reports of toxicity for this species, a number of ferns contain carcinogens so some caution is advisable[ 200 ]. Many ferns also contain thiaminase, an enzyme that robs the body of its vitamin B complex. In small quantities this enzyme will do no harm to people eating an adequate diet that is rich in vitamin B, though large quantities can cause severe health problems. The enzyme is destroyed by heat or thorough drying, so cooking the plant will remove the thiaminase[ 172 ].
Habitats Ponds and backwaters of rivers at elevations of 3 - 1,000 metres[ 308 ].
Range Widespread in tropical Africa and southeast Asia through to the Pacific including Australia, New Zealand.
Edibility Rating    (0 of 5)
Other Uses    (4 of 5)
Weed Potential Yes
Medicinal Rating    (0 of 5)
Tender Moist Soil Wet Soil Water Plants Full shade Semi-shade Full sun
Azolla pinnata Water Fern, Ferny or Red Azolla User:Laxskinn
Azolla pinnata Water Fern, Ferny or Red Azolla User:Tpa2067


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Red Azolla is a small, floating fern - one of only six, free-floating, aquatic fern species. It grows from 1 cm to 2.5 cm wide and is a bright green colour. Its colour changes to deep red when it is exposed to the sun. Azolla grows in waterways in dense patches, which can look like a green or red carpet. From a distance it may be confused with Salvinia, a noxious aquatic weed, or the scum of a blue-green algal bloom. It has a symbiotic relationship with the nitrogen-fixing cynobacterium Anabaena azollae. As such, it is used as a green manure particularly by rice farmers.

Physical Characteristics

Azolla pinnata is an evergreen Fern at a fast rate.
It is hardy to zone (UK) 10.
It can fix Nitrogen.
Suitable for: light (sandy), medium (loamy) and heavy (clay) soils. Suitable pH: acid, neutral and basic (alkaline) soils and can grow in very acid and very alkaline soils.
It can grow in full shade (deep woodland) semi-shade (light woodland) or no shade. It prefers moist or wet soil and can grow in water.

UK Hardiness Map US Hardiness Map


No synonyms are recorded for this name.


Edible Uses

None known

Medicinal Uses

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None known

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Other Uses

Other uses rating: High (4/5). Suitable for bogs, ponds and water gardens. Agroforestry Uses: The plant is used as a green manure in paddy fields, in particular to add nitrogen and organic matter to the soil[ 46 , 200 ]. The nitrogen-rich roots can be added to compost heaps, where they act as an activator to speed up the compost-making process[ K ]. Other Uses: It is a food source for waterfowl, fish, shrimp, insects, worms, snails and crustaceans. It provides habitat for many of the small organisms mentioned above. Mats of Azolla can actually discourage blue-green algal blooms. They restrict the penetration of sunlight into the water, which is essential for algal growth, and take up nutrients from the water column, limiting the availability of this food source for the algae. The mats of Azolla can be a form of biological mosquito control, preventing mosquito larvae surfacing for air. The presence of Azolla can also restrict the growth of exotic aquatic plants, including Salvinia and Water Hyacinth, as it limits the availability of nutrients to these plants.

Special Uses

Cultivation details

Succeeds in tropical and subtropical areas. It grows best in areas where annual daytime temperatures are within the range 20 - 35°c, but can tolerate 15 - 40°c[ 418 ]. Succeeds in ponds and lakes[ 200 ]. Plants are able to tolerate a very wide range of pH from 3.5 - 10.5[ 418 ]. Plants increase so prolifically that they can become invasive[ 200 ]. The presence of Azolla in a waterway is generally beneficial. However, in circumstances where waterways are extremely rich in nutrients, prolific growth may be a problem. It also is possible that thick, complete coverings of Azolla can cause de-oxygenation of the water. This can affect organisms such as fish and other aquatic plants, and the decay of the latter can lead to a strong odour. However, situations like this are rare because coverage of the waterway by the Azolla needs to be almost total for it to have a negative impact on the ecosystem

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Other Names

If available other names are mentioned here

Water Fern, Ferny or Red Azolla

Found In

Countries where the plant has been found are listed here if the information is available

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Weed Potential

Right plant wrong place. We are currently updating this section. Please note that a plant may be invasive in one area but may not in your area so it’s worth checking.

United States:Noxious weed/invasive - Alabama, California, Massachusetts, North Carolina, Oregon, South Carolina,Vermont.

Conservation Status

IUCN Red List of Threatened Plants Status : Least Concern

Related Plants
Latin NameCommon NameHabitHeightHardinessGrowthSoilShadeMoistureEdibleMedicinalOther
Azolla filiculoidesAzolla, Pacific mosquitofernPerennial0.1 0-0 FLMHSNWa00 

Growth: S = slow M = medium F = fast. Soil: L = light (sandy) M = medium H = heavy (clay). pH: A = acid N = neutral B = basic (alkaline). Shade: F = full shade S = semi-shade N = no shade. Moisture: D = dry M = Moist We = wet Wa = water.


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Botanical References

Links / References

For a list of references used on this page please go here
A special thanks to Ken Fern for some of the information used on this page.

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