Azolla has been used as a green manure crop in Vietnam and China for centuries. The ability of azolla to shade out weeds in wetland rice and taro has been noted by farmers and researchers since the early 20th century. The effect of azolla on rice yields has been the primary focus of azolla research. Increases of from 14-40 per cent were quoted in a review article (Moore, 1969). Talley et al. (1977) achieved rice yield increases of 112% over the control by incorporating one 60 kg N/ha layer of A. filiculoides into the paddy soil and an increase of 216% by incorporating one layer and then growing azolla as a dual crop with rice. Scientists in China reported increases ranging from 0.4 to 158% with an average of 18.6% from the results of 422 field experiments. (Lumpkin and Plucknett, 1980)
- Vegetative material easy to find
- Usually easy to establish
- Obtains quick cover
- Effective at shading out weeds
- Usually requires no extra fertilization beyond P applied to crop
- May increase crop yields
- Doesn't attract taro or rice pests
- Can be used only in wet fields
- Vegetative material may be contaminated with apple snails
- May require a nursery pond
- May be difficult to establish in winter
- Die back can occur during hot weather
- Weevils may contribute to die back
- Can become weedy in slow-moving waterways
Azolla, water velvet, mosquito fern
Azolla filiculoides (the species most commonly found in Hawai`i), Azolla caroliniana, Azolla mexicana, Azolla microphylla, Azolla nilotica, and Azolla pinnata.
The Internation Rice Research Institute in the Philippines maintains a collection of hundreds of numbered accessions of azolla. However, in Hawai`i there are no named cultivars of the common Azolla filiculoides that came from a chance introduction in the early 20th century (Fosberg, 1942).
Azolla is almost always propagated vegetatively since its most common mode of multiplication is vegetative. Like other ferns, it produces spores, not seeds.
When a frond reaches a certain size, the older stems detach themselves from the main stem as a result of the formation of an abscission layer.
Mature Plant Description
Azolla is a floating water fern that has a symbiotic relationship with a nitrogen-fixing bacteria. An azolla plant, generally called a frond, consists of a main stem growing at the surface of the water which has alternate leaves and adventitious roots at regular intervals along the stem. Secondary stems develop at the axil of certain leaves.(Van Hove et al., 1983). Azolla fronds are triangular or polygonal in shape, and float on the water surface individually or in mats. Plant diameter ranges from 1-2.5 cm for small species such as A. pinnata, to 15 or more for A. nilotica. At the point of attachment each branch has an abscission layer, which is important in vegetative reproduction. Leaves consist of a thick aerial dorsal lobe and a thin ventral lobe, which is boat-shaped and rests on the surface so that the frond floats. The upper lobe has a large central cavity which houses a population of symbiotic cyanobacteria (blue-green algae) named Anabaena azollae.
The optimum for most azolla is between 20 and 30 degrees C. Temperature affects both water and nitrogen content. A. fililculoides is reported to withstand temperatures as low as 5 degrees C but to be less tolerant of higher temperatures than some other species (Lumpkin and Plucknett, 1980). Van Hove et al. recommend that strain selection should take temperature into consideration.
Azolla is found in ponds, ditches, and wetlands of warm-temperate and tropical regions throughout the world. A. filiculoides was endemic to southern South America through western North America to Alaska. It is now found in Africa, Australia, Asia, and Europe. It has become a noxious weed in waterways of Europe, Africa, and New Zealand. (Lumpkin and Plucknett, 1980)
Azolla must grow in water or wet mud to survive. It dies in a few hours if it becomes dry. Water control is critical, especially for year round production. A water level which allows the roots to touch the soil surface will often cause mineral deficiencies to appear. Wind and wave action can eventually fragment and kill azolla. Maintaining low water levels and rough plowing can protect azolla from wind. In Africa, hedges, bunds, and mixed culture (with crop plants) are used to prevent wind damage. (Von Hove, et al., 1983)
Azolla can grow well in the absence of any nitrogen source. Phosphorus is the most common limiting factor in the growth of azolla. Fronds placed in P deficient solution decrease or stop growth, become red, and develop curled roots. The minimum P requirement is not known but it thrives on as little as 1.1 mg P/liter. Problems due to iron deficiency or toxicity are fairly frequent. Azolla fronds turn yellow when iron is lacking. Rapid growth is achieved with 1 ppm iron. Molybdenum requirements, though low, are higher than for most other plants.
Since azolla lives in water the following refers to the pH of the water only. Azolla can survive within a pH range of 3.5 to 10, but optimum growth is in the range of 4.5 to 7. The relative growth rate is influenced by a direct relationship between light intensity and pH with the highest growth rates achieved at high pH (9-10) and high light intensity and low pH (5-6) and low light. Nitrogen fixation is optimal at pH 6 and 20 degrees C.
Deficiency problems can be caused in neutral to alkaline water because ferric ions precipitate. There can also be competition between ferrous and manganous ions in water with a neutral pH and reduction in absorption of both iron and managanese with high calcium concentrations. At pH 4, ferric ions are so readily available that a high concentration of calcium is required to balance the increased absorption of iorn, otherwise azolla suffers from iron toxicity. (Lumpkin and Plucknett, 1980).
Since azolla lives only in wetland fields, water conditions are more relevant than soil type. Soils which have the ability to pond are usually required.
Relative growth and nitrogenase activity is at a maximum at 50% of full sunlight although the difference between growth at 50% and 100% sunlight is not that great. Heavy shading is known to decrease azolla growth to almost zero. (Lumpkin and Plucknett, 1980)
The growth rate of azolla gradually declines as salinity increases. At about 1.3% salt (33% of sea water) the growthof azolla stops and higher concentrations will kill it. In rice fields where salt concentration reaches 1480-1872 mg/l during the dry season azolla wilts. Salinity is a factor which should be looked wherever azolla is being considered. (Lumpkin and Plucknett, 1980).
Most rice herbicides kill or inhibit azolla growth. Differences in sensitivity are specific to the different azolla species. (Moody and Janiya, 1992)
A pair of sporocarps forms on the lower lobe of the first leaf at the point of a branch. They are either microsporocarps or megasporocarps that detach themselves, sink to the bottom and go dormant. When the megaspore germinates, it produces a female gamete that is fertilized by the male gamete that develops from the microspore. The resulting seedling floats up to the surface. (Van Hove, et al., 1983).
Vegetative reproduction occurs when a frond reaches a certain size. The older stems detach themselves from the main stem as a result of the formation of an abscission layer. (Lumpkin and Plucknett, 1980).
The Vietnamese recommend a rate of 0.5kg/m2, with up to 1.6kg/m2 in less than optimum conditions (low light, algal blooms, or other conditions that keep light from reaching the azolla). The rate of 0.5kg/m2 has also been used in rice fields in California. (Lumpkin and Plucknett, 1980)
- In tropical Asia azolla is traditionally cultivated as a green manure for rice in two ways. One way is to set aside 5-10% of the crop area for year-round production. The cultivated azolla is later added to crop fields as compost. In the second way, azolla is cultivated in the rice fields and incorporated before and/or after the rice crop and between crops. Ideally azolla is grown several times before rice transplanting.
- When azolla is used to suppress weeds in rice or taro production a rate of 0.5kg/m2 is used. Azolla is propagated in the field for 2-4 weeks prior to transplanting (depending on azolla growth rates which are generally higher in summer and lower in winter in Hawai`i).
- When azolla innoculum is low, azolla is overgrown by algae and weeds. Increasing the innoculum rate decreases weeds and increases rice yields. (Moody and Janiya, 1992)
- In areas where azolla may be dispersed by wind or currents, azolla should be protected temporarily by a bund or wooden floats which are gradually moved outward until the area is covered.
Year round in Hawai`i, although growth rates are lower in winter months.
There is no need for innoculation since the nitrogen-fixing bacteria is always present. (Lumpkin and Plucknett, 1980)
At this time, there is no information available suggesting that azolla has been sold in Hawaii. It is generally collected from the wild and put into a pond for propagation and distribution.
In Hawai`i Azolla filiculoides is common in ditches, ponds, and slow-moving streams where it can be collected with a net.
Days to Flowering
Days to Maturity
Azolla spreads laterally, growing exponentially until it covers the whole surface of the water. Its exponential growth rate is subject to many environmental constraints. A. pinnata doubles its biomass in 3-5 days in lab situations but in the field 5-10 days is more normal. Growth slows down as plants on the upper surface are deprived of water and nutrients, their roots unable to penetrate the lower layers. The plants in the lower layers receive less and less light. (Von Hove, et al, 1983)
When the surface of the water is filled by the very low growing fronds it begins to grow on top of itself in layers up to a few inches deep.
Adventitious roots hang in the water or, when in shallow water, occasionally penetrate into the mud.
Azolla is established by vegetative propagation. Nursery ponds are generally used to supply a large enough volume to a wetland field to ensure quick coverage of the area so shade will suppress germination of weeds. Some hand weeding may be necessary, particularly if perennial weeds have not been controlled prior to establishing the azolla. In taro lo`i pre-plant fertilizer is applied at normal rates to supply the phosphorus required for quick growth of the azolla. It has been recommended that water contain 20ppm P for optimum azolla growth. (Lumpkin and Plucknett, 1980). In experiments with azolla in California rice fields, phosphorus was applied at the rate of 7.2kg P/ha in four equal doses 7 days apart.
Occasional spot weeding may be necessary as azolla becomes established. Some species of weeds in rice have been shown to be able to penetrate the azolla cover and become established. (Moody and Janiya, 1992). Where phosphorus is limited, additional fertilization may be necessary to maintain azolla growth (see establishment section).
In Asia azolla is incorporated by trampling or ploughing. In Hawai`i it has also been rototilled into taro lo`i (pond fields).
Azolla is removed from nursery ponds using hand nets or small drag nets. It has been placed in buckets, burlap sacks or onion bags for transporting.
Chinese publications have described baskets, pushers, scoopers, scrape boards, and beaters.(Lumpkin and Plucknett, 1980)
- nitrogen-fixing green manure
- weed suppression
- feed for ducks, pigs,chickens and fish
- potential for human consumption
(Lumpkin and Plucknett, 1980)
Under Asian rice field conditions A. pinnata realistically produces 8-10 tons/ha per crop fresh weight. Yields of 1.8 to 3 tons/ha dry weight have been obtained with A. filiculoides. (Lumpkin and Plucknett, 1980)
Estimates of annual nitrogen fixation vary widely. In California rice, A. filiculoides produced 52kg N/ha in 35 days under field conditions at a rate of 1.2 kg/ha/day between 10-35 days after inoculation. The Vietnam Institute of Agriculture suggested an approximate potential of 1000 kg/ha per year. (Lumpkin and Plucknett, 1980)
A C:N ratio of 15:1 was reported for A. pinnata. Reports indicate an increase in nitrogen recovery when azolla is incorporated into soil rather than allowed to decompose in water. Azolla nitrogen is released slowly and its availability is only about 70% of ammonium sulfate. Most studies have found two thirds of the azolla nitrogen is released after six weeks (or 5-8 weeks in some). (Lumpkin and Plucknett, 1980)
Azolla as a green manure contributes organic matter to the soil. Green A. filiculoides has been reported to contain 0.43% phosphorus and 2.5% potassium. Red azolla contained only one third the phosphorus of that in green azolla and potassium was reduced by 20%. (Lumpkin and Plucknett, 1980)
Effects on Water
The Institute of Soils and Fertilizers (China) also reported that growth of azolla reduced evaporation by 11% and water salt content by 0.012-0.049%. (Lumpkin and Plucknett, 1980) Experiments in Hawai`i have recorded differences of 1 degree C and more between lo`i (ponds) with azolla and without. (DeFrank, personal communication).
Effects on Microclimate
Water temperatures have been reduced (see above).
Effects on Soil
As reported by Lumpkin and Plucknett, the Institute of Soils and Fertilizers in China found that azolla used as a green manure decreased specific gravity, increased porosity (3.7-4.2%), and increased organic matter in soils. Soil salt content was reduced by 0.014-0.048%. (Lumpkin and Plucknett, 1980)
Effects on Livestock
Amino acid analyses show lysine, methionine and histamine are probably limiting, although the protein content is as high 13% on a dry weight basis. No growth inhibitors or toxins were found in experiments with rats. Used as feed for ducks, pigs and fish.
Effects on Workers
Pest Effects, Insects
The insects which attack azolla are not those that attack rice. In Asia insects pests of azolla are controlled with the same insecticides used for rice. Insect attacks increase during the summer, especially when the temperature goes above 28 degrees C. Larvae of Chironomus, Pyralis, and Nymphula species are pests. The Chinese have also documented an Azolla Elephant Beetle and several species of snails. (Lumpkin and Plucknett, 1980) Azolla is a preferred food of Hawai`i apple snails in Hawai`i, a serious pest of taro. Care must be taken when transferring azolla from one location to another that apple snails and/or their eggs are not present. A weevil (probably Stemopelmus spp.) and larvae of the moth Agrotis ipsilon have also been reported on azolla in Hawai`i (Lumpkin and Plucknett, 1980).
Pest Effects, Nematodes
Pest Effects, Diseases
Reduction in water temperatures by azolla may help in the prevention of root diseases like Pythium spp. that are more prevalent as temperatures increase. Chinese researchers have mentioned a decay or mold disease present on azolla without naming the organism. (Lumpkin and Plucknett, 1980).
Pest Effects, Weeds
The ability of azolla to create a light-proof mat that suppresses other weeds has been mentioned since 1927 in the literature (Fosberg noted taro farmers' use of azolla in Hawai`i in 1942). Studies in the Philippines (Moody and Janiya, 1992) and in Hawai`i (DeFrank, 1995) have shown that azolla can suppress most weeds in rice and taro.
At a farm in Waiahole, Hawai`i taro was planted into a three week old azolla mat that covered the entire surface of the water. No weeding was necessary for the first three months, while in areas with no azolla, three hand-weedings were required. In this case the azolla died back from unknown causes and hand-weeding was necessary until the taro closed canopy. (Ferentinos, unpublished data).
Pest Effects, Vertebrates
Uses in the Pacific Region
No information is available in this database on this topic.
Uses in Hawai`i
- Fosberg noted taro farmers' use of azolla in Hawai`i in 1942.
- DeFrank (1995) has shown that azolla can suppress most weeds in rice and taro.
- At a farm in Waiahole, Hawai`i taro was planted into a three week old azolla mat that covered the entire surface of the water. No weeding was necessary for the first three months, while in areas with no azolla, three hand-weedings were required. In this case the azolla died back from unknown causes and hand-weeding was necessary until the taro closed canopy. (Ferentinos, unpublished data).
For More Information
1997. Taro, Mauka to Makai: A Taro Production and Business Guide for Hawai`i Growers. College of Tropical Agriculture and Human Resources, University of Hawai`i at Manoa. 108 pages.
DeFrank, J. 1995. Azolla for Weed Control in Wetland Taro Production. Video. University of Hawai`i at Manoa, College of Tropical Agriculture. 19 min. This video documents an experiment using Azolla, an aquatic fern, to reduce weeds in wetland taro. Available for purchase at http://www2.ctahr.hawaii.edu/oc/forsale/
Fosberg, F.R. 1942. Uses of Hawaiian Ferns. American Fern Journal Vol 32: pp. 5-23.
Moody, Keith and J.D. Janiya. 1992. The role of azolla in weed control in rice. Philippine Journal of Weed Science, Vol. 19: 79-102.
Lumpkin, Thomas A. and Donald L. Plucknett. 1980. Azolla: Botany, Physiology and Use as a Green Manure. Economic Botany 34(2): pp. 111-153.
Talley, S.N., B.J. Talley, and D.W. Rains. 1977. Nitrogen fixation by Azolla in rice fields. In Alexander Hollaender, ed. Genetic Engineering for Nitrogen fixation, pp. 259-281. Plenum Press, New York and London.
Van Hove, C., H.F. Diara, and P. Godard. 1983. Azolla in West Africa. West Africa Rice Development Association.
Text last updated on 9/23/02