Azolla is a small free-floating aquatic fern that grows on water surfaces. It has a symbiotic relationship with a nitrogen-fixing cyanobacterium. Azolla prefers warm temperatures, partial sunlight, and nutrient-rich water. It is used as a biofertilizer in rice paddies and organic farming to fix nitrogen and increase crop yields. Herbicides can be used to control heavy infestations, but the weevil Stenopelmus rufinasus has also been used successfully as a biological control agent in some areas.

1. Biology Morphology, and Management
of Azolla

2. • Azolla spp. are free-floating freshwater ferns.
Live symbiotically with Anabaena azollae, a
nitrogenfixing blue-green algae.
• A main stem growing at the surface of the
water, with alternate leaves and adventitious
roots at regular intervals along the stem.
Azolla fronds are triangular or polygonal Float
on the water surface individually or in mats.
Also known as duckweed ferns.

3. • Azolla is naturally found in ponds, ditches and wetlands of warm
temperate and tropical regions throughout the world. It requires
light for photosynthesis and grows well in partial shade. Generally,
Azolla needs 25 to 50 per cent of full sunlight for its normal growth.
Water is the basic requirement for the growth and multiplication
of Azolla and is extremely sensitive to lack of water. Maintenance
of adequate water level (at least 4 inches in the pond) is essential.
The species vary in their requirement of ideal temperature. In
general, the optimum is 20⁰ C to 30⁰ C. Temperatures above 37 ⁰ C
will seriously affect the multiplication of Azolla. The optimum
relative humidity is 85 to 90 per cent. The optimum pH is 5 to 7.
Too acidic or alkaline pH has an adverse effect on this fern. Azolla
absorbs the nutrients from water. Though all elements are
essential, phosphorus is the most common limiting element for its
growth. About 20 ppm of phosphorus in the water is optimum.
Micronutrient application improves the multiplication and growth.

4. • Description
• Azolla (Azolla sp.) is an aquatic fern consisting of a
short, branched, floating stem, bearing roots which
hang down in the water. The leaves are alternately
arranged, each consisting of a thick aerial dorsal lobe
containing green chlorophyll and a slightly larger thin,
colourless, floating ventral lobe. .. Azolla plants are
triangular or polygonal in shape, and float on the
surface of the water, individually or in mats. The most
remarkable characteristic of azolla is its symbiotic
relationship with the nitrogen-fixing blue-green alga
(cyanobacterium) Anabaena azollae. (Lumpkin et al.,
1980).

5. Chemical control
•Herbicides are an effective form of control for this species. The
floating fronds can be sprayed with glyphosate. Glyphosate will kill
almost all emergent and floating weeds onto which the spray is
directed. Surviving fronds may require a second or subsequent
treatment if the weed is to be eliminated. This is best carried out when
a gentle wind or currents have collected floating fronds together at
suitable points. Herbicides should be applied before a complete surface
cover has developed to increase effectiveness. If this is not possible a
repeat application may be necessary to kill any surviving plants (CEH,
2004)

6. • In rice field as dual cropping and green
• manuring.
• Used as cattle feed.
• Used as bio-fertilizer in Organic farming.
• Food for fishes.
• Food for pig, goat, poultry and other
• washed 4-5 time with fresh water.
• livestock.
• Note:- In case of livestock feed, Azolla should

7. • The most common mode of application of Azolla
in the field is as green manure or as a dual crop
along with rice. As green manure :- Azolla
collected directly from ponds/ditches is applied in
the field. A thick mat of Azolla will be formed
after application in about 2-3 weeks time and can
be incorporated in the soil. Rice can also be
transplanted in the field subsequently. Single
super phosphate (25-50 kg ha₋1 ) is applied in
split doses. Azolla application by this mode
contributes around 20-40 kg N ha₋1 .

8. • How could biological control help you?
• Research has shown the tiny 2mm long North American
weevil Stenopelmus rufinasus (right) to be one of Azolla's
main natural enemies.
• The weevil has already been used to control the weed very
successfully in South Africa, where after extensive host
range testing and climatic matching, it was released as a
biocontrol agent.
• Even the heaviest infestations were brought under control
within a matter of months, without the need for chemicals
or further control measures
• The weevils can be targeted at very specific sites where the
weed is considered a problem.

9. • Physical control is difficult as A.
filiculoides reproduces by fragmentation and,
in rivers, these fragments float downstream
and are capable of regrowth into cleared areas
within days.