Azolla pinnata

feathered mosquito fern

Salviniaceae Australia, Asia, and Africa

Container Friendly

SunFull Sun – Part Shade

WaterHigh

HardinessZones 8–11

Overview

Azolla pinnata is a small free-floating aquatic fern in the family Salviniaceae, with individual plants 0.4-1 inch (1-2.5 cm) across that form dense mats on the surface of still and slow-moving water. The triangular fronds are finely branched and covered in tiny overlapping scale-like leaves, green in cool or shaded conditions and turning red to reddish-brown under bright sun, cold, or nutrient stress. Thread-like roots hang beneath each plant into the water. The leaves house the nitrogen-fixing cyanobacterium Anabaena azollae, which allows the fern to grow in low-nitrogen water. Reproduction is mainly by fragmentation, with plants dividing rapidly to cover open water within weeks; spores form in specialised structures under suitable conditions. The species occurs naturally across Australia, Asia, and Africa in ponds, ditches, dams, and slow streams. Dense surface mats reduce light and oxygen exchange in the water below, which can affect submerged plants and aquatic life. In cultivation it is grown in ponds, aquaria, and rice systems, where the nitrogen-fixing capacity is used as a green manure. Cold winters reduce the mats, which regrow from surviving fragments in spring. Growth slows in shade and in still, stagnant water low in nutrients.

Native Range

Azolla pinnata occurs naturally across Australia, much of Asia, and Africa. It grows on the surface of still and slow-moving fresh water, including ponds, ditches, dams, wetlands, and slow streams.

Suggested Uses

Grown in ponds, dams, and aquaria as floating cover and as a green manure in rice and vegetable systems. The nitrogen-fixing fronds are used as a livestock feed supplement and mulch. Surface mats create shade and shelter for pond life.

How to Identify

Identify Azolla pinnata by its tiny free-floating plants 0.4-1 inch (1-2.5 cm) across with triangular, finely branched fronds of overlapping scale-like leaves. Colour shifts from green in shade to red or reddish-brown in bright sun and cold. Thread-like roots hang into the water beneath each plant. The triangular, feather-like outline separates it from the more rounded Azolla filiculoides.

Appearance

Size & Dimensions

Height1" - 1"

Width/Spread1" - 1"

Colors

Foliage Colors

green

red

Fall Foliage Colors

red

Bloom Information

As a fern, Azolla pinnata does not flower. It reproduces mainly by fragmentation and forms spores in specialised structures called sporocarps under suitable conditions. Mats expand fastest in warm months.

Detailed Descriptions

Foliage Description

green to red

Growing Conditions

Sun Requirements

SunFull Sun SunPartial Shade

Requires 4-12 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.0 - 7.5(Neutral)

357912

Drainagewet

Water & Climate

Water Needs

High

Frost Tolerance

tender

Time to Maturity

weeks

Care & Maintenance

Care Guide

Grow Azolla pinnata on the surface of still or slow-moving fresh water in full sun to part shade. It draws nitrogen from a symbiotic cyanobacterium and needs no added fertiliser. Warm water and bright light drive rapid surface growth, while cold reduces the mats. Surface cover is thinned by hand to maintain open water for submerged plants and aquatic animals. Mats are contained within ponds or tanks to prevent spread into natural waterways, where the fern can become a weed. Fragments overwinter and regrow as water warms.

Pruning

Thin or skim surface mats regularly to maintain open water and limit shading of submerged plants. Azolla pinnata regrows quickly from remaining fragments, so removal is ongoing through the growing season. Excess material is composted or used as a nitrogen-rich mulch.