Seasonal Wild Flowers

Latin name

Azolla filiculoides

Other names

Mosquito Fern, Fairy Fern, Fairy Moss

Family

Salvinaceae

Growing season

April to November

English distribution

It is scattered throughout the country.
.
For a map see the Botanical Society of Britain and Ireland

– Water Fern

Habitat

It is a neophyte which was first seen in the UK in the

1880s.

It grows in canals, rhynes, ponds, ditches and in

slow moving sections of lakes and rivers.

It can be very invasive and completely cover water

bodies.

It is listed as an Invasive Alien by the NNSS (GB Non

-Native Species Secretariat) and was banned from sale

in the UK in April 2014 by a EU Directive and it is illegal

to dipose of it into the aquatic environment.

The pictures shown here for a rhine (drainage ditch) in

the Somerset Levels were taken in May 2019 and this

is the first time I have seen it for several years. It was

quite common in the Lancaster canal in the early 2000s.

In some countries, especially in SE Asia, Water Fern is

encouraged to grow and is used as a nitrogen-rich green

manure.

Description

Water Fern is a symbiotic, free-floating plant of fresh water.

It is the UK's only floating fern.

The symbiont is Anabaena (possibly Nostoc) azollae,

which is a nitrogen-fixing cyanobacterium. The

cyanobacteria were previously known as blue-green algae).

Stems are up to 10cm, usually less, and the 2-lobed leaves

are on either side of the stem. The leaves are small at  up

to 2.5mm.  In spring, they are green tinged with red and

by autumn they are mainly red.

The rootlets dangle below the floating plant.

The Anabaena is found in leaf cavities and fixes nitrogen 

at a high rate. The fern inhibits the nitrogen uptake

enzymes of the Anabaena and removes the fixed-nitrogen

using an alternative uptake pathway via the uptake cells.

This means that there is no feedback mechanism to tell

the Anabaena enough nitrogen has been fixed.