Floating Ferns

Azolla species
Family: AZOLLACEAE
There’s nothing like a bit of rain to cheer us up, is there?
A plant which is pure delight after rain is the little floating fern, Azolla filiculoides. Each frond carries a drop on its waterproof back, as if it’s holding it up for us to examine, saying, as a cat does with a mouse, “see, look what a precious thing I have caught!”
As children, my brothers, cousins, and I were fascinated by the expression “cats always land on their feet”. Being scientifically inclined, we tested the aphorism with the help of my cousins’ very patient ginger mog, and established its truth to our satisfaction. Old Pickles allowed us to drop him, upside down, three times before deciding that his interests lay elsewhere. Being kindly and considerate children, we had been careful to drop him from a good height to give him time to turn over. We performed the experiment by standing on a bed and dangling him by his legs over the floor, before letting go.
A similar trick can be done for the edification of children, and without incurring disapproval from those who think that dropping cats, from whatever height, is not the way to treat the family pets. Azolla ferns have water repellent backs. If you push them, upside down, under the water, they always float to the surface, triumphantly right-side up and dry.
Each plant consists of a single frond not much more than a centimetre long, with roots on the underside. In the shade they are green, but sunshine brings out the pink in them. They are annuals. No individual plant lives for longer than a season. They reproduce by division - an unusual method among plants.
Our two locals are members of widespread species, native to Australia and elsewhere.
Pacific Azolla, Azolla filiculoides (above) grows best in winter.
“Filiculoides” means “like little threads”, and each plant of this species has a generous bunch of threadlike roots which you can see below the water in the top picture. (Click on it for a close-up look. Note that each root is a SINGLE thread, like a strand of sewing thread - no little fluffy side branches.) It is the less vigorous of the two local Azolla species, and is a delight in a garden pond, ornamenting the water’s surface with pretty dapples of coral pink and green.


Ferny Azolla Azolla pinnata (below, both pictures) has a reputation for being “the weedy one”. Gardeners who are delighted by its delicate-looking and well-mannered cousin may shudder at the thought of growing this vigorous, summer-growing plant.
You can identify it by the different roots (its hanging threads have fine side-threads) - but you usually know if this is the one you have, just because it grows so fast. If you have a smothering blanked of fern, this is your species. It's the one that forms a brick-red sheet in the summer sunshine.
On a sheet of natural water, this fern is not a problem. It provides habitat for a multitude of little creatures, which become food for fish and birds. It reduces water temperature and evaporation, so our wetlands last longer into the dry season. It does no harm to stock which drink the water.
It can be a weed, though. 
It is often seen reproducing at a crazy speed in summer, doubling its biomass every two days and making a brick-red sheet which can cover dams, impede water flow and navigation, clog pumps, and starve other plants of light and oxygen.
The cause of out-of-hand growth is usually a high nitrogen level in the water - a common modern problem of agricultural techniques which don't necessarily get the mix of chemical fertilisers just right, or animal husbandry techniques where animal wastes make their way into the environment.


Added to this, Ferny azollas don’t just guzzle the nitrogen that’s available to them in the water. They  have the ability to fix more of it, from the atmosphere - part of the secret of their rapid growth-rate. They do need help from a friend to achieve it. A nitrogen-fixing bacterium of the blue-green alga type (Anabaena azollae) lives in little cavities on the backs of their leaves.
The side effect is that those little plants convert themselves into what can be used as a rich, high-nitrogen fertiliser - and they do it for free.

Rice farmers in Vietnam add chemical fertiliser to their flooded paddy fields, when the little rice plants are newly planted out. This produces such a thick bloom of Azolla pinnata that it smothers weeds, and is said to prevent mosquitos from breeding. When the fields are dried out, the fern dies, making a rich mulch which releases its nitrogen to the crop as it decays.
We could adapt the technique, scooping plants off dams and ponds to add to the compost heap or use as mulch. Killing the plants is not a problem. As soon as they dry out, they’re dead.
Meanwhile, Azolla’s ability to draw such a lot of nitrogen out of the water, doubling its leaf area every week, has another useful function. It has proven to be a valuable tool in control of the blue-green algae which have become such a problem in our inland waters.
 Azollas can clean up infected dams and waterways, starving the algae by their aggressive removal of nitrogen. The task needs to be completed, of course, by removing the ferns from the water. Leaving them to die in situ only lets the nitrogen be released back into the water. Harvesting is a relatively easy chore because of these little ferns' surface-floating habit. There's no need to remove it completely. The bits left will keep doing their job until a slow-down indicates that there's just not so much excess nitrogen in the water any more.
This practice can also help clear up nasty-looking (and perhaps nasty-smelling) water in garden ponds.
Azolla is also used as a nutritious fodder for cattle, pigs, ducks and chickens.








Both Azolla species are very tolerant of pH variations, thriving in water from 5.5 to 7.5.