Thursday, June 5, 2008

Floating Ferns

Azolla species
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 and below) grows best in winter.
“Filiculoides” means “like little threads”, and each plant has a generous bunch of threadlike roots which you can see below the water in the top picture. (Double click on it for a close-up look.) It is the less vigorous of the two, 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) 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 plant.
However, one man’s weed is another man’s high-quality home-grown organic fertiliser. Don’t reject this one out of hand just because it reproduces 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 nitrogen.
Ferny azollas don’t just guzzle the nitrogen that’s available to them in the water. They also 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. This trick gives them high potential an as organic fertiliser which we can grow for ourselves.
Rice farmers in Vietnam add 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 (but I don’t think you should count on this). 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. The excessive use of nitrogen fertilisers causes the problem. When it runs off into bodies of water, it feeds the algal blooms. Azollas can clean up infected 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 there only returns the nitrogen to the water. Harvesting is a relatively easy chore because of their surface-floating habit.
Leaf colour is affected by sunlight, age, and available nutrients, all of which convert them from green to red. A prettier, variegated colour can be produced in new leaves, by harvesting the old. 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 species are very tolerant of pH variations, thriving in water from 5.5 to 7.5.

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