A honey bee drinks from a drop of water trapped in the bristly leaf of a floating fern.
This South American native doesn’t look like a typical fern and doesn’t behave like one either. A small drift of Salvinia minima, floating in a pool in the Jardin des Plantes, has the appearance a delicate curiosity. Its common name of water spangles adds to the air of delicacy but, gone wild in much of the USA, the plant can be seriously invasive, choking waterways and smothering native aquatic plants.
It’s easy to see how this azolla or fairy fern could quickly become a problem. Confined in a circular barrier within the pond, the floating fronds cover the entire water surface with overlapping layers. Azolla filiculoides is native to the Americas. It was introduced to the UK around 1840 as an ornamental plant and has since escaped from cultivation to blanket ponds, lakes and canals. Apart from the smothering effect on native water plants, the plant has been held responsible for disrupting flood defences, killing fish, impeding water traffic and even drowning cattle. Where the water surface is completely covered unwary animals may wander out onto the water expecting to find dry land.
Fortunately there’s help at hand in the form of a very small North American weevil which eats only this one plant. The weevil has been recorded in Britain since the 1920s, where it probably arrived after hitching a ride on imported plants. In the wild it multiplies slowly and doesn’t make much impact on the spread of the azolla. A CABI research project found that rearing the weevils in captivity and releasing large numbers early in the season can completely clear the azolla from an area of water.
The fairy fern has several close relatives found in tropical regions around the world. Some species in some places are regarded as noxious weeds, others (or the same species in different places) are welcomed as a useful part of traditional agricultural systems. According to an article on Feedipedia (supported by the FAO) azolla has for centuries been used as a biofertiliser and green manure for rice fields in southern China and northern Vietnam. It has been used to feed poultry in Peru and dairy cows in India. Recent research projects have shown that it can successfully be fed to goats, pigs and rabbits. A strange plant with a surprising history.
(Click on either photo for a closer view)