During several recent journeys through Willersey, in the north of the county, I had noticed that the ornamental pond on the village green seemed to have become largely covered by some kind of vegetative growth, much of it a reddish colour. I eventually took an opportunity to park nearby and had a closer look. It turns out that the surface of most of the pond has been colonised by the Water Fern (Azolla filiculoides), an introduced species from the Americas which was first recorded over here, at Pinner in Middlesex, in 1883. I have come across this fern from time to time but it is sensitive to our winter temperatures and I am not aware of any persistent colony in our part of the country. It is quite likely that there will be no sign of it at Willersey next year!
Water Fern is very different from any of the other ferns found in the British Isles. It does not root itself in the soil, but floats free on the surface of still waters it has colonised. The individual plants remain tiny and reproduce readily by simply breaking apart. By this means the plant can spread quickly across the surface of a suitable body of water.
Although it is not well-known in this country (as a non-native, it has been given rather limited cover in the standard field guides) Azolla is a significant member of the flora elsewhere, particularly in rice-growing areas. Pockets within the leaf lobes floating at the water surface usually house the nitrogen-fixing cyanobacterium Anaboena azollae. In China and Vietnam the native Azolla pinnata was cultivated for hundreds of years so that the nitrogen rich ferns could be used as fertiliser for the rice crop; quite recently A. filiculoides has largely taken over this role as it has proved to be a little more cold-tolerant and a great deal less susceptible to insect attack. Azolla is also used as animal feed in its native regions, and for mosquito control (principally by denying egg-laying females access to the water surface).
Like other ferns, Water Fern can reproduce by means of spores, but very little seems to be known about the extent and significance of this within the British Isles.
Elsewhere, attempts to stimulate significant spore production for commercial purposes have apparently failed, so taking advantage of the plant’s capacity for prolific vegetative reproduction remains the only viable option where cultivation is practised. It is likely that accidental and deliberate introductions by aquarists account for much of the Water Fern’s British distribution but transport of spores and plant fragments on the feet and feathers of birds is also a possibility.
There is evidence that Water Fern has become more common in recent years; this could be a reflection of our changing climate. Perhaps we should be showing more interest in this introduced fern that may have become an established resident, particularly in view of the unique features of its biology and lifestyle. It would be interesting to know whether it is, in fact, persistent at any Gloucestershire sites and whether there is any obvious pattern to its occurrence in the county.