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Rhododendron - Weed KillerWater Fern:

Azolla filiculoides (Closely similar species, sometimes found in the UK: Azolla carolinians)

Common names
Water Fern, Fairy Moss, Mosquito Fern, Red Water Fern

Introductory History
Water Fern, "Azolla filiculoides" is a free-floating aquatic fern, native to North and Central America, but is now distributed worldwide. It was introduced to the UK for aquarium and water garden use.

It is possible that it was also native to the UK in archaeological time, as spores have been found in sediments from the last interglacial period.

The date of modern introduction to these islands is uncertain, though it is mentioned in gardening encyclopaedias at least as far back as 1895. Wild populations have also been around for a long time, achieving local dominance here and there, as in the Kennet and Avon Canal in Wiltshire, which, during the 2nd World War in 1939, was so overwhelmed by Azolla as to resemble a red asphalt road. Local rumour attributed this to secret government action to hide waterways from enemy eyes!

Problems caused include shading out of suspended algae and submerged plants, thus disrupting the food web and plant/animal associations; deoxygenation leading to death of fish and invertebrates, with resultant damage to biodiversity and environmental degradation. Drainage can be impeded especially at sluices, locks and weirs. Water intakes, filters and pumps may become blocked. The possible danger of drowning of pets, livestock or small children has also to be considered as the solid appearing vegetable rafts will not bear weight and their is no limit to the depth of water on which Azolla may be found. The success of the species is helped by symbiotic association with an alga, which will be described in the following section.

Water Fern is a floating, perennial, heterosporous fern, occuring in any depth of still or sluggish water. It occludes large areas of water in the course of a season, and takes on a characteristic red colouration as temperatures fall in autumn, or when the plants are subjected to any kind of stress. Elliptic Laminae measuring up to 10cm across, are composed of minute, unequally 2-lobed imbricate, pea-green leaves, turning red or deep purple. Leaves measure 2.5mm x 1mm in size, and are membranous, with a broad margin and minutely papillose surface. The cavity of the upper lobes is filled with mucilage and the filaments of the cyanobacterium, Anabaena Azollae, which fixes atmospheric nitrogen. Its rhizomes are mostly bipinnately branched and densely covered with fronds above and bearing solitary roots below. The sori, surrounded by a sac-like indusium, are borne in pairs on lower lobes of the first fronds of a branch. Microsporangial sori globose, which measure 2mm in diameter, contain many long-stalked microsporangia. The megasporangial sori are smaller, and are reticulate at base, acorn-shaped, and contain one single megaspore.

Azolla filiculoides and its close relatives are free-floating ferns of cosmopolitan distribution. Because variations of form and growth habit are influenced by water chemistry, day length and temperature, exact certainty as to species requires microscopic analysis. Most populations in the UK are thought to be Azolla Filliculoides. Over wintering, submerged buds float to the surface to divide in spring and extend at a stupendous rate, producing large areas of water cover in one season. As with most invasive waterweeds, eutrophic conditions precipitate invasive behaviour. Spore production is also an important factor in persistence and distribution of the plant.

Sori produced during the late summer give rise to billions of vegetative spores which persist over winter and germinate the following spring, giving rise to small rooted bodies known as prothalli. These are the result of vegetative spore production. The prothalli develop egg or sperm producing structures, and eventually fertilization occurs. The fertilized egg subsequently matures and a new adult plant grows from the prothallus.

During the summer the plants appear green and moss-like, but develop the well known red colour as the mats build up and the plants are subjected to stress, or when temperatures fall in autumn

Inadvertent introduction and spread is rendered almost inevitable, as the smallest fragment is capable of instigating a colony.

As the plant is entirely free-floating, except for marginal individuals which may root in sediment, water depth does not restrict its growth. Turbulence or fast flow are not tolerated, and high flow can flush the weed out of the watercourse. Wind and moderate flows can cause the Azolla to bank up against obstructions to form thick layers.

The success of this genus, and its characteristic growth rate owes much to a symbiotic association with the alga Anabaena Azollae, which grows in the safety of the mucilage filled cavities of the upper leaves of the water fern. In return for this provision of environment, the algae fix atmospheric nitrogen, which can be easily assimilated by the fern for protein synthesis.

This nitrogen fixing property is put to good use in some rice-growing cultures, where Azolla is used as a fertilizer.

Control Methods
Azolla is susceptible to two herbicides cleared for use in the UK, Terbutryn (applied as a granular formulation) and Glyphosate. In both cases repeat applications will have to be made, if the presence of ungerminated spores is suspected, as neither chemical will destroy spores. Terbutryn will also kill submerged algae and some submerged plants, so undesirable consequential effects must be considered.

Herbicidal efficacy is reduced if the Azolla has covered the whole water surface, so prompt treatment is recommended.

Mechanical methods include harvesting by means of weed buckets, or temporarily raising the levels of flowing waters using baffle boards, subsequently removing the baffles, allowing the weed collected behind to be flushed away. These methods are likely to need repetition.

Biological control has recently been approved for use in this country. In South Africa, a biological control agent has had so devastating an effect on the plant that its threat to aquatic ecosystems was lifted in only three years, and the widespread deleterious effects on various uses of national water resources was much lessened.

This agent is the frond-eating weevil, Stenopelmus rufinasus, was imported from Florida in 1995. Following 2 years of host-specificity testing, introductions were made in 1997. Remarkable success and rapid distribution of the agent continued, with the majority of the original introduction sites being completely cleared of the weed. CABI Bioscience now offer to introduce this tropical weevil to sites across the UK, although critics remain sceptical of its chances of success and moral implications. There is a reasonable chance that it could establish in some regions, as adults remain active down to temperatures of 0°C-5°C and 50% of insects exposed to temperatures of -12.1°C for prolonged periods survived. Egg, larval and pupal stages are furthermore protected from extremes of cold by being within plant tissue. back to top

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