Duckweeds & Water-ferns
What are they?
Find a pond and the chances are there will be duckweed on it. These little plants float on the surface of still or slow-moving waterways and by late summer can often completely cover the water surface as they grow very rapidly. For a long time, duckweeds were placed in their own family, the Lemnaceae, but recent research has determined that they form a subgroup within the arum family (Araceae). They are all rather small plants, often consisting of just a single, leaf-like disc called a frond, which produce more discs vegetatively and these break off to form new plants. Their flowers are microscopic and only rarely produced.
Also included here are the mosquito-ferns (or water-ferns) which show similar behaviour in forming floating colonies of fronds that can rapidly cover the surface of a pond. They are very unfern-like and consist of a series of overlapping, scale-like fronds that are water repellent.
Where are they found?
These are plants of open ponds, lakes and slow-moving waterways, or sometimes on bare mud where a pond has dried up. Most species float at the surface but Ivy-leaved Duckweed floats below the surface in the water column.
Identification
A hand lens is often needed for identification of the species as the plants and their features are so small. Flowers are rarely produced so identification is based on features of the fronds. You should check for any obvious veins visible on the frond and whether there are any rootlets on the underside of the frond. Holding a plant up to the light to look at its cell structure can be useful. It should be noted that identification is based upon fully grown plants in late spring or summer - autumn and winter plants can be smaller and show variation in veining and are thus not reliably identifiable.
Common Duckweed Lemna minor
Native. Widespread in all types of wetlands throughout the region and typically the commonest species in most areas. Fronds 1-8 x 0.6-5mm, flattened on both surfaces and with 3 (sometimes 5) veins. Each frond develops a single rootlet. Fronds typically bright green; if there is any red colouration, it tends to be most obvious on the upper side.
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Least Duckweed Lemna minuta
Introduced from North America and first recorded in the UK in 1977. This species is now widespread in our region and care is needed to separate it from Common Duckweed, though it is probably most common in the Broads and in East Suffolk. Fronds 0.8-4 x 0.5-2.5mm, flattened on both surfaces and with just a single, obscure vein. Each frond develops a single rootlet.
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Fat Duckweed Lemna gibba
Native. Widespread in a wide range of wetland habitats but less common than Common Duckweed and typically found growing with it. Fronds 1-8 x 0.8-6mm, flattened on the top surface but strongly inflated beneath and typically with 4-5 veins. Each frond may develop a single rootlet. Fronds often with a reddish tinge which is most obvious around the margins. Note that starved or late-season fronds are often flattened and not swollen beneath. These can be very hard to tell from Common Duckweed but for the larger air spaces within the frond (more than 0.3mm across).
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Greater Duckweed Spirodela polyrhiza
Native. Found widely scattered in wetlands, especially in Fenland and Broadland but seems to be declining and is perhaps not tolerant of water enrichment or pollution. Fronds 1.5-10 x 1.5-8mm, flattened on both surfaces and with 5 or more veins. Each frond develops a cluster of several rootlets. Fronds often with a reddish tinge which is best developed on the underside and often visible from above around the margins.
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Ivy-leaved Duckweed Lemna trisulca
Native. Widespread and quite common throughout East Anglia. Fronds 3-15 x 1-5mm, flattened on both surfaces and with usually 3 veins. Differs markedly from other duckweeds in its unevenly angular leaves and in its habit of floating below the surface of the water.
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Rootless Duckweed Wolffia arrhiza
Native in southern England but only known as an introduction in East Anglia, where it has been recorded from Cambridge. Fronds minuscule, 0.5-1.2 x 0.4-1mm, globular, slightly convex on the top and greatly swollen beneath. The smallest flowering plant in Britain and easily overlooked, though the flowers have never been recorded in the UK and plants tend to replicate themselves by budding vegetatively.
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Pacific Mosquito-fern Azolla filiculoides
(Water-fern) Introduced from the Americas and occasionally escaping from deliberate introductions into garden ponds or from dumped garden waste. Has occurred sporadically at a number of locations in our region but seems not to establish itself for more than a few years. The fronds are green when they start to grow in spring and early summer but become strongly freshed with red later in the year, making them very obvious when they cover the surface of a pond. Note that there are six species of mosquito-fern which are all rather similar and require a microscope to tell apart. Only Pacific Mosquito-fern has officially been identified in East Anglia, but other species may be present.
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