Wide-leaved Pondweeds
What are they?
Pondweeds form the major part of the family Potamogetonaceae and can be split conveniently into broad-leaved and narrow-leaved species. Most of them are submerged aquatic plants, either floating within the water column, below the surface, or with leaves lying at the water surface. Some species float free within the water, while others are rooted into the mud at the bottom of the water. These latter species may often also be found surviving on bare mud in areas where the water drops below the ground surface during the drier summer months.
Where are they found?
These are mostly plants of still or slow-moving waterways and are found in ponds, ditches, dykes and similar places. A few species may also be found in faster moving water, such as in the shallower reaches of the Little Ouse at Thetford. One or two species are more salt tolerant and are thus the species more likely to be found in coastal borrow dykes and bodies of water subject to occasional saltwater intrusion, but no species are truly marine or saltwater dwellers.
Identification
Pondweeds have either long, narrow, often grass-like leaves, or broader, elliptical to orbicular leaves - or sometimes a mixture of both, with the broader leaves at the water surface and narrower leaves below the surface. The leaves are usually accompanied by a stipule - a pale or translucent appendage at the base of the petiole, where the leaf joins the stem; the size and appearance of this stipule can be important to note as well as how and where it is attached to the stem. The shape of the leaves is important and both the shape of the leaf base where it meets the petiole and the appearance of the veins should be noted in broad-leaved species. Holding the leaf up to the light to view the veins is useful.
Flowering mostly consists of variably short to long, finger-like spikes of densely packed, petalless, green or yellowish flowers which often project above the water surface. These are followed by clusters of rounded fruits, typically with a short beak at one end as the remains of the stigma tube. Species identification can be easy in some species but much more tricky in others, especially the narrower-leaved species. In most species, the flower spikes are not useful in identification, though the fruits, if present, can be useful. Flowering may occur any time from late May/early June to September, mostly peaking in June-July.
Note that hybridisation is frequent and that some plants may be very difficult or impossible to identify for certain. For narrow-leaved pondweed species, click here
Broad-leaved Pondweed Potamogeton natans
Native. Widespread and our commonest broad-leaved species, occurring in a wide range of water bodies. Floating leaves opaque, typically rich green when fully grown, up to 12x6cm in size and with a distinctive, hinge-like and often discoloured section where the petiole meets the leaf blade. Secondary veins translucent against the light. Submerged leaves opaque, linear, often long and ribbon-like with a rounded tip. Fruits 3.8-5mm long.
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Bog Pondweed Potamogeton polygonifolius
Native. Widespread but rare outside of its main strongholds in bogs and ponds on acid soils, mostly in Norfolk. Floating leaves opaque, typically green when fully grown, up to 10.5x7cm in size and without a hinge-like section where the petiole meets the leaf blade. Secondary veins not translucent against the light. Submerged leaves opaque, spoon-shaped or lanceolate. Fruits 1.9-2.6mm long.
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Various-leaved Pondweed Potamogeton gramineus
Native. Once more widespread but now probably only occurs in two sites in West Norfolk and perhaps two or three places in the Fens. Prefers acidic waters. Floating leaves opaque, up to 9.5x3.4cm in size and without a hinge-like section where the petiole meets the leaf blade. Submerged leaves opaque, variable in shape, from spoon-shaped or lanceolate on long petioles to broadly ribbon-like. Fruits 2.4-3.1mm long.
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Shining Pondweed Potamogeton lucens
Native. Widespread but generally rather scarce with the best population in the Cambridgeshire Fens and Norfolk Broads. Leaves all submerged, translucent with an obvious network of veins, up to 20x6.5cm in size and rather soft and pliant. Fruits 3.2-4.5mm long.
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Perfoliate Pondweed Potamogeton perfoliatus
Native. Widespread but generally scarce away from the Cambridgeshire Fens and the larger Norfolk rivers. A very distinct plant, producing long, trailing stems. Leaves all submerged, distinct, with perfoliate bases that wrap around the stems, up to 11.5x4.2cm in size and rather soft and pliant. Fruits 2.6-4mm long.
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Curled Pondweed Potamogeton crispus
Native. Widespread in a wide range of water bodies and more tolerant of water polution or eutrophication than many other pondweed species. Leaves all submerged, translucent and with obviously wavy and minutely toothed edges, up to 2.5-9.5x0.5-1.5cm in size and rather soft and pliant. Fruits with a longer, more curved beak than other species, 4-6.2mm long.
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Opposite-leaved Pondweed Groenlandia densa
Native. Rather scarce and declining in East Anglia, probably because of a lack of faster-moving water, which it prefers, but seemingly still frequent in parts of the Fens, around Norwich and along the Waveney. Leaves all submerged and easily told from all other pondweeds by being opposite rather than alternate and imbricate (nested within each other).
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Cape-pondweed Aponogeton distachyos
(Water-hawthorn) Introduced from South Africa as a garden ornamental. The sole member of a largely tropical group of plants, the Aponogetonaceae, occasionally recorded from open waterways where it may have been deliberately planted, or as a survivor from discarded garden waste as it is popular as a garden pond plant. The small, white flowers appear during the summer months and are carried in a highly distinctive, branched cluster, with each flower bearing just a single 'petal'.
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