Narrow-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 latter can be checked by gently pulling at the stipule to see if it parts from the stem easily. The shape of the leaf tip is important as well as the appearance of the veins in narrow-leaved species. Holding the leaf up to the light to view the veins is useful, but it can often be necessary to use a binocular microscope to fully study these features.
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 Wide-leaved pondweed species, click here
Flat-stalked Pondweed Potamogeton friesii
Native. Widespread and reasonably common in the Broads and Fens but rare or absent elsewhere. Typically occurs in calcareous waters. Leaves are mucronate at the tip with most more than 2mm wide with 3 or 5 main veins but no finer veins between them. Stems without many closely placed lateral branches. Stipules fused into a tube round the stem towards their base when young. Fruits 2.4-3mm.
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Blunt-leaved Pondweed Potamogeton obtusifolius
Native. Rare and declining with few recent records except in the Norfolk Broads. Usually in acidic, peaty waters, often in shallow ditches and ponds. Leaves are minutely mucronate at the tip with most leaves more than 2mm wide with 3 or 5 main veins but no finer veins between them. Stems with many closely placed lateral branches, producing fan-like growths. Stipules not fused into a tube round the stem. Fruits 2.6-3.2mm.
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Lesser Pondweed Potamogeton pusillus
Native. Scattered throughout East Anglia in a wide range of water body types, including slightly brackish ditches. Leaves acute to obtuse and mucronate at the tip with most leaves less than 2mm wide and with mostly 3 (to 5) main veins. Stipules fused into a tube round the stem at their base when young. Fruits 1.8-2.3mm.
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Hairlike Pondweed Potamogeton trichoides
Native. Scattered throughout East Anglia in a wide range of water body types but most frequent around the Fens and Broads. Leaves acute at the tip with most leaves less than 2mm wide and with mostly 3 (to 5) main veins. Stipules green, not fused into a tube round the stem but with the margins overlapping. Fruits 2.5-3.2mm.
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Fennel Pondweed Stuckenia pectinata
Native. Widespread and found throughout much of the region, especially in the Fens and Broads. This is the species most likely to be found in brackish water in ditches near the coast and even in upper tidal zones on the larger rivers. Leaves narrowly acute to slightly mucronate at the tip and 0.2-4mm wide with mostly 3-5 main veins. Stipules distinct, fused to the base of the leaf long most of their length with just the top free and forming a short, stubby leaf ligule. Fruits 3.3-4.7mm.
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Horned Pondweed Zannichellia palustris
Native. Widespread and found throughout much of the region, though most frequently in the Fens and nearer to the coast, in lower reaches of rivers and in fresh and brackish water. Most readily told from the other pondweeds by its mostly opposite leaves and its small clusters of flowers that are carried close to the stem rather than on long stalks. Fruits with a distinct, spike-like beak, longer than that typically found on other pondweeds and giving the species its English name.
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Beaked Tasselweed Ruppia maritima
(Wigeongrass) Native. Uncommon and mostly in the south of our region, along the coasts of Suffolk and Essex, growing in muddy, brackish pools and ditches on saltmarsh and grazing marsh close to the sea. Best told from the other pondweeds by its mostly alternate lower leaves and opposite upper leaves and its small clusters of flowers that are carried close to the stem at first, but which then elongate greatly as the fruits develop. The fruits thus appear singlely at the ends of long stalks, rather than in a tight cluster. Leaves have a midrib but no side veins.
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Common Eelgrass Zostera marina
Native. Uncommon in tidal estuaries around our coastline, where once common but now greatly declined and seldom recorded. Grows in intertidal areas that are only exposed at the lowest spring tides, making discovery of this species difficult without a dedicated search. Stems creeping through the substrate beneath the water, with leaves arising singly from each stem node and with a short shoot in each leaf axil. Leaves up to 120cm long by 1-12mm wide, typically seen lying flat at low tide. Flowering stems terminal and much branched, bearing many spathes. Flowers simple, in small, linear groups, contained within a translucent sheath.
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Dwarf Eelgrass Zostera noltii
Native. Uncommon in tidal estuaries around our coastline, where once common but now greatly declined and seldom recorded. Grows in intertidal areas that are only exposed at the lowest spring tides, making discovery of this species difficult without a dedicated search. Stems creeping through the substrate beneath the water, with leaves arising singly from each stem node and with a short shoot in each leaf axil. Leaves up to 22cm long by 0.5-1.5mm wide, typically seen lying flat at low tide. Flowering stems lateral (appearing along the length of the creeping stems, not just at its tip) and unbranched, usually in clusters of 1-3 (or up to 6). Flowers simple, in small, linear groups, contained within a translucent sheath.
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