Grasses: Freshwater Wetlands
What are they?
This page aims to make the identification of grasses a little easier by bringing together the commoner species that are likely to be found in a particular habitat. This should help you to learn the more likely species that you will come across, though it should always be remembered that occasional individuals of other species may turn up in unexpected places from time to time.
Where are they found?
Although grasses don't grow as true aquatic plants, several species occur commonly as marginal, emergent species at the edges of ponds, rivers and other permanent or temporary water bodies. And one species - Common Reed - regularly forms dense, expansive colonies of stems that create an important wetland habitat in their own right. Also included here are grasses of wet meadows in valley bottoms and in winter-wet, meadow habitats near the coast.
Identification
Identifying grasses successfully requires at least a basic knowledge of the plant parts and thus it's useful to know the names of these parts (it's much easier mentioning lemmas and glumes if one doesn't have to describe where they are every time!) To assist in this, you can click here to reference a page that gives details of these features. A good rule of thumb when working through the identification of any plant is to work from bottom to top with the whole plant and from outside to the middle with the flower parts (i.e. root, stem, leaf, flowers... sepal, petal, stamen, stigma) and this rule is generally followed here, though of course grass flowers have glumes, lemma, palea, stamens, stigma.
One important point to remember with grasses is that the flower spike can look very different according to how far along the process from flowering to setting seed that it is. Many species begin with a tight, rod-like cluster which opens out when in full flower and then closes back up again after pollination. Thus, it is important to look carefully at the parts of the florets (individual flowers) regardless of the shape of the overall flower spike, just to be sure. To get around this variability, botany books often write descriptions of grasses when they are said to be 'at anthesis'. This simply means at the point that the anthers open to release the pollen and this ensures that we are always looking at the flowers at the same point in their development. Grasses can still be identified outside of this point, but you need to understand that the overall spike might look different (the smaller flower parts will generally not change much, however). To simplify this a little, I have used the terms 'early flower spike/spikelets/florets' for spikes where the flowers have not yet reached anthesis; 'flowering spike' for plants at or around anthesis; 'late flower spike' for spikes where the flowers have passed anthesis and are progressing towards seed set. Where these terms are not used, you can assume that the flower spike doesn't change significantly in appearance over the course of the flowering period.
While this page contains the species that you are most likely to come across in most situations, it should be noted that most of them have look-alike, closely-related species. Therefore, it is recommended that you follow the link given for each species to the page where it will be compared in greater detail with possible confusion species, as a way of confirming your identification. With this group, it is also worth checking grasses found in meadow habitats (which are often in river corridors and valley bottoms) and also the Top 20 List, a number of which occur commonly in drier spots within wetlands.
Common Reed Phragmites australis
A native perennial, common to abundant in all kinds of damp places and well-known as the principle and dominant species of reedbeds, from where it is regularly cut to provide material for thatching. Flowers August to October. Plants 100-300cm in height with strong, creeping rhizomes that produce dense, single-species stands. Leaf blades 10-30mm wide, grey-green, hairless, flat and long-pointed. Leaf sheaths rounded, usually smooth, strongly overlapping around the stem. Ligule a line of long, white hairs. Flower spike narrow when young, spreading before anthesis and remaining open with age; a feathery panicle containing large numbers of spikelets. Spikelets 10-16mm, long and narrow, purplish and with 2-6 florets. Lemmas awnless but finely pointed.
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Reed Canary-grass Phalaris arundinacea
A native perennial, common to abundant in a wide range of damp places but especially fen and carr habitats and along the edges of permanent water bodies. Flowers June to August. Plants 60-200cm in height with strong, creeping rhizomes that produce dense, single-species stands. Leaf blades 6-18mm wide, green to grey-green, hairless, flat and long-pointed. Leaf sheaths rounded, smooth. Ligule membranous, 2.5-16mm long. Flower spike narrow when young, spreading at anthesis and closing again as the seeds develop; typically carried well above the leaves on upright stems. Spikelets 5-6.5mm, whitish or tinted with purple, each with a single, fertile floret and the minute lemmas of two 'lost' florets. Glumes as long as the spikelet, sharply keeled. Lemmas awnless but finely pointed. Remarkably similar to Common Reed in overall appearance, but note the membranous leaf ligule and the structure of the flower spikes. In winter, the dead leaves persist on the old stems, while in Common Reed they drop off, leaving leafless stems.
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Purple Small-reed Calamagrostis canescens
A native perennial, growing in peaty fens and wetlands, particularly in the Broads and along the Little Ouse corridor. Flowers June to July. Plants 60-120cm in height, forming loose tufts or patches. Leaf blades 3-6mm wide, bright green, gracefully arched, rather rough and with short hairs on the upperside, long, flat and pointed. Leaf sheaths rounded, smooth. Ligule membranous, 2-6mm. Flower spike 10-25cm long, narrow when young, becoming very open and spreading at anthesis and becoming narrow and upright again with age. Spikelets 4.5-7mm with a single floret, reddish-purple in colour. Glumes 4.5-5.5mm long, membranous narrowly lanceolate and fine pointed. Lemmas 2-2.5mm with a tiny awn at the tip and surrounded by a plume of white hairs that are just longer than the lemma. Compare with other small-reeds.
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Reed Sweet-grass Glyceria maxima
A native perennial, common along the margins of rivers, lakes and ponds. Flowers June to August. Plants 90-250cm in height with strong, creeping rhizomes that produce single-species stands. Leaf blades 7-20mm wide, bright green, hairless, flat and abruptly pointed at the tip. Leaf sheaths rounded towards their base, keeled towards the leaf blade, smooth. Ligule membranous but quite stiff, 3-6mm long. Flower spike feathery when young, spreading at anthesis; typically carried well above the leaves on upright stems. Spikelets 5-12mm, oblong to narrowly oblong, slightly compressed, with 4-10 florets. Glumes broadly ovate, membranous. Lemmas 3-4mm long, rounded on the back, obtuse at the tip and having 7 prominent veins. Compare with other sweet-grasses.
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Floating Sweet-grass Glyceria fluitans
A native perennial, common along the margins of rivers, lakes and ponds as well as in seasonally wet grazing marsh and low-lying ground. Flowers May to August. Plants to 100cm in height but often much less and typically forming creeping mats that spread out onto the surface of open water. Leaf blades 3-10mm wide, green, hairless, flat and abruptly pointed at the tip, flat or folded along the midrib. Leaf sheaths hairless, smooth, often purple-tinted. Ligule membranous, 5-15mm long. Flower spike 10-54cm long, typically forming a slender, wand-like panicle, the branches spreading at anthesis and later closing together. Spikelets 18-35mm, narrowly oblong, slightly compressed, with 8-16 florets. Glumes oblong or elliptic-oblong, membranous. Lemmas 5.5-6.5mm long, rounded on the back, obtuse at the tip and having 7 prominent veins; overlapping broadly at first, but later curling in at the margins. Compare with other sweet-grasses.
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Creeping Bent Agrostis stolonifera
A native perennial, abundant in grassy places, especially on damper soils in river floodplains. Flowers July to August. Plants 10-80cm in height and spreading by long, creeping stolons to form large, spreading patches. Leaf blades 0.5-5mm wide, hairless, finely pointed at the tip, rolled when young but eventually flat. Leaf sheaths rounded, smooth. Leaf ligule membranous, medium to long. Flower spike very narrow when young, open and spreading at anthesis then closing again at fruiting. Spikelets tiny, 2-3mm, clustered towards the tips of very fine branchlets with a single floret in each spikelet. Lemmas not awned. Compare with other bents.
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Marsh Foxtail Alopecurus geniculatus
A native perennial, common and widespread on damper ground, especially along river valleys and in coastal grazing marshes where it grows well in areas that are flooded in winter and poached by livestock. Flowers June to August. Plants 15-45cm in height, the stems blue-green, spreading from the base then bent at the nodes, often creeping to form spreading rafts on wet areas. Leaf blades 2-7mm wide, hairless. Leaf sheaths rounded and smooth, the uppers slightly inflated. Ligule membranous, 2-5mm long, pointed at the tip. Flower spike 15-70mm long, narrow and upright. Spikelets 2-3.3mm long, each with a single floret, densely packed in the spike. Glumes greenish-white with a narrow green stripe, free more or less to their bases and hiding the lemmas and florets. Lemmas with an awn arising from just above the base and exceeding the glumes by 1.5-3mm. Compare with other cat's-tails.
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Tufted Hair-grass Deschampsia cespitosa
A native perennial, frequent to common in woodland glades and shady places on heavier soils and land prone to being wet in winter. Flowers June to August. Plants 10-200cm in height, forming a large and dense tussock, often in colonies. Leaves 2-5mm wide, flat or rolled, green, hairless, strongly ribbed above. Leaf sheaths rounded or slightly keeled, smooth. Ligule membranous, long, up to 15mm in length. Flower spikes tall, graceful and open, 5-50cm in length with many spikelets. Spikelets 2-6mm, 2-flowered, clustered towards the ends of thin and sinuous branchlets. Glumes about as long as the spikelet, membranous and shiny. Lemmas 3-8mm, membranous, toothed at the tip.
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Meadow Fescue Schedonorus pratensis
A native perennial, but also included in grass seed mixes in new sowings. Widespread but rather thinly distributed and most typically a constituent of grassland on old commons, but also frequent in water meadows and floodplains. Flowers June to August. Plants 45-90cm in height, sometimes forming tussocks, but may also be loosely scattered, especially where regularly mown or grazed. Leaf blades tough, long strap-like, 3-8mm wide, hairless and glossy beneath, rough on the margin and tapering to a fine point. Leaf sheaths rounded, not fused together around the stem and ending in two, wing-like auricles that do not have bristles on their margins. Ligule membranous, short. Flower spike narrow when young, spreading at anthesis and with the shorter of the two lowest branches bearing no more than two spikelets. Spikelets 10-20mm with 5-14 florets. Lemmas awnless or rarely with an awn no more than 1.4mm long. Compare with other fescues.
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Meadow Barley Hordeum secalinum
A native perennial. Widespread in our region in heavier soils, especially where prone to winter flooding and notably frequent in coastal grazing marshes. Flowers June to july. Plants to 80cm in height, forming loose tufts with non-flowering shoots present at flowering time. Leaf blades 2-6mm wide, lightly hairy or smooth. Leaf sheaths rounded, the lower ones hairy, the uppers hairless and with a very short auricle or without auricles. Ligule membranous, very short. Flower spike upright but not stiffly so, 2-8cm long. Spikelets each with a single floret, attached to each other in groups of three, the larger middle floret bisexual, the smaller outer two male or sterile. Central florets with lemmas 6-9mm long and narrow, with an awn up to 12mm in length. Care should be taken in separating from Wall Barley, which can be common on floodbanks in some places. Compare with other barleys.
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Purple Moor-grass Molinia caerulea
A native perennial, favouring wet, peaty ground in fens, dune slacks and damp heaths. Flowers July to September. Plants 15-130cm in height, forming tight tussocks. Leaves 3-10mm wide, flat, rough at the margins and usually with a few long hairs towards the base; long-tapered to a fine point. Leaf sheaths rounded. Ligule a ring of stiff hairs. Flower spikes rather variable, but typically well-branched, narrow and wand-like, at first stiff and straight, later arching, up to 65cm in length. Spikelets 3-7.5mm, 1-4 flowered. Glumes shorter than the lemmas and typically purple-tinged. Lemmas 3-6mm, firm and smooth. A distinctive, late-flowering grass, forming dense tussocks of tough leaves topped with long, wand-like flower spikes, all of which persist well into winter as brightly straw-coloured clumps.
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Whorl-grass Catabrosa aquatica
Native. A patch-forming perennial, growing on damp mud in water meadows and alongside rivers and ponds, often forming floating mats. Generally uncommon except along section of the Bure and Wensum in Norfolk and parts of coastal Suffolk. Flowers May to July. Plants 5-75cm in height with stems often bending upwards from a prostrate base. Leaf blades 2-10mm wide, hairless and smooth, blunt at the tip. Leaf sheaths compressed, smooth. Leaf ligule membranous, 2-8mm long, whitish. Flower spike open and spreading at anthesis with the branches typically angled slightly downward. Spikelets 3-5mm, loosely 1-3 flowered. Lemmas not awned. The flowering spike is rather like the larger meadow-grasses in general appearance, but the florets are loosely carried in the spikelets (not densely-packed) and the habitat is unlike that of the meadow-grasses.
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Manchurian Wildrice Zizania latifolia
Introduced from eastern Asia as an ornamental and recorded from a garden site in the Norfolk Broads, where it is spreading rapidly and may become established further afield. A plant of wetlands with complex flowerheads that have male and female spikelets carried in different panicle branches. However, the species has not yet been recorded flowering in the UK. Vegetatively, plants may grow to two metres in height with sword-like leaves up to 5cm wide and bearing serrate margins and midrib, capable of inflicting a cut. Leaf ligules membranous, 5-15mm long.
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