Plant of the week – September 20th 2021 – Common Reed (Phragmites australis)

The Common Reed is said to be the most widely-distributed of all flowering plants. It is recorded in every continent except Antarctica. In Britain it has the distinction of being the tallest non-woody native plant. It is often 3 metres tall but can be up to 5 metres elsewhere in the world. Phragmites is commonly seen by lakes and rivers, ditches, marshes, swamps and fens. It can form huge populations (reed-beds), as it does in East Anglia. It needs a good supply of nutrients and a pH that is not too acidic (greater than 4.5, and preferably near-neutral). It grows best in still and shallow waters but tolerates slightly salty conditions and can survive even in metal-polluted waters.

Phragmites australis in flower, Kipi Koovi, Saaremaa, Estonia. Photo: Chris Jeffree.

It has served humankind very well. Most famously, it was the main material used traditionally by thatchers throughout Europe to cover roofs. It can be used as fodder, for fence making, or as a source of cellulose. In other cultures it has been used for pens, calligraphy and Japanese flutes. Fishermen may unwittingly be using it whenever they cast their line into a pond – it is often used to make the float. As we learn to reduce our consumption of plastics, look out for drinking straws made from Phragmites stems instead of non-degradable plastic ones.

Distribution of Phragmites australis – all continents except Antarctica. Image from GBIF.

Another way in which Phragmites is becoming increasingly useful to humans is in the design of constructed reed beds for treating waste-water. When planted on a suitable substrate, sewage-contaminated water is purified and can be discharged safely into rivers (Drizo et al 2000). Other species can be used, but Phragmites is preferred for its high tolerance to adverse conditions.

Phragmites reed bed, Dunsapie Loch, Edinburgh. Photo: Chris Jeffree.

What are the secrets of its success? It has an underground rhizome system, which anchors the plant to the sediment and stores starch, from which it grows vigorous shoots in the spring. There can be as many as 100 shoots per m2 and a shoot may grow 3-4 cm in one day. It can also sprout creeping shoots, above water level (stolons). A clump can extend 10 metres in one year, given favourable conditions. Some of the highest rates of biomass production ever recorded are from Phragmites. However, its remarkable growth rates make it both a friend and a foe. In Britain, it is definitely a friend of conservationists.  Reed-beds form the habitat of several rare birds: the most important are the bittern, marsh harrier and bearded tit. But it has become a foe of conservationists in North America where it is thought to be an invasive alien (although this alien status is disputed). There, it outcompetes native vegetation and reduces biodiversity.

It is more common in England than elsewhere in Britain. However, Spence (1964) reported pure stands of Phragmites in 15 Scottish sites from Sutherland to Galloway (and there are surely more). When not in pure stands it grows with Sparganium minimum (Least Bur-reed), Equisetum fluviatile (Water Horsetail), Ranunculs lingua (Greater Spearwort) and various other macrophytes. It doesn’t tolerate high elevation sites: its limit in Britain may be 445 metres above sea level.

The upper leaves are usually blown to the lee side of the stem (like a flag). It happens because the inner surfaces of the long leaf-sheaths are unusually well-polished (Arber 1934).

I have often admired this plant: the sight of it swaying gently in the wind and the sound of the leaves and culms rubbing against each other never fails to raise my spirits. I was pleased to be involved in research on Phragmites in 1992. We investigated the impact of very high CO2 on plants. We worked at a site in Tuscany, central Italy, where CO2 escapes naturally from vents at a mineral spring. In the early morning the concentrations of CO2 near the vent were highly toxic, over 70%, and the air temperatures rose to over 30 oC (a mini-greenhouse effect). The early-morning conditions of no air movement meant that the CO2 accumulated. Being denser than air, it formed an invisible ‘lake’ with which we amused ourselves by floating balloons on the ‘lake’ (see the image below, and Van Gardingen et al 1995). The ‘soil’ appeared to be little more than carbonate rocks. Conditions were so severe that we were unable to use our instruments until later in the day (temperatures of our gas analysers exceeded their specified operational limits). Next to the CO2 source, the only plant species surviving these extremes was Phragmites australis. A little further away, Agrostis stolonifera (Creeping Bentgrass) appeared. Even further away, the vegetation was quite diverse. I thought this was a nice demonstration of the extreme resilience of the Common Reed.

Shorter forms of Phragmites growing at a CO2 vent in Tuscany, Italy. The CO2 forms a ‘lake’ on which we were able to float balloons. Photo: Chris Jeffree.

“Reed’ is a general term that can refer to many different species. Perhaps at school you recited the poem by Elizabeth Barrett Browning. “What was he doing, the great god Pan, down in the reeds by the river?” Yes, Pan was making his pipes from reeds; perhaps Phragmites, although there are other possible species. In the biblical story, Moses first appears as a baby in a floating reed basket. The English translation of the reed is ‘bullrushes’ but it could have been made of Phragmites. More likely, it was Cyperus papyrus, a tall sedge, known simply for parchment-making as ‘papyrus’ . Reed boats are made even today in South America, and the ancient Egyptians used them too. The best reeds to be found in these parts of the world are probably not Phragmites; in Egypt they would have been Cyperus papyrus. Reed instruments in the modern orchestra are a different matter. They use Arundo donax, which is quite similar to Phragmites.

Oh, if you are interested in thatching in Scotland take a look at this site: many different materials have been used: https://www.buildingconservation.com/articles/thatch-scotland/thatch-scotland.html.

References

Arber A (1934) The Gramineae. Cambridge University Press.

Drizo A, Frost CA, Grace J and Smith KA (2000) Phosphate and ammonium distribution in a pilot scale constructed wetland with horizontal subsurface flow using shale as a substrate. Water Research, 34, 2483-2490

Packer JG et al. (2017) Biological Flora of the British Isles: Phragmites australis Journal of Ecology, 1123-1162. https://besjournals.onlinelibrary.wiley.com/doi/full/10.1111/1365-2745.12797

Spence DHN (1964) The Macrophytic Vegetation. Pages 306-425 in The Vegetation of Scotland, ed. By JH Burnett. Oliver and Boyd, Edinburgh.

van Gardingen PR, Grace J, Harkness PD, Miglietta F & Raschi A (1995). Carbon dioxide emissions at an Italian mineral spring: Measurements of average CO2 concentration and air temperature. Agricultural and Forest Meteorology73, 17-27. https://doi.org/10.1016/0168-1923(94)02176-K

John Grace

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