Species showcase: Cottongrass

‘Sedges have edges’ is a mantra that many of us learn at university or during our early conservation careers! And cottongrasses certainly follow that trend - despite their name they are in fact a type of sedge from within the sedge family, Cyperaceae. 

Introduction 

Cottongrasses (Eriophorum species) are flowering plants that truly stand out on peatlands at the start of summer due to their distinctive white, fluffy seed heads that resemble cotton wool. Whilst grass leaves are flat, sedge leaves are v-shaped in cross-section and their flowering stems are not round but instead have angles and are typically triangular in cross-section. Cottongrasses are perennial and have been known to live up to 120-140 years in the Arctic tundra¹. They flower in April or May, but their small flowers are inconspicuous compared to the cotton-like seed heads that develop over the summer (May-September) and transform the appearance of the peatlands they inhabit. The fluffy fibres of the tufty seed heads help in wind dispersal of the seeds and insulate the reproductive organs of the plant by trapping heat, allowing cottongrasses to thrive in cold environments.  

Cottongrass species prefer open, wet habitats such as peat bogs, fens and bog pools from lowlands to mountains. These plants are widely distributed across the northern hemisphere, being particularly abundant in Arctic tundra environments that support extensive peat bogs. Around 25-30 species of cottongrass have been identified globally, and four can be found in the UK, ranging in height from 15-75 cm. 

Cottongrasses of the UK  

Common cottongrass (Eriophorum angustifolium): This is the most widespread species in the UK. This species prefers acid conditions, develops 1-7 seed heads and is often found growing in shallow bog pools alongside Sphagnum species. You may come across this species all around the UK, but it is especially well-established in northern and western regions. The tips of the leaves of E. angustifolium typically turn a bright red colour towards the end of the summer.  

Hare's-tail cottongrass (Eriophorum vaginatum): Also widespread in the UK, it is easily distinguished from other species because it is the only cottongrass that develops a single, upright seed head. E. vaginatum can form tussocks on acidic peatlands². The dense tussock growth form is particularly pronounced in drier, degraded peatlands and tussocks confer a degree of resilience to fire. You are less likely to see this species in the south-east of England, but it is widespread in the north and west.  

Broad-leaved cottongrass (Eriophorum latifolium): Less common across the UK, this species develops 2-12 seed heads. E. latifolium prefers a more basic soil and is often found in calcareous, stony mires and base-rich meadows². This species is more common in Scotland and northern England, but also occurs in scattered communities in Wales and parts of southern England. This species can appear similar to common cottongrass but the tips of the leaves typically turn a yellow, rusty colour as opposed to the bright red of the more common species.  

Slender cottongrass (Eriophorum gracile): The rarest cottongrass species in the UK, slender cottongrass forms 3-6 seed heads and is found in wet neutral to slightly basic mires². It is listed as ‘Near Threatened’ on the IUCN Red List of Threatened Species³. In the UK, it has been recorded in a few locations scattered around England and Wales. 

It can be a challenge to tell cottongrass species apart, especially if the seed heads have not yet developed. A detailed identification guide with visuals can help in the field. Sedges of the British Isles by Jermy et al. (2007)² describes all four cottongrass species found in the UK, and apps such as the Explore Moor app, developed by Moors for the Future, help to identify the two most common species.  

Cultural and historical importance  

The little fluffy white seed heads, waving in the wind are so conspicuous on the open moors that it’s not unsurprising there have been multiple uses of cottongrass over the centuries. Cottongrass was used as a raw material for textiles in central Europe in the 18th and 19th centuries⁴. The fibrous material occurred as mats in peat deposits, which was used in peat fibre factories to produce yarns, weaving mats, horse blankets, isolation cords, adsorbent wadding and even paper.  

Mixing cottongrass fibres with wool or cotton created a useful cotton substitute, as cottongrass seed head tufts are too brittle to be used on their own. The seed head tufts have also been used to stuff pillows in Sussex, England, to make wicks for oil lamps and candles⁵ and as fodder for animals. During World War I, cottongrass was collected and used in Scotland to dress wounds⁶. 

Outside of Europe, the edible seeds and stems of common cottongrass have been used in traditional Native American cuisine⁷, whilst the leaves and roots have been used for medicinal purposes due to their astringent properties.

A host, a home and a meal: the role of cottongrass in peatlands 

On peatlands, cottongrasses often extend above the Sphagnum moss layer. Their presence contributes to the structural diversity of an open peatland and creates a unique microclimate that is beneficial for Sphagnum by regulating light, temperature, and humidity⁸. Like Sphagnum, cottongrass is a key peat-forming species. 

Hare’s-tail cottongrass is the main foodplant of large heath butterfly larvae (Coenonympha tullia) which have also been found to feed on common cottongrass⁹. Large heath butterflies are widespread in boggy habitats in parts of Scotland and Ireland, but the species has significantly declined in England and Wales. The stems of common cottongrass also support caterpillars of the purplish-brown Haworth’s minor moth (Celaena haworthii) which inhabits fens and upland acid moors⁹. The flower buds that appear in early spring are attractive to grazing animals. Cottongrass also acts as a host to a range of fungal species, such as Myriosclerotinia ciborium, Hysteronaevia advena, Lachnum imbecille and Lophodermium caricinum¹⁰. 

Indicator of peatland condition 

The dominant growth form of cottongrass on a peatland can depend on the available moisture and competing species in the environment. A thick Sphagnum layer that carpets a healthy, wet peatland usually features single-stem growth forms of cottongrass, whilst the lack of Sphagnum on peat surfaces under drier conditions encourages the growth of dense cottongrass tussocks¹¹. Draining of peatlands reduces the abundance of plants that depend on waterlogged conditions, including cottongrass, and favours non-wetland species like heather. Drainage is considered one of the causes of decline in common cottongrass population in the lowlands – for example, most populations have been lost in Oxfordshire¹².   

Tussock-forming cottongrass species are also highly fire-resistant, and short return times of human-induced management fires (10-15 years) tend to encourage dominance of hare's-tail cotton grass at the expense of other peatland species. Although cottongrasses are important peat-forming plants, the tussock growth form is usually associated with initial stages of peat formation and thus often becomes dominant after a peat bog has suffered a set-back (such as a fire or grazing)¹³. Peatlands that have lost their moss-rich carpet to fire struggle to retain water, as water can easily flow over the surface between the tussocks that now dominate the landscape. Hare's-tail cottongrass is therefore used as an indicator species that helps to assess the condition of its habitat. In a healthy blanket bog, hare’s-tail cottongrass should not exceed 75% of the vegetation cover – higher values suggest that the condition of the bog is not ideal¹⁴. 

Cottongrass – a methane chimney?  

Like many sedge species, cottongrass stems contain spongy tissue called aerenchyma made up of large air spaces. This tissue allows gases to travel from the atmosphere down to the roots, providing the cottongrass roots submerged in water-saturated, low-oxygen peat with a supply of oxygen. However, the stems can also act as 'chimneys' and allow methane (CH₄) to move in the opposite direction and escape from the peat to the atmosphere; this can increase methane loss from the peatland. On the other hand, the leakage of oxygen through the roots into the peat can allow for local oxidation of methane to CO₂, reducing methane emissions from the area¹⁵. Sedge-dominated areas of peatland are generally associated with higher methane emissions compared to areas with other peatland vegetation, but this loss of carbon could be offset if the sedge communities absorb more atmospheric CO₂ than is released from the peat¹⁶. Methane emissions from peatlands are natural due to the waterlogged nature of the peat, and sedge species such as cottongrass play an important role in the gas exchange between peat and the atmosphere.  

Cottongrass in peatland restoration 

Due to their resilience, cottongrasses are one of the plants that are used to help restore areas degraded by erosion. Cottongrasses are effective nurse crops, stabilising the bare peat and facilitating the growth of other species, such as Sphagnum. In fact, common cottongrass is usually one of the first species to naturally recolonise eroded blanket bogs, establishing in the more stable, sheltered areas of the damaged peatland.

Cottongrasses can also be used in restoration of drained peatlands: low stone dams have been used in parts of the UK to block larger gullies and trap peaty sediments to restore the hydrological function of peatlands. Planting cottongrass behind dams where sediment has settled may speed up the blocking effect as the roots help to stabilise the peat¹⁷. However, whilst cottongrasses tend to be resilient, they are not immune to peatland degradation and drainage. Climate change and pressures from land use change can lead to reductions in, or even loss of cottongrass populations, as is evident today in Oxfordshire.    

From windowsills to wetlands: GroWet helps to save historic freshwater habitats 

Initiatives exist that work to ensure peatland species like cottongrass will continue to thrive in the UK. GroWet is a citizen science initiative from national wildlife conservation charity Freshwater Habitats Trust. It gives people of all ages and from any background the opportunity to help save historic wetland and freshwater habitats in a hands-on way. 

More than 1,000 people have been involved so far by growing rare plants at home or in community centres. Once the plants are big and strong enough, Freshwater Habitats Trust staff and volunteers plant them out in wild places. With many ‘lost’ plant species being reintroduced, GroWet is bringing back wildlife to regenerate freshwaters, fens and other wetlands. 

The GroWet plants include 15 native species which were once plentiful but are now in decline – such as common cottongrass, which is now scarce in Oxfordshire. Some species are so rare they are at risk of becoming extinct in England without the sort of help being offered through GroWet.  

The initiative is part of Freshwater Habitats Trust’s vision to build the Freshwater Network and create a wilder, wetter, cleaner connected network of freshwater habitats to reverse the decline in freshwater biodiversity. GroWet started in Oxfordshire in 2022 and schools and community groups in Buckinghamshire can now get involved. Freshwater Habitats Trust has developed a schools pack to engage children as young as five with GroWet, which you can find on the GroWet page of the Freshwater Habitats Trust website. 

Watch the GroWet video on YouTube: GroWet - the Oxfordshire-Buckinghamshire Freshwater Network (youtube.com).

References:  

1. Thomas DN. Arctic ecology. 2021. Hoboken, NJ: Wiley-Blackwell

2. Jermy AC, Simpson DA, Foley MJY, Porter MS. Sedges of the British Isles. 2007. BSBI Handbook no 1: Edition 3. 

3. IUCN. Eriophorum gracile (Slender Cottongrass). Eriophorum gracile (Slender Cottongrass) (iucnredlist.org) [Accessed 13th May 2024]. 

4. Korhonen R. The suitability of the cotton grass fibres for the raw material of the textiles - studies from Finnish peatlands. Suoseura - Finnish Peatland Society. 2012; 63(1): 15-18. 

5. Svanberg I. The Use of Rush (Juncus) and Cotton-Grass (Eriophorum) as Wicks: an Ethnobotanical Background to a Faroese riddle. Svenska Landsmål och Svenskt Folkliv. 1998; 323:145-157. 

6. Beardshaw C. The Natural Gardener: Lessons from the Landscape. 2003. Trafalgar Square.

7. Moerman DE. Native American Food Plants: An Ethnobotanical Dictionary. 2010. Timber Press.  

8. Titterton P, Maynard S, Fry C. A report into the impacts cutting cotton grass (Eriophorum spp) has on the growth rate of Sphagnum moss spp. 2022. Moors for the Future Partnership, Edale, UK. 

9. Butterfly Conservation. Large Heath. Large Heath | Butterfly Conservation (butterfly-conservation.org) [Accessed 10th May 2024]. 

10. Hallgrimsson H and Eyjólfsdóttir GG. Íslenskt sveppatal I – smásveppir [Checklist of Icelandic Fungi I – Microfungi]. Fjölrit Náttúrufræðistofnunar. Náttúrufræðistofnun Íslands [Icelandic Institute of Natural History]. 2004.  

11. Lindsay R, Birnie R, Clough J. IUCN UK Committee Peatland Programme Briefing Note No 2 Peat Bog Ecosystems: Structure, Form, State and Condition. 2014. 2 Biodiversity final - 5th November 2014.pdf (iucn-uk-peatlandprogramme.org) 

12. Freshwater Habitats Trust. Common Cotton-grass. Common Cotton-grass - Species Directory - Freshwater Habitats Trust [Accessed 10th May 2024]. 

13. Lindsay R, Birnie R, Clough J. IUCN UK Committee Peatland Programme Briefing Note No 8: Burning. 2014. 8 Burning final - 5th November 2014.pdf (iucn-uk-peatlandprogramme.org)  

14. Joint Nature Conservation Committee. Common Standards Monitoring Guidance for Upland Habitats. 2009. Common Standards Monitoring Guidance for Upland habitats (jncc.gov.uk) 

15. Agethen S, Sander M, Waldemer C, Knorr K-H. Plant rhizosphere oxidation reduces methane production and emission in rewetted peatlands. Soil Biology Biochemistry. 2018; 125:125–135. 

16. Baird AJ, Holden J, Chapman PJ. A Literature Review of Evidence on Emissions of Methane in Peatlands. Defra Project SP0574. 2009.  

17. Thom T, Hanlon A, Lindsay R, Richards J, Stoneman R, Brooks S. Conserving Bogs: The Management Handbook. Conserving Bogs The Management Handbook 2nd Edition.pdf (yppartnership.org.uk)