Sedum acre in full flower. Image: Chris Jeffree.
The yellow star-like flowers of Sedum acre give delight at this time of year. Sedum belongs to a Family of succulent plants, the Crassulaceae (the stonecrops) famous for their unusual mode of photosynthesis, whereby they close their stomata during the day and open them to absorb carbon dioxide at night. More about that later.
Sedum acre, detail of flowers (star-like, floral parts are in fives). Image: Chris Jeffree.
Sedums have long been collected as ornamentals and grown in rock gardens. They are decorative, hardy and some are almost indestructible. Recently they have become one of the favoured groups of plants for those who design ‘green roofs’. Sedum acre in particular is often preferred, being undemanding and much easier to look after than a grass sward. Sedums have become big business, being used on the rooftops of industrial buildings such as the 22,000 m2 roof at the Rolls-Royce Motor Car factory in England, whilst in some European cities these plants can be found between tramlines with the claim that they beautify the track and absorb the noise.
S. acre has colonised this roof spontaneously. Image: Chris Jeffree.
Sedum acre forms an evergreen mat in stony habitats including rock outcrops, walls, cliffs, shingle, dunes and wastelands. It needs very little soil and is sometimes seen alongside pavements (Ballinger 2021). It can tolerate saline conditions and has been recorded at disused industrial sites including old lead-mine workings (Grime at al 1989) and oil-shale bings in West Lothian (Harvie 2006). The name, ‘sedum’ is from the Latin verb ‘to sit’, and indeed it forms a cushion over hard surfaces on which a weary traveller might sit. This species avoids acid soils and wetlands, and does not grow in the shade.
S. acre, growing on coastal shingle in Ayrshire, 8th June 2024, not quite in flower yet. Image: John Grace
It spreads by creeping stems to form a loose patch. Its fleshy leaves taste peppery, and hence the name ‘biting stonecrop’. The chemical compounds that give it the ‘punch’ are two alkaloids called sedamine and pelletierine, related to the highly poisonous coniine of Hemlock Water-Dropwort. Consumption of Biting Stonecrop is not recommended, although it was been used as a medicinal plant in former times. A cautious tasting to confirm identification will do no harm.
Sedum anglicum growing on a cliff at Bennane Head, Ayrshire on 1st June 2024. Image: John Grace.
Despite formidable chemical defences, the plant is a food source for several lepidopteran species. For example, at Holyrood Park in Edinburgh the larvae of the rare micro-moth Glyphipterix equitella feed on the leaves of both S. acre and S. anglicum, entering the leaves and ‘mining’ the juicy contents (Currie and Cathrine 2018). It is presumed that evolution has equipped the larvae with a mechanism to detoxify the peppery compounds.
S. anglicum growing with Wild Thyme Thymus polytrichus. Image: Chris Jeffree.
Stace’s 2019 New Flora of the British Isles lists eleven species of Sedum, but only three are native. The natives are S. acre, S. anglicum and S villosum. All three are declining according to the BSBI’s 2020 Atlas. The ‘odd man out’ of the three is Sedum villosum – it prefers colder, wetter habitats and has ‘headquarters’ in the hills of the Scottish borders. It is commonly seen in bryophyte-rich wet-flushes in sub-montane parts of Scotland and Northern England. It is also found at much higher elevation (over 1000 metres) on Ben Lawers in Perthshire. Its decline in the Scottish border region of Berwickshire was well-documented by Braithwaite (2010), who suggests various changes in land management are the underlying cause of decline.
Sedum anglicum in full flower. Image: Chris Jeffree.
Sedum anglicum is like S. acre but its petals are white, tinged pink. It prefers the more acid substrates found in western regions, and thrives on west-facing cliffs, despite being sprayed with saline during storms.
Most plants open their stomata to capture CO2 by day to form a 3-carbon compound, phosphoglycerate (the process called ’C3 photosynthesis’). But there are variants of this. One of them is widespread in succulent plants, particularly those in the Families Crassulaceae and the Cactaceae. It is called Crassulacean Acid Metabolism (‘CAM’) and occurs in about 7% of plants. CAM is a remarkable adaptation to enable water conservation in arid regions. The stomata close in the day to avoid water loss and open at night when the air is humid and the potential for evaporation is low. At night, CO2 diffuses into the leaf tissues to be incorporated into malic acid (a C4 compound). At daybreak the stomata close and the malic acid is broken down to release CO2 inside the leaf which is then captured by the same series of enzyme reactions as occur by day in C3 plants.
Sedum villosum, Hairy Stonecrop, a distinctly northern species. Leaves are 3–8 mm long and usually reddish and generally covered with hairs. Attribution: Manfred Morgner CC BY-SA 3.0 http://creativecommons.org/licenses/by-sa/3.0/, via Wikimedia Commons
In the 1970s, researchers in Darmstadt in Germany investigated CAM in Sedum acre (Shuber and Kluge 1981). They found it could do both C3 and CAM photosynthesis, switching the relative proportions according to conditions. In dry periods it would behave mostly in CAM mode but with plenty of moisture it would additionally perform C3 photosynthesis. ‘The best of both worlds’ was achieved.
Distribution in the British Isles of the three native species of Sedum: S.ac Sedum acre, S.an Sedum anglicum and S.v Sedum villosum. For S. villosum the map has been enlarged to make clearer the decline (see the key). From BSBI Maps.
As with many succulents the leaf surface of Sedum acre is waxy. I found reference to this in a report of the use of Sedum as a lubricant in pre-industrial Sweden. According to Svanberg (2006) people would lubricate cart-wheels with slugs (Arion species), whilst squeamish children used Sedum instead to ensure good running of the wheels on their toys. Sedum was also used as a polish.
The three species have remarkably different global distributions. From GBIF.
Sedum acre has been used as a medicinal plant, and various Sedum species may well have potential in the future. The web site of Plants for a Future does however strike a note of caution:
I trawled the internet for possible herbal remedies and found rather little. I was however struck by one offer (on Amazon.com): “Sedum Natural Herbal Extract for Sauna, Massage, Broom Soaking”. The ratings for the product were not very positive. The firm selling it seems to be Latvian. I used to enjoy a sauna when I visited friends in Finland but I was never offered Sedum.
References
Ballinger B (2021) Checklist of the Urban Flora of Scotland. Botanical Society of Scotland.
Braithwaite ME (2010) A wake-up call for the future of Britain’s montane flora? Sedum villosum in decline in Berwickshire. BSBI News 115, 15–17.
Currie N and Cathrine C (2018) Glyphipterix equitella (Scop.) survey at Holyrood Park in 2018.
Grime JP, Hodgson J and Hunt R (1989) Comparative Plant Ecology. Kluwer.
Harvie B (2006). The importance of the oil-shale Bings of West Lothian, Scotland, to local and national biodiversity. Botanical Journal of Scotland, 58, 35–47. https://doi.org/10.1080/03746600608685105
Schuber M and Kluge M (1981) In situ studies on crassulacean acid metabolism in Sedum acre L. and Sedum mite Gil. Oecologia 50, 82–87. https://doi.org/10.1007/BF00378797
Svanberg I (2006). Black Slugs (Arion Ater) as Grease: A Case Study of Technical use of Gastropods in Pre-Industrial Sweden. Journal of Ethnobiology, 26(2), 299-309. https://doi.org/10.2993/0278-0771_2006_26_299_bsaaag_2.0.co_2
© John Grace
Botany in Scotland 