Geranium sanguineum – 4th September 2023 – Bloody Crane’s-bill

This is one of our prettiest wild flowers, and you will probably see it if you visit areas where the parent rock is lime-rich, especially with rocky outcrops. I’ve seen it at Arthur’s Seat (Edinburgh), Bennane Head (Ayrshire), Ardrossan (Ayrshire) and the Burren (County Clare, Ireland). I have distant memories of seeing it at the Great Orme (Denbighshire) and the Avon Gorge (Bristol). These places are all fairly near the sea, with base-rich or limestone rocks, and they are often windswept. It doesn’t mind the weather; it’s tough.

Clump of G. sanguineum at Inis Oirr, County Clare, Ireland. Photo: Chris Jeffree

Richard Mabey, in his Flora Britannica, says it is “one of the great signature plants of the northern limestone, especially the Yorkshire and Derbyshire Dales….”.

You will also see it in rock gardens all over the country, perhaps not always in its ‘pure’ wild form. The striking colour of its flowers, the long flowering season and its drought tolerance combine to make it a favourite with gardeners. There are many internet sites offering plants. The flower colour of the wild type is variously described as ‘vivid purplish-red’ or magenta. There are rare albino forms too. Cultivated forms are sometimes shades of pink.

G. sanguineum growing in a gryke on limestone at Carran Turlough, the Burren, County Clare. Photo: Chris Jeffree

It is a perennial with short rhizomes persisting and resprouting each year to form conspicuous clumps on many rocky hillsides. It has the largest flowers of our native Geranium species (petals 14-22 mm), and they are borne on longish stalks which wave around in the breeze. These large cup-shaped flowers attract bumble bees, hover flies and saw flies (Philipp and Hanson 2008).

G. sanguineum, flower structure. Five sepals and petals, stem hairy. There are two whorls of five stamens which are deployed serially – there is an inner blue set which is releasing pollen. Behind it a white set that is reflexed but will come forward to release pollen later. Photo: Chris Jeffree

The leaves are dark green and deeply cut, but in late autumn they usually turn red, as in many Geranium species. The English and French names include ‘bloody’ (or French ‘sanguin’), referring I think to the colour of the autumnal leaves although the flowers might be considered ‘bloody’ too.

Morphology of leaf: palmate and deeply dissected. Photo: Chris Jeffree

What is known of its ecology? I found rather little published research in my trawl of Google Scholar and the Web of Science. Most of the research has been on the possibility of finding new compounds to treat cancer and influenza but the results have been inconclusive. Surprisingly, there is very little about the ecology of this species and no major review or PhD dissertation.

Leaves in autumn. Attribution: Le.Loup.Gris, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons

I turned to PLANTATT, the data set compiled by Mark Hill and others, with ecological indicators derived from Heinz Ellenberg’s work in Germany (see here).  Then I stumbled upon a new and very similar data set, by Tyler et al. (2021) drawn up using information from vascular plants found in Sweden. Both data sets provide ‘scores’ to indicate tolerance of plants to light, moisture, soil reaction (acidity), nitrogen requirement and salt tolerance. For G. sanguineum both sources show the species to be found in nitrogen-poor soils, with little or no acidity, and tolerant of drought. The two data sets differ only in the stated tolerance to shade and saltiness. The Swedish data set indicates significant tolerance to shade and some degree of tolerance to salt.

These indications may be compared with the habitats mentioned by Stroh et al (2020) in the BSBI Atlas 2020. These authors state:  

“…base-rich grassland and scrub, open rocky woodlands, coastal cliffs and stabilized sand dunes; mainly on the coast but also inland on limestone pavements and cliff ledges, and in chalk and limestone grassland. As a native it has a curiously uneven distribution, and is often restricted to localized substrates such as dolerite and serpentine. Commonly grown in gardens, and frequently occurring as a garden escape or throw-out on grassy banks, verges, refuse tips and waste ground”.

Left, expansion of the style after petals have been shed. Photo: Chris Jeffree. Right, appearance immediately after ballistic ejection of seeds (from wikipedia, attribution is Geranium sanguineum by Pethan July 19, 2005 Mosel, Germany).

Seed dispersal in the Geraniaceae has intrigued naturalists and researchers alike (Yeo 2008, Abraham and Elbaum 2013). G. sanguineum has a mechanism known as ‘ballistic ejection’ whereby the seed is flung out by a spring action. The process starts once flowering is over; then, the style elongates to form a column. The fruit nestles at the base of this column, surrounded by the sepals. As the whole structure dries, tensions are set up in the tissues of the column until a critical point is reached when the outermost tissues (‘beak’) break away from the column, tearing the fruit into its five separate seeds. This happens in a split second, ejecting the seed and leaving the empty case at the end of the beak as shown in the image above.

I tried to observe this during one evening at the Bennane Head, but failed (I believe it was too late in the day). I did pick one that had already ejected, and took it home. I found that the angle of the beak could be adjusted even at this late stage by varying the humidity of the air (see the images).

Demonstration of the hygroscopic nature of the beak tissues. On the left, a specimen newly collected from the field; on the right, appearance after suspending the same specimen over water. Bringing it back into the dry air reversed the process.

The global distribution pattern shows how it has migrated to North America, presumably as an ornamental plant that has escaped gardens and become naturalised. There are a few records in Australia and New Zealand.

G. sanguineum, global distribution according to GBIF.

The distribution in Britain and Ireland is patchy, with an emphasis on coastal situations. Inland, the picture may reflect, in part, the pattern of horticultural trade. There are many locations where it was once recorded but now has disappeared. Analysis of these cases may show co-incidence with changes in land use.

Distribution of G. sanguineum in Britain and Ireland. From BSBI/Maps

What use is this plant, other than brightening the rock garden or herbaceous border? It is enjoying a ‘new life’ as a component of vertical gardens and roof top plantings. Its long flowering season, tolerance to drought and low requirement for nitrogen fertilizer, make it especially suitable.

References

Abraham Y and Elbaum R (2013) Hygroscopic movements in Geraniaceae: the structural variations that are responsible forthe coiling and bending. New Phytologist 199, 584-594.

Philipp P and Hanson T (2008)The influence of plant and corolla size on pollen deposition and seed set in Geranium sanguineum (Geraniaceae). Nordic Journal of Botany.

Stroh PA et al. (2020) Geranium sanguineum L. , in BSBI Online Plant Atlas 2020.

Tyler et al. (2021) Ecological indicator and traits values for Swedish vascular plants. Ecological Indicators 120, 106923.

Yeo PF (1984) Fruit-discharge-type in Geranium – its use in classification and evolutionary implications. Botanical Journal of the Linnean Society. 89, 1-36.

©John Grace