There are over 400 species of Geranium in the world as a whole, and we have 30 of them in the British Isles (Stace 2019). They are known as cranesbills because the fruit resembles the head and beak of a Crane, with its own special explosive method of dispersing seeds. Many have very small pink (sometimes bluish) flowers and it can be hard to tell one species from another.
Flowers, leaves and fruits of G. lucidum. Leaves are sparsely hairy, petals are 8-10 mm long. Images: ©Chris Jeffree
The Shining Cranesbill is one of my favourite cranesbills. Given the right angle of the sun, it really does ‘shine’ as its name suggests. Smith et al (2000) describe it as being ‘scarce’ in Edinburgh and the Lothians and it was probably always rather scarce in the rest of Scotland. I started noticing it less than 10 years ago, but now it seems to be on the increase. Perhaps the increase is simply because it is an attractive little plant that people want in their gardens or on their walls.
Image to show the arrangement of leaves, the ‘shine’, and the red pigmentation. Photo: ©John Grace
It is a native plant, first described in 1633 by Thomas Johnson, the botanist and soldier, famous for revising Gerard’s Herbal by correcting mistakes and adding eight hundred new species. He called it Geranium saxatile, and saxatile means ‘living or growing on or among rocks’. He found it growing on the banks of the highway leading from Guildford to London. Today, according to Stace (2019), it is found on ‘bare ground, rocks, walls and stony banks, mostly on calcareous ground’. Unsurprisingly, given the prevalence of urban structures made with calcareous mortar, it has become a city-dweller. The new Atlas says its habitat includes “the base of mortared walls, churchyards, roadsides, waste ground and railway ballast”. It is also found as a ‘weed’ in gardens and allotments, but I’ve noticed that many people tolerate this weed above all others, regarding it as an interesting guest, bringing colour to the garden especially in the month of May. It likes rather open conditions, and it tolerates drought quite well, but it needs soils of rather high pH with good fertility. Thus, most gardens and allotments suit it quite well, but being physically rather delicate it doesn’t withstand much competition from the more thuggish perennial species that commonly invade and shade.
Masses of plants, showing the invasive tendency, Leith sewage works. Image: ©Chris Jeffree
It is however regarded as a noxious weed in many parts of the North America, having been introduced there in 1971. I find it hard to believe that our rather humble weed could ever be as devastating as claimed by one Canadian authority in their Invasive Species Alert Sheet. They assert the dangers it poses as:
Displaces native vegetation through aggressive and rapid spread, decreasing foraging capability for wildlife and decreasing local plant biodiversity, also increases erosion.
Threatens species at risk by dominating habitats and altering vegetation.
Dominates understorey in home gardens, parks, and other habitats.
Can contaminate plant nursery stock and be accidentally introduced.
Are they even talking about the same plant as we are? Well, they are, according to the photographs in their article. I presume this to be yet another example of how a species becomes more vigorous when transported to a far-away region, away from the normal pests and diseases of its native land. But I have never seen diseased or infected plants of this species around here (Scotland). There are records of only two caterpillars that are known to feed on it. The Royal Horticultural Society says it is susceptible to powdery mildew; but in general there appear to be no significant biotic controls. It may of course be limited by soil and climatic conditions here. Jeremy Bartlett, in a very recent blog, has suggested it may be spreading to more acid soils.
Growing on a tiled roof. Image: ©Chris Jeffree
The seed dispersal mechanism of cranesbills is intriguing. You can watch a short video here. In Edward Salisbury’s 1961 book Weeds and Aliens some of the variations in the structure and mechanism are explained. He writes that seeds (or in the case of G. lucidum the carpels that contain the seeds) are projected “as much as 20 feet” (i.e. 6 metres). The forces involved and the exit velocity must be considerable. Several papers have been published in physics journals but I confess I haven’t understood them yet. A full analysis would have to include the surface tension forces that develop as water evaporates, and the potential energy locked away (like a coiled spring) in the microfibrils of the cell walls.
Flowers and fruit of Shining Cranesbill growing at my allotment. Photos: John Grace
I suspect many of the seeds are thrown only short distances from their parent. The plants often appear as small groups, year after year in the same place. As they are annuals, the repeated annual appearance implies weak dispersal (unless of course, they are sometimes covert perennials despite being labelled ‘annuals’ by nearly all authors). Dispersal by humans on feet or motor tyres may explain the gradually increasing records in the BSBI’s map of the British Isles.
Distribution of G. lucidum in the British Isles, from BSBI Maps. Left: pre-2000, right: from 2000 to now.
The global distribution pattern, from the GBIF website, suggests G. lucidum has not travelled as much as many of the species we’ve looked at. There are a few recent records from Australia and New Zealand on the GBIF map but these are not shown in Plants of the World Online, which may be a little out-of-date.
Global distribution of Geranium lucidum according to GBIF.
The challenge of identifying one cranesbill from another is considerable. Differences in leaf shapes are subtle, but in the still-much-used earlier flora (Clapham et al 1987) and in a BSBI blog you can find helpful line drawings. Copyright law prevents me from copying them to this blog, but I know my colleague Richard Milne is working on a photographic comparison which we may be able to show soon.
References
Bartlett J (2023) blog Shining Crane’s-bill, Geranium lucidum.
Clapham AR et al (1987) Flora of the British Isles (3rd Edition). Cambridge.
Smith PM et al (2000) Plant Life of Edinburgh and the Lothians. Edinburgh University Press.
Stace CA (2019) New Flora of the British Isles. C&M Floristics.
Botany in Scotland 