Plant of the week – September 13th 2021 – Purple Oxytropis (Oxytropis halleri)

This is a native species in the pea family (Fabaceae). It is rare in the UK, where it is confined to Scotland.

Purple Oxytropis (Oxytropis halleri) is a hairy perennial with a tuft of 9-15 pairs of leaflets with a single terminal leaflet. The hirsute flowers are in clusters of 6-10 and are usually purple, although occasionally white. The lower petal has a sharp point. This species flowers between May and July. O. halleri is larger than the slightly more common Purple Milk-vetch (Astragalus danicus), with which it is sometimes confused.

Oxytropis halleri in full flower at Shandwick, Easter Ross. Photo: Brian Ballinger

Oxytropis halleri usually grows in base-rich locations, but there are exceptions to this. It is often on steep unstable slopes, although also occurs on shingle. Elsewhere in Europe it is a mountain plant, but in Scotland it has an unusual distribution, being seen on a few mountains in Perthshire and Argyll  but also on the coast, mainly in the north.

I first saw this species on Ben Vrackie in Perthshire where it grows near the summit with some other rarities. It took a couple of visits to locate it, although it is quite near a path. It is also on a few of the other mountains in this area.

In contrast it is also to be seen on coastal sites, mainly in north Scotland, apart from an outlier in the south-west. A location on the Forth was lost long ago with the building of the railway.

Site at Shandwick, Easter Ross.

The first coastal location I visited some years ago, is where it grows on the north coast near Bettyhill, on shingle and the low cliffs.  My main memory of this visit is of nearly getting stuck on the cliff whilst trying to photograph it. It also is recorded further along the north coast towards the west.

My main recent involvement has been with the three coastal sites in Easter Ross, where it has maintained a precarious presence. Here it grows on a steep coastal site on the sandstone cliffs near Shandwick, where a good size population persists. This is a very limited area and the slope is unstable. Nearby are Rock-rose (Helianthemum nummularium) and Carline Thistle (Carlina vulgaris). This site is prone to gorse invasion, which, if not managed, could be a serious threat.

Oxytropis halleeri appearance after flowering. Photo: Brian Ballinger

Another well recorded site is on the steep and unstable cliffs by the North Sutor near the entrance to the Cromarty Firth. A scattered population of plants persists here and access to monitor them is hazardous because of the difficult terrain. Nearby is Bloody Cranesbill (Geranium sanguineum). Ivy and Gorse grow here and are likely to be a threat, being liable to spread.

Oxytropis halleri: distribution in the British Isles. Reproduced from BSBI

The third Easter Ross population, further south on the Black Isle by Eathie, slowly grew smaller over repeated assessments early in the 2000s and appeared to have become extinct recently. A re-introduction using native stock grown by the Edinburgh Botanic Garden was undertaken but the outcome is not certain so far. This site has been damaged by inappropriate conifer plantation and is liable to Gorse and scrub invasion. Access up a crumbly slope is very difficult which makes management a problem. Unfortunately, the conifers were planted before the SSSI designation was in place.

Some of these coastal sites may be vulnerable to temperature rises with global warming but it would be shame to lose this very attractive plant.

References

Butler, K. (2009) Wild Flowers of the North Highlands of Scotland. Berlinn, Edinburgh.

Scott, M. (2016). Mountain Flowers. Bloomsbury, London.

Wigginton Ed (1999). British Red Data Book. I Vascular Plants. JNCC. Peterborough.

Brian Ballinger

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