As it’s restricted to those areas in southern England where there is chalk and limestone, I hadn’t seen Horseshoe vetch (Hippocrepis comosa) until my recent visit to the Isle of Portland, or perhaps I should say that I hadn’t recognised it. The plant was, no doubt, growing all around me, especially when I explored the former quarries, but I hadn’t looked at it closely enough to realise it was different from the Bird’s-foot trefoil that it superficially resembles.

Both are members of the pea family, so the individual flowers are similar but the 5 to 12 individual flowers of Horseshoe vetch grow together in whorls, a structure that helps to identify them. I wondered whether this shape was the reason for their common name, but, in Flora Britannica, Richard Mabey notes that each flower develops into a pod ‘which breaks up into a number of horseshoe-shaped segments’.

As I noted in yesterday’s blog post, Horseshoe vetch is the larval plant for the Adonis blue butterfly (Polyommatus bellargus), which is why the populations of that butterfly are restricted to where the plant grows. The leaves of Horseshoe vetch are also used as a food plant by the larvae of the Chalkhill blue and Dingy Skipper butterflies, though the latter species uses several other plant species, meaning its distribution is not as restricted as the Adonis and Chalkhill blues.