Western Gorse Ulex gallii is a member of the Fabaceae, or pea family with 2 fused lower petals which are keel shaped, 2 wing petals and a standard petal at the top. There are 3 main species of gorse in Britain which are Common Gorse Ulex europaeus, Western Gorse Ulex Gallii and Dwarf Gorse Ulex minor.
Ulex europaeus is the most readily identifiable being more robust and having larger petals and calyx. The other two are both dwarf plants but are not so easy to differentiate. Both species are fairly plastic in growth form and variable in size, and calyx length seems to be the best guide.
‘Perhaps the best character is calyx length, which it is usually possible to examine all year round since they remain on the plant: mean calyx length of less than 9.5mm …are found in populations of U. minor and mean values greater than 9.5mm in populations of U. gallii..’ (Webb pp127-8)
Stace also describes not only the calyx length but also the shape of teeth on the lower calyx lip.
Fortunately in Britain the 2 species have markedly different distributions (though this is not the case on the continent) with U gallii being more or less restricted to the Western seaboard and U minor mainly in the South East of England. However note this quote from the Plant Crib
Calyx length scarcely changes between anthesis in late summer and dehiscence of the pods in the summer following, so it a very useful character. Identifications using single measurements are likely to result in 5-10% misidentifications, whilst means of 10 measurements will almost always place a plant unambiguously. Ulex gallii is much more frequent in the west, but does occur rarely in the east, and has been under-recorded in northern Scotland.Ulex minor is now increasingly being found outside its traditional stronghold of SE England, and could be more widespread still (Rich and Jermy)