Vernal Violets

Last week I received my copy of the BSBI's new handbook Violas of Britain and Ireland, by Mike Porter and Michael Foley. It's a pretty little photographic guide, and as it's the time of year for many violets I thought a blog post would be apposite. I hope it prompts a few readers to buy a copy and/or to have a closer look at the violets growing around them.

In Oxfordshire we have ten of the fifteen UK Viola species and six of the eleven hybrids. The species have been variously divided at subspecies and variety level but these have not traditionally been recorded in the county, or records are only of the nominate species (e.g. Viola tricolor subsp. tricolor). The other infraspecific taxa tend to be found in habitat types that we don't have (e.g. the dunes favoured by Viola tricolor subsp. curtisii), but four of the at least six varieties of the sweet violet Viola odorata have been recorded. I will just talk about the eight taxa (five species) I have found around Islip - were I to venture to Otmoor then I'd be able to add a few others, such as the rare fen violet Viola persicifolia.

As quite a large genus by British standards it is best to start by dividing the species up into groups: first the pansies and violets, and then subdivisions of the latter. This is how all the keys work and is easy to remember.

Violets

Colour variants of Viola odorata: the deep violet and hairy var. odorata (top), and white and hairless var. imberbdis (bottom). These seem to be the commonest variants hereabouts.

The violets (Section Viola) are all perennials and have reduced stipules, which are usually small, thin or membranous, and have fimbriate margins. One of the first things to look at when confronted with a violet is the sepals: if they are short and rounded at their tips then you are looking at either the sweet violet V. odorata or the hairy violet V. hirta; if long and pointed then some sort of dog violet.

The mebranous, fimbriate stipule of a violet, from what I assume is a cultivar of Viola riviniana, never having seen wild plants with this purple colouration

The blue-tinted flowers of Viola hirta, here growing in chalk grassland.

Sweet or hairy?

The sweet violet is so-named for its sweetly-scented flowers that appear early in spring, but the most distinctive feature is its round, cordate and bright green leaves. It is also stoloniferous, forming often large patches in sheltered grassy places such as hedgebanks. The flower colour varies from violet to lilac, rose to white - most of these forms are recognised taxonomically as varieties. For example var. imberbis has a white corolla with a pinkish spur and the petals are hairless.

The hairy violet favours alkaline soils and has quite large, attractive, blue-tinted flowers, contrasting with the purples/lilacs of many of the other species. Of course it is also a hairy plant, with a dense covering of patent hairs (at right angles) to the petiole. The leaves are the more typical pointed heart shape of most violets and it is a clump-forming plant. I expect it was once commoner on the limestone-derived soils around the edge of Otmoor, but I've been able to find only one in the whole 10km square (SP51).

The sweet and hairy violets do hybridise (V. x scabra) but it has rarely been recorded in the county, once since 2000. It is quite a vigorous plant and may be out there in limestone areas.

Common and early dogs

Now we come to the most similar species, the common and early dog violets, V. riviniana and V. reichenbachiana. Thus far I have only been able to find these in the village itself; as most of the verges and hedgebanks hereabouts are very rank perhaps these few are all that's left. The early dog violet is mostly a plant of ancient woodland and hedgerows in England and lowland areas of Wales, and scattered across Ireland. The common dog violet is a more cosmopolitan species, being found across the UK and grows in a variety of habitats, including the habitats of V. reichenbachiana but also heathland and a variety of grassland types, particularly maritime grassland on cliffs in the west.

The two species are quite well separated when they're behaving themselves (see the caption below) but intermediates do occur, but very rarely has anyone recorded their hybrid (V. x bavarica) in the county. If you find intermediate populations then I'd be most interested to learn of them; perhaps you'll also be able to identify one or the other of the species more confidently nearby. A table of comparative features between these three taxa can be found in the handbook. Also in the Plant Crib.

Viola reichenbachiana with (top) thrown-back upper petals, dark spur and small sepal appendages (flaps at the top of the sepals), and (bottom) narrow upper petals and little-branched veins on the lower petal. V. riviniana has a spur paler than the rest of the petals, broad upper petals and more branched veins on the lower petal, and the sepal appendages are conspicuous.

Pansies

The pansies (Section Melanium) differ from violets in being mostly annuals and having hollow stems and very leafy stipules. Islip has two pansy taxa, the field pansy Viola arvensis, which is a very common plant and grows abundantly in the arable fields hereabouts, and the garden hybrid V. x wittrockiana, which grows on the allotments. The true wild pansy V. tricolor seems to have declined greatly in Oxfordshire; I've yet to see it in Islip where it has been recorded in the past, or indeed elsewhere in the county. 

The diminutive yellow or cream-coloured Viola arvensis, a common arable weed. Wikimedia Commons