Dog Violets are well known to be a difficult group due to variation and hybridisation. Classic ID features as described are often hedged with words like most, often, not always, sometimes....
A few photographs mainly of flower front views to illustrate the difficulties. Species covered are:-
Common Dog Violet ( V. riviniana)
Early Dog Violet ( V. reichenbachiana)
Heath Dog Violet ( V. canina)
Common Dog Violet ( V. riviniana) A classic example with typically wide upper petals. Photographed in Hardwick Wood, Cambs on 2nd May 2016. The veins on the lower petal are clear right to their tips and extend about half way down the petal. The white throat has the slightly darker purple band at its edge.
The wide upper petals can often be used to separate Common from Early DogViolet which typically has 'Rabbit Ears' that is narrow upper petals.
Next an example of a Common Dog Violet with atypical narrow upper petals.
This example has very narrow upper petals but you can just see the white spur which is a good indicator for Common Dog Violet in comparison to Early Dog Violet. These narrow flowered versions can make up to 10% of a Common Dog Violet population. The veins are not quite as clear as in the first example, not as clearly forked or as many but still more typical of Common than Early Dog. The white throat has the typical darker band at termination. This photo was on 18th May 2016 at Gamlingay Great Heath. No Early Dog Violets were found at this location, so the possibility of this feature being an example of a hybrid was discounted but Botanists seem to fall into two camps, Broad Species converts believe hybrids are very rare while others believe hybrids are common. In practice the distinction between species variation and hybridisation is very difficult. A small percentage of hybrids are fertile and back cross to parents to give intermediate features (Ref Hybrid Flora of the British Isles).
Are these narrow petal versions in fact hybrids?
(Update:- Recent research from Europe shows hybrids between Common and Early Dog Violets do not occur, except very rarely. A separate blog covers this see Violets label section) In Britain it is probably very rare and over recorded. Hybrids occur with other species of Dog violets like Heath Dog Violet.)
If all narrow upper petal variants consistently had other features which were towards the Early Dog Violet set, this might be believable but the example below shows a flower with quite narrow flowers but classic veins consistent with Common Dog Violet. Taken 5th May 2013, Oxwich Wood, Gower.
The photo right has been selected as the veins are very long reaching beyond 75% of petal length. It is another Common Dog Violet photographed on 5th May 2013 , Gower, South Wales. The flower has the wide upper petals and a really strong clear and forked vein pattern. This flower has quite small sepal appendages , about 1mm in length (slightly less than the stem thickness) and a almost white spur tinged with the slightest hint of purple.
Next photo is
Early Dog Violet ( V. reichenbachiana) which can often grow alongside Common Dog Violet, as in Hardwick Wood where this photo was taken. To date, all violets found at this site have been a distinct species and without any obvious hybrids but I will look again next season.
The dark purple spur can be seen behind the upper petals as the first indication that this is a Early Dog Violet ( however some Common Dog Violet can also have dark purple spurs which is not often mentioned in the field guides). There are only five veins and the tips are not clearly defined tending to blur out in the dark purple terminal band. This is typical of most Early Dog Violets. The upper petals are quite narrow but many have even narrower petals.
Second photo of a Early Dog Violet was taken in Eversden Wood, Cambs on 31st May 2014. I have added a side view showing the long dark spur, pointed sepals and sepal appendages ( very small upper three, lower much longer showing another variable feature. Sepal appendages can be useful as an ID feature when large or small compared to stem thickness. They are often in between at about same as stem thickness ( 1-1.5mm ) so there is overlap in this ID feature.
This flower has classic Early Dog Violet bold veins, thick not forking very much and a bit blurred at the tips where they just go beyond the white throat. This flower does not have the terminal darker band at the junction of the white throat and the purple of the outer petal. This flower also has the narrow upper petals 'Rabbit ears' look.
The third photo of a Early Dog Violet was selected as it has veins which are forked and unusually are quite clear but still blurred at their tips. The number is still limited to three main veins and a couple of outer faint ones. Still more typical of Early than Common but only just.
The veins are quite short extending only a third of petal length. This and the limited number of major veins plus the lack of forking does seem to be a consistent difference between the species. You can just see the dark purple spur behind the upper petals in this photo. The number of veins is yet another variable, Early Dog Violet generally has less but I suspect if you look at enough, some will not conform to a simple number count method of separation.
Next stage is to go back and look at Common Dog Violet in Ireland and consider the even more difficult Heath Dog Violet. The photo to the left has a pure white spur, quite wide petals and very clear veins that are well forked and extend beyond the purple band terminating the white throat. This photo was taken at Lough Allua in County Cork, Ireland on 15th May 2015. It was considered to be a Common Dog Violet.
Very similar to the previous photo except veins are much longer, going approximately two thirds down the lower petal. A feature not outside the range of Common Dog Violet. There is
no throat terminal purple band present. This photo was taken on 15th May 2015 on the South shore of Lough Allua in Co. Cork. It was regarded as a candidate for
Heath Dog Violet ( V. canina) although Lough Allua is well known for hybrids between Common Dog, Heath Dog and the rare Pale Dog Violet.
With any Violet ID, all features of the plant have to be taken into account from leaf shape, extent of hairs on leaf, sepal appendages, shape of the side teeth on stipules and structure. In this case leaf shape is much longer than wide and some but not all leaves did have the squared off base ( not heart shaped) . The side view shows a short spur, white with a slight yellow green tinge plus very large sepal appendages.
The main problem with Heath Dog Violet is finding one in the first place. For example in Cambridgeshire it has not been seen at its last stronghold of Newmarket Heath for several years. It is now effectively no longer present and declining rapidly in many places. Lough Allua has its problems with Hybrids so a trip to the New Forrest was undertaken.
The next photo was taken on 23rd May 2015 in the New Forest and was considered to fit with Heath Dog Violet with long leaf shape and stipules with forward pointing teeth. The flower shows very thin veins with limited forking in this case. No dark purple band at the edge of the white throat. This ID was going well until we found more plants with quite pale flowers and this raises the question of hybrids with Pale Dog Violet ( V. lactea). Heath Dog Violet is often stated to have very blue flowers but in Cork some Common Dog Violets also have blue flowers. In Europe it can have a real spread in flower colour (rarely white through to purple) plus it has two sub-species ssp. canine and sap.montana.
The final photo is a
Pale Dog Violet ( V. lactea) taken on 16th May 2015 at Lough Allua, Cork.
I am looking forward to the BSBI handbook on Violets due out in early 2017 as I am sure it will be a great help with this difficult group. In addition the recent 'Hybrid Flora of the British Isles' covers hybrids and adds information from Europe which can be very different from Britain.
Peter Leonard, Rampton, Cambridgeshire
December 2016
