A good clue is in the names of these close species, Hairy Violet has hairy leaf stems whereas Sweet Violet has either no hairs or very short hairs. Although this is basically true, in practice it is often difficult, not only because Sweet Violet is very variable but also these two close species do hybridise.
As with many challenges there is nothing like a little practice to gets ones eyes tuned in by closely looking a few plants of each species. The real problem is the hybrids can occur anywhere and are probably more common than the distribution map would indicate. Intermediate plants will occur and many will not be the vigerous F1 type but long term stable variations produced by back-crossing. The BSBI Plant Crib 1998 states these hybrids can be quite variable and not easy to tell from either parent.
Basic ID Features.
First step is to check the sepals are blunt ended to exclude all the dog violets. This is (as far as I can tell) an absolutely reliable feature and also Dog Violets are a separate enough never to hybridise with the Hairy/Sweet Violet pair. See previous blog for diagrams of this feature.
Second feature of the Hairy/Sweet pair is that all stems come from a basal rosette unlike Dog Violets that have stems that divide (with leaves and flowers).
Sweet Violet (and the hybrid) do push out stolons ( shoots that stay close to the ground which the form another basal rosette a few cm from the parent plant.) The presence of stolons is diagnostic for Sweet Violet ( also the hybrid often have them). Small plants of either species of course may have not developed stolons yet, also they can be hard to find if the plant is growing in grass.
What is Hairy.
Above photo shows leaf of Hairy ( Viola hirta) and Sweet Violet ( Viola odorata). The leaf stem ( petioles) are clearly different in the length of the hairs. If all leaves were this simple.
First problem is that not all leaf stems in Hairy Violets have these long hairs, many have no hairs especially early grown leaves. I have seen plants that have only one stem hairy and four without hairs, so it must be possible that occasional plants could completely lack this feature. Fortunately at least one stem usually shows it true hairiness.
Photo above shows Hairy Violet basal rosette with lower leaf stem showing no hairs ( lower right) and other stems with very prominent hairs. The key feature of these hairs is the length at about 50% of stem thickness (or even slightly more up to about 1mm long) and the way they sick out at 90 degrees (patent). Best to look for the new leaves that are also hairy and have the most hairy stems. Go by the most hairy stems as the indicator.
Example of really hairy leaf stem. Leaf shape tends to be more pointed than Sweet Violet which has a more rounded end. Any plant with these more pointed longer leaves is worth a closer look as it is a good pointer for Hairy Violets, however some pure Sweet Violets can have untypical leaf shape very similar to Hairy. Leaves can be quite hairy as well as stems. Both species have hairs on the margin of the leaf.
Three examples based on the most hairy leaf stems present on a plant.
Photo above is a Hairy Violet leaf stem with typical hairs about 50% of stem thickness and pointing out at right angles (patent) giving it a quite spiky appearance. Some plants can have even longer hairs.
Photo above is a Sweet Violet. Short length hairs (15% of stem thickness) are about as long as these hairs get on Sweet Violet. many plants have no hairs on leaf stems.
Photo above is a hybrid known as Viola x scabra Intermediate length hairs (25% of stem thickness) can occur on some stems of Hairy Violet but if this the longest hair type present probably indicated a hybrid Hairs approx at 45 degrees to stem which gives a much less spiky appearance. Combination of quite long hairs at about 45 degrees seems a very good feature of hybrids. Hybrids are often vigorous with large leaves but not always.
The highly vigorous F1 hybrid normally have these medium length hairs at 45 degrees.
Example follow to show this.
14th April 18. Bedford Gap, Fleam Dyke, Cambs. Example from vigorous hybrid clump showing long angled hairs.
Same plant , example of flower. The very pale throat of this example is found in both Hairy Violet and the hybrids. It is also probably present in some Sweet Violet. In fact flower colour and amount of violet streaking in the throat or whether lateral hairs are present or not does not seem to help in separation of Sweet and Hairy Violets. Just too much variation present. At the tip of the orange conal appendages which terminate the anthers ( the pale part just visible ) is the style which is normally bent down at the tip. On the right of this photo is a part eaten flower as some bug has decided not to enter by the front door to get at the nectar but just eating through the spur. Quite a common route.
How do you spot a F1 hybrid, see next photos. Not that difficult. Group of Hairy Violets at Madingley. 30th April 2014. Both Sweet and Hairy are present at this site plus a few vigorous hybrids.
Secondary feature Sepal appendage direction.
The leaf stem hairs are the key ID feature however the BSBI Handbook no 17 , Violas of Britain and Ireland lists a couple of interesting secondary features to help in separating these species. One is that the sepal appendages in Sweet Violet are patent ( that is they stick out from the stem) whereas the sepal appendages of Hairy Violet are appressed. An immediate problem is that sepal appendages are very variable, the top one might be smaller than the side ones and they can vary in shape etc. Having taken quite a few photos of this feature, provided you only use the top sepal appendage it does seem to work. At least it holds up in the two Cambridgeshire sites I have checked , Fleam Dyke and Madingley. I suspect that this feature is not 100% reliable?
Example of a Sweet Violet V. ordorata var. dumetorum below
Sweet Violet sepal appendages which run parallel to the stem. In this case all the appendages are about the same in size and shape.
Sepal appendages of a Hairy Violet. Not quite the best angle but the appendages tend to bend into the stem rather than staying parallel.
Sepal appendages of a Hairy Violet ( Possibly a hybrid) which shows appendages adpressed. This plant has the upwards end of the spur which is a common feature of Hairy Violets.
This plant was on a roadside verge near to Dungate Farm TL5599 5277 and a whole group of plants showed intermediate leaf hairs.
Based on the non patent hairs this look like a candidate for a Hybrid despite none of the plants were showing any F1 vigour. The other strange feature was the leaf shape was not that pointed and more like Sweet Violet. The combination of leaf shape and leaf stem hairs does suggest these plants are hybrids.
Another example of a non-vigorous hybrid.
15th April 2018. Taken at East Hatley, Churchyard on a Cambridge Flora Group walk and found by Steve Hartley who immediately noticed that this small group of plants were probably hybrids.
Photo of flower follows.
Secondary Feature, Position of bracteoles on flower stem.
Mentioned in the new Viola BSBI Handbook as a feature to distinguish between Sweet and Hairy Violet, I took some measurements of 10 plants at Madingley of each species and found it works. Sweet Violet has the bracteole above the mid point whereas Hairy has them below the mid point.
In fact there was no overlap so this looked like good news and we could forget all those problems with stem hairs. Unfortunately increasing the sample to other sites started to point to a more complex picture, with many plants having the bracteole at 50 % whether Sweet or Hairy. Having said that plants with the bracteole at 30% of stem hight or less were always Hairy and plants with bracteole at 70% up were always Sweet Violet so there is definitely something going on.
Example of a white flower Sweet Violet ( var. dumetorum) showing bracteoles just below the flower ( the tiny green leaf like structure on the flower stem).
Example of Hairy Violet with bracteole at 20% of stem height. Hard to see but just possible.
Conclusion.
Based on a few years attempting to learn about the various species of Violets I have to conclude that nothing is straightforward with the identification of these species. Dog Violets are well known to present problems but probably less known is how hard it can be to separate Sweet and Hairy Violets.
Some populations can be straight forward but many are not and are probably stable hybrids. The main feature which is missed in the field guides is that Hairy Violets only have some leaf stems that are fully hairy and this adds a massive variable. This in addition to the hybrid problem really can make ID difficult.
Peter Leonard. Rampton, Cambridgeshire. 20th April 2018
