This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
As with the Sweet Violet no fossil seeds of this species have been found. It is confined to the cold Temperate Zone, in Europe, N. and W. Asia, extending as far as N.-W. India. It is absent in Wales from Brecon and Radnor, Pembroke, Cardigan, Merioneth, and from Mid Lancs, and the Isle of Man, but elsewhere it is universal. In Scotland it does not occur in Roxburgh, Berwick, Haddington, Edinburgh, Fife, Forfar, Kincardine. From Forfar it ranges to the south of England, and is found at a height of 1000 ft. in Yorks. it occurs also in Ireland.
The Hairy Violet is found on dry banks, and in woods, in hilly country as a rule, being addicted more especially to drier conditions than the Sweet Violet, which thrives best in the shade, but this species may also be found in damper stations in woods in low-lying situations. There is less likelihood of this species being spread artificially, and it has a less wide range.
The habit is prostrate like that of the Sweet Violet, which also has no erect stem, the leaves arising from the rootstock directly. The leaves are likewise heart-shaped, but in this case the stoles or trailing stems with buds are absent or very short, and the bracts are below the middle of the flower-stalk. Moreover, the whole plant is hairy, or roughly hairy, giving it a greyer, less green, appearance when dry.
The flowers are not fragrant and less dark blue than those of the Sweet Violet, but the spur is long and hooked, and the anther spurs are linear. The plant flowers later than the Sweet Violet. The capsules are pendent in this as well as in the latter, and in each the spring flowers do not produce seeds, while the later cleistogamic flowers without petals do. The flowers vary in number of spurs, some having sack-like ends to the petals or rudimentary spurs.
Photo. Flatters & Garnett - Hairy Violet (Viola Hirta, L.)
The Hairy Violet is not more than 6 in. high. The flowers may be found from April to May or June. The plant is perennial, increased by division of the root.
The flower is pollinated in the same way as Viola odorata (which see). Both are conspicuous flowers, and have two types of flower, spring (usually infertile) and autumn, the latter cleistogamic and fertile though apetalous. One difference is the marked absence of scent, but as it is not so usually a woodland or shade species this is the more readily to be explained. The spur is long and hooked and two anther-spurs are lance-shaped.
The seeds of the Hairy Violet are dispersed by the plant's own agency, the flower-stalks hanging down when the capsule is ripe, and the seeds are sown in the ground around the parent plant. The seeds are also dispersed by ants. The capsule opens by three valves.
Hairy Violet is a sand-loving plant, requiring a sand soil with a very little humus, in this differing from V. odorata.
Puccinia violae, Urocystis violae, Peronospora effusa, Thielavia basi-cola are fungal parasites.
The Lepidoptera Argynnis paphia (Silver-washed Fritillary), A. adippe (High-brown Fritillary), and A. aglaia (Dark-green Fritillary) feed on it.
The specific Latin name hirta means hairy, alluding to the hairy leaves, stem, or leaf-stalk.
In Viola odorata numerous stolons or soboles are thrown out which trail over the surface and root at intervals. In V. hirta they are not prostrate, and do not root at intervals. The roots of both are covered with tubercles when advanced. The leaf-stalks are smooth in V. odorata, hairy in V. hirta, and give it quite a downy, silvery appearance. The leaves are much alike, and V. odorata has hairs below, but they are more numerous in V. hirta. The leaf of the Sweet Violet is glossy above, and the leaves are longer, not so heart-shaped. In V. odorata the bracts or leaflike organs are above the scape, in V. hirta below. The Hairy Violet flowers a week later than V. odorata, and the flowers are not so deep a blue, nor do they smell. They both produce barren spring and fertile autumn flowers.
Essential Specific Characters:43. Viola hirta, L. - Stoles absent or short, bracts below the middle of flower-stalk, leaves hairy, cordate, petioles with spreading hairs, flowers light blue, scentless, spur linear.
1 These are pink, fleshy, swollen, and when the flower-stalk lengthens it may bury the ripened capsule in the loose soil. The pendent capsules are due to the practically non-existent stem (which is very short), so that they are not raised up.