This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
This is a low-growing species with many underground creeping stems, by which it extends itself and forms carpet-like masses of a lively green. The stems are angular, and clothed with acutely egg-shaped leaves toothed on the margin. The flowers, produced in great profusion, are large, pale purple, and furnished with a conspicuous awl-shaped spur. Adapted to either the rockwork or mixed border, preferring a little shade and ample supplies of moisture during the growing season. The yellow-flowered V. Zoysii of some catalogues is regarded by some botanists as a variety of calcarata under the name V. c. flava. Flowers from early spring throughout the summer in moist situations. Native of the Alps of Switzerland.
This is very near in character to the last, but is a more vigorous plant, and further distinguished by its broader and less deeply toothed leaves, and the more upright tendency of the stems. It is now a well-known plant in flower-gardens, having been extensively tried for some years as a dwarf bedding plant, and most conflicting reports have been made regarding it. When it is successful, there can be but one opinion as to its merits; it is very beautiful, but it is successful as a massing or edging plant only in moist soil and seasons. There are several varieties of greater or less pretensions for being improvements on the normal form, but chiefly marked by different shades of the purplish colour of the original. The best that has appeared is the one named "Perfection," a very distinct and handsome plant with large Pansy-like flowers of a bright purplish blue, yellow-eyed, and more strongly fragrant than the reputed parent; but it has so little in common with cornuta, beyond the horn, that there are grounds for questioning the alleged parentage. It is as unlike cornuta in its power of resistance of drought as in most other particulars. During this excessively droughty season it has looked fresh and bloomed profusely, while cornuta has been "done brown" for weeks.
Cornuta is a native of the Pyrenees.
This is another unsuccessful candidate for parterre honours of recent introduction. It is a native of mountain pastures in Wales and the north of England and west of Scotland. It grows in rather a straggling manner, rising 3 or 4 inches high, with weak stems and small oblong egg-shaped leaves. The flowers are bright yellow, with a few black lines radiating from the centre on the lower petals. Although it succeeds better in the majority of dry soils and aspects than Viola Cornuta, yet it is not so flori-ferous as that species, and has disappointed many in the expectations raised regarding its adaptibility to summer bedding-out when first introduced for that purpose. It is a pretty little gem, creeping over rockwork, or in the front line of a partially-shaded moist mixed border; but in bright blazing parterres it is eclipsed, and very often burnt up, and does not supply effectively the much - desiderated dwarf bright yellow edging plant. The variety grandiflora has, as its name implies, larger flowers than the ordinary form, and is somewhat of an improvement also in the matter of habit being slightly more vigorous.
Flowers continuously from May till September.
(To be continued).