This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
This popular genus is characterised by having a double involucre, no pappus, and a large scarious bracteole at the base of each floret. It was named in honour of a Swedish botanist named Dahl, and contains probably not more than half a dozen species, all of which are natives of Mexico.
1. D. variabilis. Common Dahlia. - This appears to be a variable plant in nature, and has received several names supposed to indicate distinct species, but they are now generally united under the above designation. There were two tolerably distinct forms originally introduced : one, frustranea, in which the outer involucral bracts are spreading; and the other, super-flua, having them reflexed and also producing seed more freely. The latter variety was introduced into this country as early as 1789 by the Marchioness of Bute, but soon lost, and not reintroduced till early in the present century. Little care, however, seems to have been bestowed upon them even then, for, until 1814, when some more plants were imported from France, we read of no progress having been made in raising new varieties. It was first introduced into France about the year 1800 and cultivated for its tubers; but it was not destined to become famous for economical produce. Soon, however, it engaged the attention of numerous horticulturists, and founded its reputation as an ornamental plant of the first order. In the wild state the central or disk florets are small, tubular and yellow, and the marginal or ray-florets only conspicuous and highly coloured in some shade of scarlet. But every successive sowing brought forth new variations in colour, and gradually the disk-florets were metamorphosed, assuming the same shape and colour as the outer ones, until at length the 'perfect flower' of florists was attained, in which all the florets are similar, forming an almost spherical head, erroneously termed a double flower (fig. 126). The Dahlia indeed offers one of the most striking instances of the variability of species under domestication, which is exemplified not only in the modification of the disk-florets but also in the wide range of colours. But so little is understood of the real nature of vegetable colouring matter and the cause of its variability, and to what influences the changes must be ascribed, that we cannot correctly estimate the importance of this phenomenon. We know that pure white flowers exist, and that various shades and tints of yellow, scarlet, and purple, and combinations of these colours, are common; but we are not sure whether these colours are not also found in natural varieties. There is evidently a limit in the production of colours, as nothing approaching blue has been observed in all the varieties raised. Perhaps chemistry may some day tell us why.
Fig. 126. Dahlia variabilis. (1/6 nat. size.)
Fig. 127. Dahlia coccinea. (1/6 nat. size.)
The culture of the Dahlia has probably been carried to higher perfection in England than in any other country, for almost without exception the varieties grown are of English raising. Besides the old tall race, a new one of dwarfer stature and another bearing smaller flowers have sprung into existence.
The varieties of the different classes range from 1 to 6 feet or more in height, and the flowers, or more properly flower-heads, from 2 to 5 inches in diameter. The comparatively recent Bouquet or Pompon Dahlias furnish the best varieties for a small garden. In some of these the florets are beautifully fringed, as in the pure white variety Guiding Star. For detailed information respecting the numerous varieties we must refer the reader to the catalogues of our great Dahlia growers.
In addition to the above species there are two or three others occasionally seen, including D. coccinea (fig. 127), a rather taller plant with large scarlet sometimes semi-double flowers; and D. imperialis, of recent introduction, and better adapted for a large conservatory than the open air. The latter attains a height of 8 to 12 feet, bearing numerous smaller bell-shaped heads of pure white flowers, with a crimson spot at the base of each floret.