This section is from the book "The Standard Cyclopedia Of Horticulture Vol2", by L. H. Bailey. See also: Western Garden Book: More than 8,000 Plants - The Right Plants for Your Climate - Tips from Western Garden Experts.
Practically all of the named varieties of dahlias have come from one immensely variable species, usually known as D. variabilis, but more properly as D. rosea. For garden purposes, however, a second form of great importance, D. Juarezii, the parent of the Cactus forms, must be kept distinct. There are other species cultivated to a slight extent. It is curious that these showy plants should be closely related to a common weed, the beggar's tick, of the genus Bidens; but other species of Dahlia have leaves whose forms pass gradually into those of Bidens. Other close allies are Cosmos and Coreopsis. Cosmos flowers are some shade of purple, rarely white in wild nature, and only one species has yellow flowers; Coreopsis has yellow flowers only; Bidens, yellow or white; and none of these genera has produced double - flowered forms of the first importance. Dahlia has all these colors and more, being far richer in bright reds, and lacking only sky-blue and its closely related hues, which are seen to perfection in the China asters.
Although dahlias are popular plants, especially in old gardens, they are destined to still greater popularity from the new "Cactus," "Decorative," "Peony-flowered," and "Collarette" types. There exists a prejudice against dahlias in many localities in which these new types have never been seen. This prejudice is part of a reaction against formal and artificial flowers in general. The old-time dahlias were round hard and sriff like a ball. The new-time dahlias are flatter, and tend toward loose, free, fluffy chrysanthemum-like forms. The dahlia has now become immensely variable.
Of the important and very variable florists' flowers, the dahlia was one of the latest to come into cultivation. The first break of considerable importance in the wild type occurred about 1814. Up to that time there were perhaps a dozen well-marked colors in good single-flowered varieties. Dahlias had been cultivated in Europe since 1789, and it is a curious fact that they showed signs of doubling the very first year of their European residence; but it was not until twenty-five years later that a marked gain in doubling was made. The dahlia seemed to be undeveloped until 1814, when the era of doubling began. Before another twenty-five years had passed, the dahlia had sprung into the front ranks of garden plants. In 1826 there were already sixty varieties cultivated by the Royal Horticultural Society. In 1841, one English dealer had over 1,200 varieties. Today it is not uncommon for the leading tradesmen to keep 500 to 1,000 distinct varieties. In the absence of good records, it is conjectured that over 3,000 different names of varieties have been published in the catalogues. Most of the varieties are the Show and Fancy types, which are as spherical and regular as possible, and differ only in color.
At first the distinction between the two types seems to have been the same as that between "self-colored" and "variegated" flowers in general. Lately, for purposes of exhibition in prize competitions, the following arbitrary distinction has been adopted: A Show dahlia (Fig. 1210) is often of one color; but if the edges of the rays are darker than the ground-color, the variety may be exhibited in the Show section. A Fancy dahlia (Fig. 1211) always has two or more colors, and if the rays are striped or if the edges are lighter than the ground-color, the variety must be exhibited in the Fancy section. The two types reached full perfection certainly by 1840, and after that date the improvements were mostly in matters of secondary importance. Most of the longest-lived varieties belong to the Show and Fancy type. These types held full popularity until about 1879, when the first Cactus dahlia appeared in England with a promise of new and freer forms. This form is the one which is perhaps farthest removed from nature, and it is probably so highly esteemed largely because the most work has been spent on it.
A reaction against formalism in all departments of life and thought set in about the time of the American Civil War. It was in the sixties that the Japanese chrysanthemums did much to emancipate the floral world. With dahlias the reaction came much later and has proceeded more slowly. The first Cactus dahlia was so called because of its resemblance in form, but chiefly in color, to the brilliant crimson-flowered Cereus speciosissimus, a well-known garden plant (which is known in the present work as Heliocerus speciosus). The name is now highly inappropriate because the color range of the pure Cactus type has been extended to include all of the important well-defined colors of which the dahlia seems capable. The original Cactus dahlia was named Dahlia Juarezii, after President Juarez, the "Washington of Mexico." It was pictured for the first time in the Gardeners' Chronicle for 1879, and this interesting picture is here reproduced in a reduced size in Fig. 1207. The type is still cultivated under the same name and in all essentials seems to be unchanged.
Forms of the Cactus dahlia are shown in Figs. 1212, 1213.
The origin of the Cactus type, as of all the other types of dahlias, is uncertain, and our efforts to secure full and definite information upon some of the most interesting points may perhaps always be baffled.
A Dutch dealer secured a root from Mexico that produced one plant which is the parent of all the Cactus forms. It is not known whether the seed which may have produced the original root came from a wild or a cultivated flower. It has been said that seedlings of D. Juarezii have produced in cultivation forms approaching the Show type of D. rosea. The reverse process is also said to have taken place, but full, authoritative and convincing statements are wanting. In the garden, D. Juarezii is exceedingly distinct from the florists' forms of D. rosea. It is usually a slenderer, taller and longer-jointed plant, with much handsomer and more delicate foliage, the leaves being narrower than in the coarse and almost ugly foliage of the old forms. It has another peculiarity of growth, which is still one of the most serious defects in the true Cactus type: the plants tend to hide some of the flowers beneath their foliage. This comes about in a curious way. At a node between two young leaves there commonly appear, at about the same time three new growths; the middle one develops into a flower with a naked stalk only 2 or 3 inches long, while the side shoots quickly overtop it and repeat the same threefold arrangement.
The other most serious objection to the true Cactus type is that it does not stand shipment well and does not last so long as a cut-flower as the Show dahlias.
The Decorative or Cactus Hybrid types are numerous, and their popularity is more modern. They have been largely seedlings from show flowers. Their rays are rarely, if ever, recurved at the margins. All the other types of dahlias are well defined, and a single picture of each one will represent its type with sufficient exactness. No one picture, however, can give any conception of the great variety of forms included in this more or less open horticultural section. The name Cactus Hybrid means practically "miscellaneous," and is analogous to the "Japanese" section of chrysanthemums. It is on this section and the pure Cactus type that the greatest hopes for the future of the dahlia are based.
Dahlias considered to be of true Decorative type are those possessing broad flat and nearly straight petals, arranged somewhat irregularly; but the flowers are not spherical in shape like the Show dahlia, but are inclined to be flat and massive, and are always full to the center. Dahlias of this character score a greater number of points at exhibitions.
The Colossal dahlia is the basis of much discussion, especially at exhibitions, the cause of debate being that these dahlias are in reality not classified; that is, the same variety is exhibited in one display as a Show dahlia, and in the next as a Decorative dahlia; but in reality there should be a Colossal class for this type of dahlia. This type, if it may be so called, has large cupped but not quilled rays or petals; the flowers are 5 inches and over in diameter, and spherical in shape; they therefore partake of both types, but are sufficiently different to spoil the harmony, when exhibited in either the Show or Decorative class. "Le Colosse" is the first of this type of dahlia, and hybridization has given a large number of seedlings, which are almost identical in form, shape, and size, the most prominent being at present American Beauty, Giant Purple or Royal Purple, J. K. Alexander, Surpasse Colosse, and Janne (Yellow) Colosse.
The Pompon type is a small form of the Show and Fancy types. It has the same colors and the same form, but the flowers are smaller and more abundant. As a rule, the smaller the flowers the prettier and more individual they are. The larger they are, the more they suffer by comparison with the Show type. Perhaps their greatest point is their productiveness. When profusion is the main idea, not great size and quality, the Pompons are the favorite type of dahlia for cut-flowers.