This article is adapted from the book The Dahlia: Its History and Cultivation by Richard Dean, Robert Fife, John Ballantyne, Stephen Jones, William Cuthbertson, and Leonard Baron. It was published in 1903 by Macmillan Company in London and New York.
Although the original work is in the public domain, this article has been edited for the modern reader and contains new material.
This version is Copyright © 2024 by Steve K. Lloyd and may not be reproduced without permission.
The Dahlia's Natural Habitat
The Dahlia belongs to the largest of all the orders, namely, Compositae, which contains about 10,000 species. The order is divided into a certain number of tribes, the Dahlia being classed in the Helianthoideae, or that to which the Sunflower and our wild Bur marigolds belong [Bidens laevis, also known as Begger’s Tick. ~Ed.].
These tribes are again divided into genera, of which the Dahlia forms one, having received its generic name from the Abbe Cavanilles, who first scientifically described it, and named it the Dahlia in honor of his friend Dahl, the Swedish botanist.
Many species of the Helianthoideae have their leaves opposite, a characteristic which obtains in the Bur marigolds, the only representatives of the tribe indigenous to Britain.
The Dahlia, it is said, grows wild in sandy meadows in Mexico, whence it was brought to Europe about a hundred years ago. It is perennial in its growth, but flowers freely from seed the first year.
As it cannot resist frost there is no chance of its ever establishing itself as a garden escape in this country. One night's frost in autumn is sufficient to blacken and destroy the bloom and foliage of every plant exposed to it.
It has been known to appear in some places a second year from roots left in the ground, but this can only happen under special circumstances in some mild and well -sheltered locality. Plants may succeed for a year or two in growing from seed, but sooner or later, the frost puts an end to their existence. As a result, their growth in a semi-wild state cannot be permanent.
The Dahlia's Botanical Characteristics
The root is tuberous and fasciculate — that is, the tubers all spring from the collum or neck at the base of the stem. The tubers vary considerably in size and shape in the different species and varieties. It is said that the roots of the Dahlia are used by the native Mexicans for food. Would it not be possible to use the surplus roots in this country for a similar purpose? They must contain a large amount of nourishing matter, which might thus be turned to a most useful and profitable purpose.
The stem is woody and branching, and has short, slightly hispid, hairs. In growth it is inclined to be somewhat decumbent, then ascending, which allows it to send up a number of strong branches.
The leaves are opposite, as already indicated, and are without stipules. The leaf-stalk or petiole is naked in some species, but in others it is winged.
The leaves in the cultivated varieties are exceedingly variable, being entire, lobed, or pinnate, with the leaflets often pinnatifid. In some cases they are almost bipinnate.
The Cactus Dahlias have their leaves more broken up than the "Show" and "Pompon" varieties. The margins are serrate and the serrations finely spinulose, more especially in the Cactus varieties. The under surface of the leaves is also slightly rough.
The peduncles or flower-stalks spring from the axils of the leaves, generally in pairs, and bear a single flower each, although there are instances on record when, occasionally, the double dark red variety has borne two flowers on a peduncle. This is, however, of rare occurrence.
The primary involucre is composed of five green persistent fleshy bracts, which are so much reflexed that they point downwards. They are ovate or ovatelanceolate in form in the Pompon and Show varieties, but linear-lanceolate in the Cactus.
The secondary involucre is composed of the outer row of bracteoles or palese, each ray and floret having a bracteole at its base, which is equivalent to a calyx. Very few species in Composite have this scale or calyx, which is therefore quite a distinguishing feature of the Dahlias, whose more common form is that of simple or branched hairs termed the pappus.
These scales form a protection to the young florets in the same way as an ordinary calyx. The outer row is generally composed of eight or nine scales, green at the base, and transparent from the middle upwards. The inner rows are all transparent, but some of the more outward ones are slightly tinged with green.
The Single Dahlia is, no doubt, representative of the plant in its original wild state But due to cultivation, the florets are all, or nearly all, developed into rays, in the double varieties.
The Dahlia rays are folded in a peculiar manner, the one side overlapping the other. They are quite flat in bud, but assume afterwards a round form, thus giving the tubular appearance so characteristic of some of the varieties.
This tubular arrangement is best seen in the double show blooms. In the single varieties the ray is folded in the same manner when young. When fully expanded it becomes either flat or saucer shaped and a little reflexed at the tip.
The tendency of the ray to become revolute is seen wonderfully developed in the Double Cactus Dahlias. In these varieties the ray while young is also folded inwards and flat, but, when fully developed, is rolled backwards so that they too have quite a tubular appearance. These rays are also of various lengths, which make the blooms look irregular.
The florets of the Dahlia are perfect, and the tube or corolla is five-lobed, and contains the five anthers and one pistil with a double stigma. The five anthers are united into a tube, and the stigmas have to force their way through them. In doing so the anther tube is carried upwards out of the floret by the force of the stigmas. After the stigmas get completely through the tube, the latter is withdrawn inside the floret again by the elasticity of the anther filaments.
The stigmas are large, and covered on their outer sides with long transparent hairs about half-way down from their upper ends. Further down are the papillae of the stigmatic surface proper, arranged in two rows. These papillae seem to gradually change into hairs as they ascend. This arrangement of the stigma is similar to that of the asters.
Dahlia Seed Propagation
The seed or fruit is from a quarter to half an inch in length and of a dark color, with a few short white hairs scattered over it. During the maturing of the fruit, the scales or bracts all close compactly together, forming a complete protection to it. They are persistent to the fruit until it is dispersed. They must play an important part in the distribution of the seed, as they are so large that in a breeze of wind they will carry it a considerable distance.
The adaptations which many species in the Compositae have for helping them in their distribution are most wonderful, and without doubt account for the fact that the Compositae contains a larger number of species than any other order.
The fruit in many instances is peculiarly well-adapted for being distributed either by the wind, as in the Dandelion and Dahlia, or by birds and animals, as in the Bur marigold.
The seeds of many species are provided by nature with the means to carry them long distances where they may find new habitats.
When they have reached their destination, they are able, by means of barbs, to work their way down through grass or any other obstruction into the soil.
In the Dahlia the receptacle is convex while the plant is in flower, but becomes flat and often concave in fruit.
The Dahlia may under certain conditions be self-pollinated. As already stated, the stigmas are large and have to force their way through the staminal tube, and in doing so rupture the anther cells. When they emerge from the tube they carry up with them on their stigmatic surfaces large quantities of the pollen, thereby exposing it to the necessary conditions for causing fertilization.
To ascertain whether the Dahlia is self-pollinated or not, a number were isolated with a fine netting so as to exclude all, except the very smallest insects, and in each case the plants flowered freely and set seed. In this locality the Dahlia is visited by the red-tail and other bees, which will cause cross pollination and thus account for the great variety of colors in blooms of plants raised from seed. Probably other insects besides bees visit the Dahlia, but so far as we are aware there is no record of it.
The Dahlia is regarded as the symbol of "instability," because it was introduced to Europe during those great social and political upheavals which culminated in Waterloo. But may it not with equal propriety be taken as a type of instability because of its great variability, and the readiness with which it lends itself to producing new varieties through the agency and ingenuity of the florist.
In the "Double" all the florets, or nearly all, are developed into conspicuous rays. There can be little doubt that the changed conditions under which the plant is cultivated by gardeners has much to do with this variability.
Many plants, however, when cultivated under similar conditions do not show the same readiness to sport in so many directions. It is owing to this changeable character that the plant has become so deservedly popular.
An enormous number of varieties are already in the market, and there seems to be no limit to them. New ones appear every year.
This naturally leads us to ask, "Have all these varieties of 'Single,’ ' Double,' 'Pompon,' and 'Cactus,' been raised from a single wild species?" Evidently so.
Some authorities are inclined to regard the two or three wild varieties as separate species, but strong doubts seem to be entertained regarding this, as their distinctive characters are not permanent enough.
At first sight one would be inclined to think the "Cactus" varieties had been developed from a different species because their foliage is more broken up, the primary bracts of their involucre narrower, and also the scales or palese. But notwithstanding these differences, there is every reason to believe that they have all been developed from a common wild species.
The Double Cactus evidently was partially developed that way in Mexico before it was introduced to Europe. And the so-called Single Cactus Dahlias were raised by Mr. E. J. Lowe. F.B.S., of Chepstow, from an ordinary single variety (Stella bianca by name), crossed with pollen of D. Mercki.
It would have proved most interesting had there been a reliable record kept by those who have raised the different forms of this most interesting plant, but horticulturists are not noted for keeping correct records of their doings, and consequently many important facts as to how new varieties of plants are developed are lost to the scientific world.
