How Double Dahlias Are Made: A Study of Plant Life

Originally published as The Evolution of the Dahlia: Chapter One (1918)

Original text by J. J. Broomall is in the public domain.

Compilation, editorial notes, introduction, biography, and revised text copyright © 2026 by Steve K. Lloyd. All rights reserved.

Broomall opens The Evolution of the Dahlia with the principle that underlies much of what follows: variation makes plant improvement possible. From there he turns to plant reproduction, insect pollination, cultivation, and inheritance in an attempt to explain how the single dahlia became double. His 1918 explanation is characteristically empirical and speculative, drawing freely from his experience with dahlias, greenhouse cucumbers, and other plants.

Equally important is the distinction he establishes between permanent inherited character and temporary failure. A double dahlia, Broomall insists, does not truly revert to a single form. When it produces imperfect flowers, there is a cause to be found in its conditions or treatment. That conviction becomes one of the recurring arguments of the later chapters.

Broomall himself would revise another major claim made here. By Chapter Four, he had abandoned repeated pollination as his principal explanation for the origin of doubleness and instead emphasized abundant plant food and cultivation. The 1918 explanation is retained intact so that the development of his thinking remains visible.

Chapter One closes by promising that the causes of temporary failure will be explained in the “next chapter.” That promise is fulfilled by Why Double Dahlias Are Not Always Double, originally Chapter Two.

The text printed here is from J. J. Broomall’s 1918 Dahlia Guide.

J. J. Broomall, Pioneer Dahlia Breeder of the Pacific Coast

For years I have been advocating the teaching of gardening in the public schools. This having been neglected in the past, there appears to be an urgent need of a school for grownups. I believe all professional plantsmen and dealers will agree with me that one of the greatest drawbacks to the trade is the ignorance of many people as to how plants should be treated. Ignorance very often leads to failure—it means a dissatisfied customer, and that is poor business.

I contend that the study of plant life is the most necessary and vital of all studies, as well as the most interesting and fascinating. Our existence depends upon plant life; the better we understand plants, the better will we fare. And bear in mind the only limit to the possibility of improvement in plants is our knowledge of their nature and requirements. If by writing the following article I shall awaken the interest of the reader and cause him to desire a better understanding of plants and their possibilities, then I shall not have written in vain.

A Study of Plant Life

By far the most important factor in the improvement of plants is variation. It is this that makes all that follows possible. Nature seldom makes two plants exactly alike. In their natural or wild state, the variation may be so slight as to pass unnoticed; but when brought under cultivation, the change in environment and plant food not only accentuates the difference that may have existed, but causes the plant to “break” into new and widely divergent forms.

It is by taking advantage of these tendencies and intelligently co-operating with nature that man has procured more beautiful flowers, more luscious fruits, more nutritious vegetables, and this is only the beginning. The only limit to what may be done with plants is our knowledge of them.

One of the most important questions in connection with the subject of plant improvement is sex in plants. Here, as elsewhere, Nature is not confined to any one rule. For we find in one class of plants the male blossoms are borne on one plant, the female on another. Then there are plants where both male and female blossoms occur on the same plant. In some instances the flowers are so nearly alike that it requires an expert to distinguish the difference; in others the greatest difference in form, color and size exists. As an example, take the hazelnut, the male blossoms of which are much in evidence, but I venture to assert that not one person in ten thousand has seen the tiny red female flower.

In a very large class of plants (and this includes the Dahlia) the male and female organs occur in the same flower.

The study of plant life is one of constant surprises. Not the least of these is what might be called the inter-dependence of Nature; that is, that many plants are as dependent on insects for their perpetuation and existence as the insects are dependent upon the plants for their food. But there are many exceptions to this rule.

For instance, take Indian Corn. The tassle sheds its pollen upon the silk and the ear is made fertile without the aid of any insect. On the other hand is the Fig, which depends on the services of one small insect alone, and that insect finds its only home and breeding place in the Wild Fig.

There are many plants, however (and this includes the Dahlia), dependent upon insect visitation, to which it is immaterial what kind of an insect it is provided it visits the flower. In this, what might appear to be a haphazard arrangement, Nature seems to have provided against possible failure by providing a superabundance of pollen, so that if an infinitesimal fraction of it reaches its proper destination, fertilization is assured.

The primary cause in the production of the Double Dahlia is repeated pollenization due in the great majority of cases to increased insect visitation. This is much more liable to occur to plants under cultivation from the fact that animal manures are generally used in fertilizing the soil, and these manures often contain and very often are the breeding places of insects.

To illustrate this point, I will refer to the growing of hothouse Cucumbers. In growing Cucumbers under glass, either a hive of Bees must be kept in the hothouse or the blossoms must be pollenized by hand. The writer, when engaged in the business, depended on the latter method.

When the Cucumbers were coming into full bearing, a heavy coating of cow manure was applied to the benches. A few days afterward swarms of little flies appeared, and as long as those little flies lasted it was not necessary to pollenize by hand. Those little flies performed the task much more thoroughly than I did; they did not miss a blossom.

The second and most important factor in the production of double flowers is intensive cultivation; and I have seen flowers increase in doubleness that could not have been caused by pollenizing. I have noticed this not only in Dahlias, but in other flowers, particularly in roses and chrysanthemums.

INHERITABILITY, since Double Dahlias were first obtained, is probably the greatest contributing cause to the further production of them, and to me it is the most inexplicable—how a flower that has acquired a habit that is unnatural and detrimental to its welfare should be capable of transmitting the habit to its progeny; for if we accept the theory that self-preservation is the first law of Nature in plants as well as in animals, then this is certainly contrary to that law, for doubleness in flowers hinders seed production, and in the case of the Dahlia, called Gee Whiz (which refuses to show an open center), prevents it altogether. Thus it would appear that man can so mold the habits of plants to his will as to cause them to commit race suicide, or perhaps it might be called Floricide.

Fixity—I have often been asked, do Dahlias “go back”? And I have heard some growers assert that they do “go back.” If by this is meant will they revert to their single state, I answer most emphatically NO; a Dahlia once double will always be double; what may sometimes appear to be a reversion is only a temporary failure, due to various causes, some of which I will endeavor to explain in my next chapter.

It is true that the habit of doubleness is less strongly developed in some flowers than in others; but such flowers, if given proper treatment and under favorable conditions, will come as perfect as they ever did. I have found it a difficult matter to obtain perfectly double flowers from Juarezii (the original Cactus Dahlia), yet it is no worse in this respect now than when I first grew it nearly 20 years ago.

Now, to sum up in brief, Double Dahlias are caused, first by repeated pollenization, later by inheritance, and last, but not least, by good cultivation. And allow me to say, and to reiterate, this good cultivation is the most important of all; it is the keystone of the causes that make perfect flowers possible.

In the foregoing article I have endeavored to give you the benefit of 20 years’ experience in Dahlia growing. In presenting my conclusions to the public I not only expect but invite criticism. And as an inducement for you to write me your opinion of my statements, I will give, free of charge, a tuber of the Cactus Dahlia Gee Whiz to the person sending the best review of this article; to the one sending second best review I will send a tuber of Gladys Sherwood.

Answers must be received before May 1st, 1918.

Yours faithfully,

J. J. BROOMALL.