Why Double Dahlias Are Not Always Double

Originally published as The Evolution of the Dahlia: Chapter Two (1919)

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.

Chapter Two takes up the problem promised at the close of Chapter One: why a dahlia whose doubleness is supposedly fixed may nevertheless produce imperfect flowers. Broomall’s answer is characteristically practical. He looks first to nourishment and soil condition, then to competition, shade, crowding, cultivar vigor, breeding, climate, excessive moisture, and heat. Failure, in this account, is not evidence that the plant has permanently “gone back,” but evidence that something in the plant or its environment has interfered with full expression.

The chapter also introduces several ideas that Broomall would revisit repeatedly: his distrust of excessively close breeding, his belief that wider crosses tend to produce greater vigor, and his conviction that California conditions could confer unusual strength on dahlias. These claims belong to Broomall’s historical understanding of heredity and environment and are preserved as such rather than silently recast in modern genetic terms.

The Chapter One–Chapter Two connection is one of the strongest surviving links in the numbered series because Broomall himself announces the subject before turning to it here.

The text printed here is from J. J. Broomall’s 1919 Dahlia Guide.

One of the most frequent causes of imperfect Dahlia blossoms is starvation or near starvation. The soil may be deficient in plant food, or it may be in such an unfavorable condition that the nourishment it contains is not readily available to the plants.

Weak growing varieties planted near stronger growing plants are sure to produce imperfect blooms and few of them. Too much shade is another thing to be guarded against. Dahlias should not be planted in a lawn; the treatment necessary for a well kept lawn is wholly unsuitable for Dahlias. They cannot be expected to do their best unless they occupy a strip of cultivatable ground at least three feet wide (4 or 5 feet would be better); overcrowding means inferior flowers.

To guard against unfavorable conditions, begin right by having the soil in the best possible condition when you plant, and then plant the right kind, avoiding varieties with an inherent weakness of growth and blooming qualities. Many of the finest English Cactus Dahlias are deficient in this respect.

To Henry Cannel of Swanley, England (who first exhibited Juarezi, in 1882), is probably due the credit (more than to any other one person) for having started the Cactus Dahlia on its way to world-wide popularity.

While English growers have taken the lead in the past, and have produced some of the most beautifully formed flowers the world has ever seen, it is an undisputable fact that many of them are so weakly in habit that it requires all the skill of the professional gardener to grow them to perfection, and they are exceedingly liable to prove disappointing in the hands of amateurs.

The reason for this weakness is that English growers have been breeding and selecting too close to the line; inbreeding is as certain to result in weakness in plants as it is in animals. On the other hand we find the strongest growers among those Dahlias whose parents differ the greatest; thus it is that the strongest growers are found among the Hybrid Cactus, the Decorative, and Peony types.

Decorative and Peony Dahlias are really true hybrids as they are the result of crosses between the Cactus and the Show Dahlias, or the descendents of such crosses. It will be found nearly always, where the parent flowers differ most widely in form and character, their progeny will be the most robust; such flowers will be a success and a pleasure where more weakly varieties will prove disappointing, except under the most favorable conditions.

For instance, should such Dahlias as Frederick Wenham or H. L. Brousson be planted within three feet of any strong growing hybrid, the former will not produce a flower worth looking at, as the stronger grower will absorb the greater part of the plant food and moisture, and completely overshadow their weaker neighbors.

It is a well established fact that California Dahlias lead the world for size of flowers, and vigor of growth; while this is due in part to hybridization as explained above, there is another cause contributing to this most desirable effect that can be stated in one word: CLIMATE; the long growing season under the most favorable conditions has a tendency to develop the Dahlia to its fullest capability, so that they acquire an amount of energy and vitality that would scarcely be possible under a less favorable environment.

It is most gratifying that California bred Dahlias, having acquired such desirable characteristics, retain them when transplanted to other climes, as has been proven by the many delighted Eastern growers who have tried them.

There are other causes of imperfect flowers which I will notice briefly. Overwatering, especially in heavy soils, is liable to cause Dahlias to show an open center, for the reason that perfectly healthy root action is scarcely possible if the ground becomes sodden and sour.

Another cause that is not always possible to guard against is hot weather; extreme heat not only causes premature development, if long continued it injures the buds, so that perfect flowers cannot be expected until later buds shall have had time to grow and mature. At such time plants require an abundance of moisture, and if a shading of cheese-cloth could be provided, the ill effects of such conditions would be reduced to the minimum.

1. Historical genetics and environmental inheritance. Broomall combines practical observations about cultivar weakness and environmental response with biological assumptions characteristic of his period. His claims that inbreeding necessarily produces weakness, that widely differing parents nearly always produce the most robust offspring, and that vigor acquired under California growing conditions is retained in other climates are preserved here as part of his historical interpretation rather than as modern genetic principles.

2. Broomall’s changing explanation of doubleness. Chapter Two continues Broomall’s 1918 argument that an established double dahlia may produce imperfect flowers temporarily without permanently reverting to a single form. His explanation of how doubleness first originated changed over time. In Chapter One he gave repeated pollination the primary role, followed by cultivation and inheritance. By Chapter Four in 1921, he instead attributed the first double dahlia principally to a superabundance of plant food. He repeated that later explanation in The Magic of Growing Plants in 1930.