JARS v40n1 - Exbury Azaleas - From History To Your Garden

Exbury Azaleas - From History To Your Garden
R. Christian Cash
Temple University
Ambler, Pennsylvania

How many times have you walked through a garden center and found the majority of the deciduous azaleas referred to as "Exbury Azaleas"? Are all of these plants in their various dazzling colors truly Exburys? In many cases the answer is no. Many of the plants are often close relatives of the Exbury azalea. Further observations at other nurseries or garden centers might reveal other hybrid names such as Knaphill and Ghent. What are these Knaphill, Ghent, and Exbury Hybrids? Why has the name Exbury become associated with so many of these plants? The following historical account is an attempt to clarify the answer to these questions and to discuss the development of Exbury azaleas and their kin.
Many of the hybrid deciduous azaleas available today are the result of over 160 years of hybridizing utilizing numerous plants native to the United States, Europe, and Asia. The story of these hybrids begins with the discovery of deciduous azaleas on the east coast of the United States. Around 1738 these plants were beginning to be introduced to European gardens. Plant hunters such as John Bartram and Andre Michaux were known to have collected and identified azaleas in the United States. These collections were distributed to wealthy Europeans and were considered precious treasures for their gardens.
Much of the early hybridization was accomplished in Ghent, Belgium around 1825. A baker by profession and a gardener by hobby, Monsier P. Mortier was a pioneer in the breeding of these azaleas. Mortier's exact methods and crosses were kept secret. It is assumed that he was the first to be successful at crossing the American azaleas, R. calendulaceum , nudiflorum , arborescens , and viscosum , with the European R. luteum .
The city of Ghent, Belgium was to become the center for the breeding and propagation of these plants, hence the evolution of the name Ghent hybrids. A botanically specific name was also applied to this collective group of hybrids: x R. gandavense . It is referred to as a collective group of hybrids because Mortier started the work and it was later continued by such people as Verschaf-felt, Davies, Cassel, Vuylsteke, and Van Houtte. Interest in these plants was so high that only eleven years after Mortier's first cross there were recorded 107 clones of Ghent Hybrids. (Bowers, 1936)
So what was interesting about these Ghent hybrids? They were plants with bright colors, a light fragrance, and a range of bloom time from early to late spring. They were also suited to colder gardens where many other azaleas failed.
A few of these plants that are still commercially available include: 'Coccinea Speciosa' - deep orange flowers, 'Daviesii' - flowers white to pale yellow with a sweet fragrance, 'Narcissiflora' - a bright double flowered yellow.

Development of Exbury Azaleas

The next important developmental step in the hybridizing of Exbury azaleas was the introduction of the Chinese Azalea ( Rhododendron molle ) and Japanese Azalea ( Rhododendron japonicum ) to Europe. Both of these azaleas produced brilliantly colored, large, open faced flowers, a trait lacking in the azaleas known to exist at that time.
The crossing of the Chinese Azalea with the Japanese Azalea was performed in Ghent, Belgium and Boskoop, Holland around 1880. Today, these plants are referred to as the Kosterianum hybrids, Anthony Koster initially creating these plants. These hybrids were then crossed with the Ghent azaleas to produce a new strain of plants commonly known today as the Mollis azaleas Many of these hybrids were initially developed by Anthony Koster and Sons of Holland and Anthony Waterer Sr. of Knaphill Nurseries in England.
What was unique about the plants of the Mollis and Koster strains of azaleas? They produced plants that displayed larger, brighter, most exposed flowers. A few of the common Mollis azaleas include: 'Hugo Koster' - which develops large reddish orange flowers with an orange blotch, 'J.C. van Tol' - large red flower with a yellow blotch. 'Miss Louisa Hunnewell' - orange yellow flower.
The last important ingredient for the Exbury azaleas was the introduction of Rhododendron occidentale from California in 1851. In and of itself the plant was not looked upon with much enthusiasm. Anthony Waterer was one of the only people to realize the potential of this plant as a hybrid parent. (Street, 1959) In Rhododendron occidentale Anthony Waterer found more scent, a well developed flower presentation, longer flowering period, and bright autumn color. Apparently, since the plant did not develop exceptional flower color it was overlooked by others. With the acquisition of this plant, Anthony Waterer and his son, Anthony Waterer Jr., attempted to breed and select new plants with superior qualities.
Some of the qualities that needed improvement were:
1)  Ghent azaleas sometimes had their flowers hidden among the leaves, a residual trait from the Swamp Azalea. The flowers of this group of plants were also comparatively small.
2)  Mollis azaleas flowered a little early making them susceptible to spring frosts. This group of plant also had little scent or fall color.
3) Rhododendron occidentale was pale in flower color. (Street, 1959)
After working with these groups of plants a strain of azaleas was developed by Anthony Waterer Sr. and his son that became known as the Knaphill azaleas. The origin of these plants was around 1880.
"The resulting Knaphill azaleas seemed to have everything: size, scent, glorious autumn tints, and a scheme of flower colors ranging from flame, orange-red, pinks and yellows to pale cream and white with gold or orange blotches. (Bowers, 1936)
Meanwhile many others began to hybridize azaleas with Rhododendron occidentale , Anthony Koster for one, and Slocock of Goldsworth Nurseries another. Eventually many of these hybrids were used by the Waterers to improve the Knaphill azaleas.
Now the question arises - Why, if these plants were so perfect, didn't the Knaphill name become more popular than the Exbury name? The answer might be in the fact that of all of the new and exciting plants the Waterers developed few were offered to the public. Many of the Waterers' favorite plants were kept for their own pleasure. Thus, the public was not to realize the bounty of the hybridization efforts until many years later.
In the 1920's, Lionel de Rothschild was to lead the way in the hybridization of the deciduous azaleas. In 1919, he purchased the estate of Exbury. Over the next two decades Rothschild would plant over one million rhododendrons, hybridizing plants on a larger scale than anyone in previous years.

R. 'Klondyke'
R. 'Klondyke'
Photo by R. Christian Cash

His work with deciduous azaleas began with the acquisition in 1920 of a batch of Knaphill azaleas from Anthony Waterer Jr. Until that point in time, the Waterers had shared few of their azaleas with anyone. Rothschild would begin to develop the Exbury strain from this group of azaleas.
Among this group of plants was a pink and yellow flowered plant named 'George Reynolds'. Rothschild took an immediate liking to the plant and used it as a parent in a great many crosses. Rothschild's objective with the continued hybridization of these plants was to develop azaleas with clearer more intense flower color and presentation. To accomplish this, pinks were crossed with pinks, yellows with yellows, and oranges with oranges. (Berrisford, 1964)

R. 'Berryrose' R. 'White Swan'
R. 'Berryrose'
Photo by R. Christian Cash
R. 'White Swan'
Photo by R. Christian Cash

In 1924, Rothschild's first group of hybrids flowered but he still felt these could be improved. These were again crossed and the next generation of plants was to begin flowering in 1930.
The mid-late 1930's saw the advent of the Exbury azaleas "going public". In 1934 'Hotspur' azalea was featured in "The Gardeners Chronicle" - the major publication in horticulture of public interest at the time.
The Exbury azaleas had the most public exposure at the 1937 Chelsea Flower Show. The exhibit was judged best in the show by an "amateur" and the azaleas received wide acclaim with another glowing report in "The Gardeners Chronicle". From this point on the Exbury azaleas would continue to gain notoriety in Europe and abroad. After World War II, the Exbury azaleas would become very popular in the United Sates. "It is probably true to say that in 1956, the Exbury strain of Knaphill azaleas was better known in the United Sates than it was in" England. (Street 1959) The nurserymen would need thousands of plants to meet demand. The result was that most plants would be grown from seed and named by flower color. Previously, most plants were cloned from the parents by a method referred to as as layering, a slow procedure providing few plants. By growing plants from seed nurserymen could provide many plants quickly. although the resulting plants would not be genetically identical to the parents. Frederick Street stated in the book, Azaleas , the resulting "seedling Exbury azaleas are all good, many excellent... And they are at last available to everyone." (Street 1959)

R. 'Cecile'
R. 'Cecile'
Photo by R. Christian Cash

The story of the Exbury azalea does not end here. The generosity of Lionel de Rothschild and the increased availability of his hybrids has lead to the continued production of improved hybrids. A few of the recent hybrid strains that have made use of Exbury plants as parents include: Edgar Stead's IIam strain, the Windsor hybrids originating as a gift to King George VI, and Edmund de Rothschild's Solvent strain.
Now we can answer the question - What are Exbury azaleas? They are a group of plants with superior flowering qualities that have been developed by some very dedicated people. They are continually being improved to develop new plants that are wonderful and different. Plants from around the world have contributed to the development of these hybrids and resulting plants are providing our gardens with unmatched color, fragrance, and beauty.

Bibliography Berrisford, Judith. Rhodendrns and Azaleas, St. Martins Press, N.Y., 1964
Bowers, Clemnet Gray. Rhododendrons and Azaleas, McMillan Company, N.Y., 1936
Street, Frederick. Azaleas, Cassel & Co. Ltd, London, 1959

Cash, R.C. 'Exbury Azaleas', Philadelphis Flower Show Phamphlet, 1985

R. Christian Cash is Assistant Professor of Horticulture at Temple University, Ambler, PA