The Ironclad Rhododendrons

Lest we forget—the Past is Prelude

RICHARD MURCOTT the New York Chapter

Published in the NY Chapter newsletter about 1990.

We now hear very little about a group of hybrid rhododendrons that, when I started collecting and growing this genus in 1962, were the mainstay of the plants available. The Dexter Hybrids were very sparingly available, really just sold as a favor to the purchaser. There were other cultivars available, but the basic selection of rhododendrons available in garden centers were the Ironclads.

To understand where they came from and how they were designated, a little history is necessary. Rhododendrons started to be hybridized in England in the early 1800’s. The English had by this time obtained seed and plants of a few species native to Europe, the Caucus mountains and the American species catawbiense and maximum. When the English saw the blood red flowers produced by plants grown from seed of the Asian species they went gaga. Their blooming about 1850 started a hybridizing frenzy. Many hybrids of ponticum, caucasicum, catawbiense, maximum and arboreum were created.

The Centennial Exhibition of American Independence was held in 1876 in Philadelphia. It was there at that exhibition that the American public first came to see hybrid rhododendrons as 3,000 hybrids were brought in from England and used throughout the grounds as landscaping plants. They were immensely popular. These plants were sold to local gardeners in the Philadelphia area at the end of the show and I don’t doubt that some of them or propagations of them are still growing there.

Nurseries began to get many inquiries regarding hybrid rhododendrons. This started a 50 year love affair for these plants, mainly by the wealthy as the plants were very expensive. Many nurseries began to import plants and soon it became evident that not all hybrid rhododendrons were both beautiful and hardy. The enormous variations in the climate on the East Coast made selection of cultivars frustrating. A plant perfectly hardy in Philadelphia would succumb to the cold during its first winter in Boston. This feature was not lost to the Arnold Arboretum. They had imported many named hybrids from Anthony Waterer and had discovered the remarkable variability of the plant’s hardiness.

After many years of observing these plants growing at the Arboretum, and with consultation with nurserymen also growing them, E.H. Wilson, in 1917, published a list of “Ironclad Rhododendrons.” The original list consisted of fourteen cultivars. The “Ironclad” referred to their ability to withstand severe winter conditions. This list is appended to the end of this article. In 1927 the list was shortened to twelve and became known as “Wilson’s Dozen” or “the ironclads”.

The publication of this list had many lasting effects on rhododendrons. First, it made everyone aware that rhododendrons varied in their hardiness and that proper cultivar selection was most important for success. Second, even though the title didn’t say so, everyone thought that the cultivars on the list were the only hardy rhododendrons. This was certainly not so. They were the hardiest of the plants imported from Waterer! There were certainly other hybrids from Holland, Germany and yes, even America. (The Parsons Nursery in Flushing had named several very hardy hybrids) that equaled their hardiness. And lastly, it permanently placed the Waterer name to the top of the list of important rhododendron hybridizers. Once again, this shows that someone other than the hybridizer is usually responsible for the placement of the hybridizer in the history of rhododendrons. (Another fact to remember is that the embargo prohibiting the importation of plants with soil on their roots was enacted about 1917. That effectively cut off the importation of newer European hybrids that might have competed with those on the ironclad list.) If it weren’t for the publication of this list, I wonder if anyone now would know the name of Waterer!

Of course, asexual propagation of rhododendrons in the early 20th century was very slow. Grafting or layering were the only two ways available. They both had drawbacks. Grafting suffered from understock growth, understock compatibility, and required highly skilled personnel to be successful. Layering took at least a year and could only be done every third year to the same plant. Even though propagation was slow, with the passage of time, rhododendrons were roaring in the Roaring Twenties.

There are a relatively small number of individuals who really had an enormous impact on rhododendrons. One such person was Paul Vossburg. Paul developed a love for the plant while working at Hicks Nursery from 1917 to 1942. During the war, when he was working at an aircraft factory, he developed a technique to root rhododendrons. He perfected the use of indole-3-butyric acid that Professor Zimmerman at the Boyce Thompson Institute discovered in 1939.

After the war Paul worked for Westbury Rose propagating rhododendrons by the tens of thousands, an unheard of feat in that day. What cultivars was he propagating?—the Ironclads, of course. He knew of many large plants on the north shore of Long Island from which cuttings could be taken. (He, by 1950, had become a member of, and the propagator for, the Dexter Committee too, but that is another story.) I guess that the heyday of the Ironclad was just about when I started in 1962. Westbury Rose must have had 100,000 of them growing in their nursery. It was really all that was available in retail nurseries.

What happened to them, you ask? Dexter is what happened. There aren’t too many sensible people going to grow an Ironclad when they can grow one of Mr. Dexter’s hybrids. Over the last thirty years, the Ironclads have disappeared on Long Island and really anywhere that the climate allows the Dexters to be grown. I doubt that a complete collection of Ironclads exists today in a private garden. I do think that ‘Mrs. C.S. Sargent’ is one of the best of the Ironclads that even stands up to a Dexter, and I would encourage anyone to grow it, if you can find it.

Space, and my knowledge of the subject, does not allow me to talk too much of the experience hybridizers have had using the Ironclads. Clem Bowers, another giant in the history of rhododendrons, was one person to extensively use the Ironclads. He wanted to create hybrid rhododendrons hardy in the Binghamton area of New York State where the winter temperatures go to -40°. Since the Ironclads were the hardiest hybrids we had, he thought that he would cross each of them with each other of the Ironclads. So in 1926 and 1927 he made the 196 crosses and gave the seed to Hicks Nurseries in Westbury to grow on. The devastating winter of 1931/1932 decimated the seedlings. Those that survived to bloom were hopelessly poor. The only Ironclad to bloom in the spring of 1932 was ‘Atrosanguinium’ making it the hardiest of the Ironclads.  Bowers’ experience with these really gave the Ironclads a terrible reputation for hybridization. I can remember Paul Vossburg telling anyone who would listen not to use the Ironclads in hybridizing. (Please do not think that Clem’s use of the Ironclads was his only sojourn into rhododendrons; he made hundreds and hundreds of other crosses, finally settling on the use of R. maximum in evergreen hybrids and the selection of super-hardy clones of deciduous azaleas as examples of the genus rhododendron hardy in Binghamton.)

Well, we all know that many hybridizers have been successful with the Ironclads, but usually only when one of the parents in the cross is not an Ironclad. Paul himself made one of the finest hardy crosses on Long Island—’Meadowbrook’. He crossed ‘Pink Everestianum’ with ‘Mrs. C.S. Sargent’ to get ‘Meadowbrook’. Paul told me that out of 135 seedlings of that cross, just three were of any value and ‘Meadowbrook’ was certainly the best of the three. Now Paul was always very insistent that you remember that ‘Pink Everestianum’ was used in the cross, not the common ironclad clone of ‘Everestianum’. Maybe the pink clone has a genetic makeup that is more conducive to creating successful hybrids.

I do think, though, that for Eastern coastal U.S.A. the Ironclads are history. It is fun and really nostalgic to remember back to the ‘good old days,’ but those varieties just don’t compare to what we can grow now.

A list of the Ironclad Cultivars:

Album Elegans

Album Gradiflorum


Caractacus **

Catawbiense Album

Charles Dickens


H. W. Sargent **

Henriette Sargent

Lady Armstrong

Mrs. C. S. Sargent

Purpureum Elegans

Purpureum Grandiflorum

Roseum Elegans

** These two cutlivars were removed, making a total of 12