Rhododendron Breeding at Swarthmore
Dr. John C. Wister, Swarthmore Pa.
The Rhododendron Collection of the Arthur Hoyt Scott Horticultural Foundation of Swarthmore College was started in the 1930's. From a small beginning of a few dozen plants it has now grown to include (without counting Azaleas) over 40 species, 200 named varieties, and over 300 numbered selections of hybrids of Dexter, Gable, Nearing and other American breeders.
A few kinds bloom in April, and many in early May. The peak of bloom is about in mid-May and the flowering display continues spectacular until the first of June. Then, in the 1940's and 1950's, there was a gap with no Rhododendron flowers until about June 20th when our native Rhododendron maximum began to bloom.
This gap of two to three weeks was particularly unfortunate here because it was during these weeks that the greatest number of people visited our Campus for Commencement and the Class Reunions.
To try to fill these weeks with Rhododendron flowers, a breeding program was started in 1953, and was continued in 1957 and 1958, and in 1962 and 1963.
To begin our breeding for late flowers we had plenty of plants of Rhododendron maximum. and a few plants of R. discolor which bloomed with it or slightly later. We had also plants of two unnamed late blooming hybrid strains. In 1939 we had purchased from Joseph B. Gable two small seedlings of a cross of R. maximum x R. discolor that had been made by G. G. Nearing. After a wait of nearly ten years they produced their first flowers. Both looked like improved forms of R. maximum. BUT, and in spite of having two such late blooming parents, the flowers opened the first week of June, the exact time that we needed them most.
The second unnamed late blooming strain of hybrids were crosses of the century old British variety 'Lady Eleanor Cathcart' (itself a hybrid of R. maximum x the tender red R. arboreum) with R. decorum. Over a dozen un-bloomed seedling of this cross came to us from the garden of the late C. O. Dexter of Sandwich, Massachusetts, in 1945. When these bloomed some of the flowers were white and some various shades of pink. They opened very late-some on June 12th to 15th, and some as late as June 21st or 22nd.
And finally in addition to all these we had a number of plants of a very good late flowering pink Rhododendron which we had nicknamed and labeled "Andorra Pink."*
*We had purchased these about 1931 from the then famous Andorra Nurseries of Chestnut Hill, Philadelphia under the name of "Maximum Roseum." This, of course, is the accepted botanical name of the typical form of the wild Rhododendron maximum-Linnaeus var. roseum-Pursh. The Andorra plant, however, is not a pure R. maximum. It is evidently a hybrid and the name is now considered a synonym of the variety 'Ponticum Roseum' (also of unknown origin but supposedly a hybrid of R. ponticum and R. maximum.)
The confusion about the name "Maximum Roseum" is made worse by the fact that one or more much earlier blooming paler pink cultivars have been distributed under the same name.
It seemed obvious that if we crossed all these plants back and forth that we would get some late blooming plants. It seemed equally obvious that we needed in addition the use of other parents to give us more color.
We therefore saved pollen of May blooming species and varieties of other colors. Among those that proved most successful in giving us late flowers of various shades of pink were:
R. adenopodum, R. haematodes, and the varieties 'Dr. Dresselhuys', 'H. W. Sargent', 'Michael Waterer', 'Mrs. C. S. Sargent' and 'Nova Zembla'.
We had good results also from the pollen of some of our Dexter hybrids, particularly 'Scintillation', Dexter 201 (possibly the one later named 'John Wister') and one unnamed selection of George Gillies.
On the other hand we had poor plants or colors resulting from the pollen of:
'Amphion', 'Atrosanguineum', 'Caractacus', 'Charles Dickens', 'James MacIntosh', 'Lady Clermont' and 'Purple Splendor'.
We had either no seed or no germination from the pollen of: 'Everestianum', 'F. D. Godman', 'Ignatius Sargent', 'Parsons Gloriosum', 'Purpureum Elegans' and 'Roseum Elegans'.
In all, from about 500 crosses, we grew many thousand of plants. Hundreds and hundreds of these, of course, were utterly without merit and were heaved out. On the other hand a surprising proportion gave us the late flowers we wanted to carry our blooming season along over the former gap and were plenty good enough for massing in the woodlands of the College and of the Tyler Arboretum in nearby Lima.
We now have plants that bloom in sequence from early June, to mid-June, to late June and into early July. In a few instances, in years of not too hot weather, we have had flowers last us until July 10 or 12 and once even to July 15.
These plants seem very important to us. We know perfectly well that they will never become important, or even acceptable commercial varieties because the public demand is for May or early June flowers not for late ones. But we are greedy! We want Rhododendron blooms as long as possible! We hope that at least some future breeders will be interested in making further crosses to get better trusses, finer flowers of greater substance, and a greater color range-more and better rose pinks and salmon or apricot pinks, reds (and yes, even purples!).
We have been enjoying and are keeping for further enjoyment and further testing, plants from over a hundred different crosses. From some of these we have made special selections and recorded blooming dates and one or two word color remarks. We have designated Swarthmore clones by numbers and Tyler Arboretum clones by capital letters.
As of 1970 we had over a hundred such plants to keep for further study. We ought to cut this number drastically in the next year or two because the flowers are too much alike. Further selections should be made based on size (big, medium or little) and on natural shape of plant (to eliminate any ungainly plants), and on good foliage color. We are watching particularly the bronze coloring of the spring foliage which seems to characterize seedlings tracing back to R. discolor crossed 'with the Dexter 'Lady Eleanor Cathcart' and R. decorum hybrid.
As we cut down to more reasonable numbers we will try to enlist the help of other institutions and individuals to get their opinions. We hope that half a dozen or more clones worthy of propagating and distributing may - emerge to help lengthen the blooming season throughout June in this area and, farther north perhaps, into mid or late July.
If only a very few clones get that far we shall feel that all the effort. time and labor spent on them will prove to have been well worth while.