In the Beginning
Dr. H. Roland Schroeder, Evansville, IN
The story of the rhododendron is international, although it centers around Great Britain. Despite the fact that the climate is perfect for growing these shrubs, no rhododendrons are native to the British Isles. Plants and seeds were collected from various parts of the world and transported to England where they were raised and propagated, hybridized and distributed all over the world.
The first species of rhododendron to be cultivated was Rhododendron hirsutum . It is native to the central and eastern Alps and northwestern Yugoslavia. It is found on limestone formations and is one of very few that can tolerate an alkaline soil. It likes dry, open, rocky soils. R. hirsutum is sometimes confused with R. ferrugineum , but there are many differences even though R. hirsutum is also erroneously called the "Alpine Rose". R. ferrugineum is the true "Alpine Rose".
R. hirsutum was introduced to Great Britain in 1656 by John Tradescant who was believed to be a Huguenot refugee. He presumably obtained a plant from Charles I'Ecluse, court gardener to Maximilian II at Vienna.
The second rhododendron to reach England (this does not include the Rhododendron species commonly identified as azaleas) was "Rose Bay of the Carolinas", R. maximum .
Peter Collinson, a Quaker haberdasher, carried on trade with the American colonies and was put in touch with John Bartram, a farmer of Pennsylvania, who had an intense interest in botany. Collinson financed Bartram's expedition and R. maximum was one of 150 new plant species Bartram uncovered. By this extraordinary conquest Collinson obtained the title of King's Botanist in America for Bartram.
R. maximum did not become widely planted because of several faults. The flowers were small, the trusses loose, and new growth hid the flowers. The foliage was nice and the plant bloomed in June which was a plus for England, since they have occasional frost in April and May. R. maximum also has a blotch which is transmitted to its offspring and can be seen in some of the hybrids of today.
An early rhododendron to arrive on the scene was R. ponticum . It reached Great Britain in 1763 from Gibraltar. This was probably the subspecies Baeticum being found in Spain between Cadiz and Gibraltar. This also occurs in Portugal. There are some minor inconsistent differences between this subspecies and the one from the Black Sea and Balkan areas.
R. ponticum's main use was for grafting garden hybrids, but the ponticum understalk is very vigorous and sends up suckers and can overpower the scion. The flowers are various shades of mauve, an unacceptable color to many rhododendron enthusiasts. The antipathy toward this color may be partly sociological, since the first fast dye discovered was mauve which is a synthetic color. The result was that at one time the only cheap colored clothes for women were mauve.
R. ponticum can be very attractive when planted in a shadowed woodland with white flowered shrubs. R. ponticum is not heat tolerant and very susceptible to Phytophthora cinnamomi in the United States on the east coast and middle west. It also is not as cold hardy as one might think and is given a rating of -5° F.
Captain Hardwicke in 1796 discovered the first of the Himalayan rhododendrons. This was a tree-like form named R. arboreum . However it was not until 1815 that R. arboreum was grown in England, and it did not flower until 1825.
The red variety of R. arboreum is responsible for many of the red and pink colors in our garden hybrids. R. arboreum is fairly tender but does not seem to transmit its tenderness to its offspring.
The next rhododendron introduced to the British Isles came from high up in the Caucasus Mountains, R. caucasicum . This is a shrub varying in color from pale cream, almost yellow to deep pink varieties.
R. caucasicum was first received at Kew Gardens by Sir Joseph Banks as a gift from the Russian collector, Count Puschkin in 1803. It is thought that R. 'Cunningham's Sulfur' is a good yellow form of the species. Plants obtained from a height of 8,000 feet above sea-level are hardy. R. caucasicum is a compact shrub with a long flowering period. If the early flowers get frosted there are more to come so as to insure more blooms for the season.
Another species introduced from the Himalayas was R. campanulatum . It has a very neat habit of growth with flowers varying from rosy-purple to near blue. There are some forms that are hardier than others and have withstood zero temperatures in the vicinity of London, England.
The species that proved to be the most important in producing hardy hybrids was R. catawbiense . It flowered for the first time in England in 1813.
John Fraser and son, while collecting plants for Emperor Paul of Russia in the American colonies, found R. catawbiense growing on Roan Mountain and also along the Catawba River.
The great advantage of this rhododendron is its habit of growth and extreme hardiness which it transmits to its offspring. By transmitting its hardiness, its hybrids can be grown in most temperate zones of the world. It is the ancestor of many fine garden hybrids currently in commerce despite its shortcomings. The flowers are small and contain a mauve pigment and the truss is loose.
This was the material at hand for the nurserymen and hybridizers to work with at the beginning of the nineteenth century, namely, R. ponticum , R. catawbiense , R. caucasicum , R. arboreum , R. maximum and R. campanulatum . By crossing, re-crossing, selecting and reselecting, the hybridizers developed some fairly nice rhododendron hybrids that have withstood the test of time. R. 'John Walter' ( arboreum x catawbiense ) and R. 'Fastuosum Flore Pleno' ( catawbiense x ponticum ) are two first rate garden hybrids for our colder climates.
The hybrids with the most R. caucasicum , catawbiense , ponticum , and maximum blood are sometimes called the "Ironclads" and have been the mainstay for our colder gardens and are now being used with the newer Sino-Himalaya species to produce hardy, more exotic flower trusses and shrubs for future gardens.