Breeding Rhododendrons for the Gulf South
John T. Thornton
During the early 1970's, I attempted to grow many species of rhododendrons from Royal Horticulture Society and ARS Seed Exchange seed. I also attempted to grow many hybrid crosses from ARS Seed Exchange seed as well as several hundred commercial hybrids. Most of the hybrid crosses from the ARS Seed Exchange were not successful and most of the commercial hybrids were not satisfactory. Several rhododendron species, however, grew well along with a few hybrids. Having become hooked on growing seedlings, I decided to embark on a hybridizing program.
Many problems are encountered when attempting to breed and grow rhododendrons in the gulf south. Due to our heavy clay soils, the water table is on top of the ground here during rainy weather. People cannot have cellars in their homes because they fill with water during wet weather.
Rhododendrons classified as evergreen and deciduous azaleas thrive here when planted at ground level. The horticultural rhododendrons, however, must be planted in built-up beds because their roots will not grow and survive below the wet weather water table. It would probably be unproductive to attempt to breed rhododendrons that would grow without built-up beds. This would involve changing the basic physiology of these plants.
Many factors adversely affect rhododendrons in this area. Long hot summers, dry air, warm winters and fungal diseases play havoc with rhododendrons that naturally grow in the clouds at high elevations in the mountains. The low elevation species of rhododendrons that often grow along rapid streams show more promise for adaptability in a climate such as ours. Rhododendrons of a more northerly or high elevation origin often bloom out of season in the fall or winter. Some bloom so late that their flowers are of poor quality. Some rhododendrons do not seem to get enough cold stimulation to properly open their flower buds. Those that bloom after May 1 here usually have poor quality flowers due to the heat. Rhododendrons open their flower buds over a long period of time as compared to evergreen azaleas which can successfully flower between frosts. The expanding flower buds of rhododendrons that bloom in March should ideally be frost resistant.
Ten species have been chosen as likely candidates for producing plants adaptable for this area. They are R. hyperythrum, R. pseudochrysanthum, R. morii, R. formosanum, R. simiarum, R. arboreum, R. arboreum ssp. delavayi, R. ponticum, R. chapmanii, R. minus, R. formosum and R. formosum var. itelophyllum.
Nine secondary species may make important contributions for hybrids adaptable to this area. They are R. decorum, R. fortunei, R. diaprepes, R. maximum, R. catawbiense, R. metternichii, R. adenopodum, R. makinoi and R. griersonianum.
| R. hyperythrum
Rhododendron Species Foundation garden.
Photo by Russ Gilkey
R. hyperythrum, R. pseudochrysanthum, R. morii
These Taiwan natives are very similar in flower. Typically the flowers are pink in the bud, opening white. They bloom in early April. It is interesting to note that this pink, opening white flower color in rhododendrons seems to be confined to a small area of southeastern Asia encompassing Anhwei, China (R. anwheiense) Yakushima, Japan (R. yakushimanum) and Taiwan.
Most of my early hybridizing was done with a pure white R. hyperythrum. This white R. hyperythrum seemed to have very powerful "bleaching" genes. When bred to deep reds, pinks and purple, the resulting flowers were mostly pink in the bud, opening white.
Two plants of 'Westbury' x white R. hyperythrum have been named 'Ruby Magee' and 'White River'. They are pink in the bud, opening white.
Two plants of 'English Roseum' x white R. hyperythrum have been propagated, one has been named 'Charles Loomis' and the other, ERH1 will be named later. These are extremely vigorous, dense plants, pink in the bud, opening pink and quickly fading white.
'Van Nes Sensation' x white R. hyperythrum has produced two plants that are being propagated. 'Lisenne Rockefeller' is a dense, spreading, low-growing plant with light green leaves and white flowers. VNSH1, to be named later, is an extremely vigorous, tall-growing, spreading plant with large white flowers that have purple dots.
'Trilby' x white R. hyperythrum produced two plants named 'Tarn Morris' and 'Jim Lynch'. These are low-growing plants with deep pink buds, opening pink and holding their color.
'Antoon van Welie' x white R. hyperythrum has produced vigorous large leaved plants. One plant with silvery pink flowers has been named 'Opal Thornton'. Another pink and at least one white will be named. Plants from this cross seem to be quite drought resistant.
Rhododendron maximum x white R. hyperythrum has produced low to medium, open-growing plants with large attractive foliage. Flowers are white. One plant will be named.
Rhododendron metternichii Oki Island form x R. hyperythrum has produced dwarf, dense, spreading plants with flowers deep pink in the bud, opening white. These plants resemble and rival R. yakushimanum in beauty. Plants of this cross have been shown in full flower at the New Orleans spring garden show and have been the star of the show.
'Crest' x R. hyperythrum (cross by Dr. Peter Valder of Australia) produced an upright, medium-growing plant with large delicate white flowers with purple dots. This plant has been named 'Lorell Loomis'. There is not a hint of yellow in the flower.
'Janet Blair' x a pink, opening white R. hyperythrum has produced plants with exceptionally attractive pink flowers that hold their color.
Recently I made several crosses using a deep pink R. hyperythrum that holds it pink flower. This deep pink R. hyperythrum crossed with red hybrids has yielded a high percentage of plants with red flowers.
I have several plants from crosses involving the Ben Nelson form of R. pseudochrysanthum. These hybrids closely resemble the R. hyperythrum crosses except they usually are lower growing.
'Rochelle' x R. pseudochrysanthum produced a low-growing, spreading plant with bright pink flowers. This plant has been named 'Beulah Black'. It is very floriferous and seems to be a good parent for further breeding.
Rhododendron morii produces low to tall growing hybrids similar to the R. hyperythrum plants. 'Janet Blair' x R. morii produced an upright, medium-growing plant with dark green foliage and light pink flowers.
R. formosanum, R. simiarum
Rhododendron formosanum also hails from Taiwan. It belongs to the Argyrophyllum series and has white to pink flowers that are variably fragrant. My plants of R. formosanum were severely damaged by the cold winters of the early 1980's and have only recently started blooming. Hybrids from this species grow well, but have not flowered.
Rhododendron simiarum, the monkey head rhododendron, comes from Hong Kong and other areas of southeastern Asia. It belongs to the Argyrophyllum series of rhododendrons and grows at low elevations. According to Peter Cox, R. fokiensese, R. fordii and R. youngii are probably synonymous with R. simiarum. Rhododendron simiarum has been described as "pretty in a quiet way." It reportedly survived for many years in a hot, old Australian garden.
I have been unable to grow R. simiarum; however, I have plants from ten crosses involving the species. Hybrids of this species have a long juvenile stage. They typically grow ¼ to ½ inch annually for the first five years, then ½ to 1 inch annually the second five years. At ten years of age, plants grow one to four inches annually. A plant of R. hyperythrum x R. simiarum which was 12 inches tall at thirteen years of age, grew 6 inches its fourteenth year. A plant of 'Bibiani' x R. simiarum which was 16 inches at thirteen years, grew 9 inches its fourteenth year.
Rhododendron simiarum hybrids tend to bloom in May during hot weather and have unattractive flowers with "muddy" colors. Plants that bloomed in 1985 after a cold winter and spring seemed to have more attractive flowers.
'Belle Heller' x R. simiarum are upright, low-growing plants with dense, dark green foliage and pink to ivory white flowers with a yellow blotch. Rhododendron fortunei x R. simiarum are upright, dwarf to low-growing plants with light lavender flowers and medium green leaves.
'Bibiani' x R. simiarum plants are low-growing upright to spreading plants with attractive deep green foliage with silvery indumentum. Flowers are deep pink. These plants seem to be completely cold hardy here even though 'Bibiani' is quite tender.
'Grierosplendour' x R. simiarum are dwarf plants as wide as tall with dark green leaves and lavender flowers. Rhododendron hyperythrum x R. simiarum are dwarf, upright plants with rigid leaves and pink flowers in late April. 'Blue Ensign' x R. simiarum are upright, low-growing plants with light lavender flowers that do not open completely.
'Caractacus' x R. simiarum is a dense spreading dwarf with silvery indumented leaves and bright pink flowers. 'Mrs. C.S. Sargent' x R. simiarum is an upright, low-growing plant whose flower buds do not open. This is characteristic of 'Mrs. C.S. Sargent' itself in our climate.
'Annie Dalton' x R. simiarum is a low-growing, spreading plant with pink flowers in mid-April. 'Cavalcade' x R. simiarum is an upright, extremely floriferous plant with salmon flowers.
These R. simiarum hybrids will not have commercial value because they grow too slowly and have poor quality flowers. The late flowering and late new growth suggest that they might do better in the upper South. The hybrids have dark green foliage that remains for two to three years. The leaves are held upright which is typical of the Argyrophyllum series. The foliage tends to stay dark green in the winter. They seem to be quite drought and disease resistant. They might be excellent for further breeding. None of my second generation plants have bloomed. ('Rochelle' x R. pseudochrysanthum) x (R. hyperythrum x R. simiarum) has yielded vigorous attractive dwarf plants. 'Princess Elizabeth' x (R. hyperythrum x R. simiarum) have produced low to medium plants with attractive foliage. (R. fortunei have produced dense, spreading, twiggy plants with dark green foliage. ('English Roseum' x R. adenopodum) x ('Grierosplendour' x R. simiarum) has produced vigorous, tall-growing plants with dark green foliage.
The Drought of 1977
During April, May and June of 1977, we received essentially no rainfall. The weather during May was unusually hot. I had many plants growing over an area of several acres, most of these plants were not adequately watered during this period and many plants died. The R. griersonianum hybrids, 'Anna Rose Whitney', 'Lucky Strike', 'Vulcan', 'Cavalcade' and others seemed particularly hard hit.
Rhododendron fortunei suffered greatly and several plants died. Plants of a large leaved form of R. decorum seemed to suffer less than R. fortunei, but these plants suffered dieback for several years following the drought. Four-year-old seedlings of a small leaved form of R. decorum from H.L. Larson seemed to be quite drought resistant. This small leaved R. decorum resembles the hybrid 'Caroline' in leaf and flower, and blooms quite late. Rhododendron diaprepes 'Gargantua' seedlings seemed to suffer during the drought, but recovered without die back. Hybrids of R. fortunei x R. ponticum seemed to be distinctly more drought resistant than R. fortunei. Rhododendron maximum seemed to suffer, but recovered without damage. 'English Roseum', 'Roseum Elegans', 'Janet Blair', 'Westbury' and 'Caroline' suffered moderately, but recovered completely.
R. arboreum, R. arboreum spp. delavayi
The R. arboreum hybrids, 'Bibiani', 'Dame Nellie Melba' and 'Ivery's Scarlet' seemed to suffer not at all during the drought. Equally drought resistant seemed to be four-year-old seedlings of R. arboreum ssp. delavayi Broderick Castle form and 'Scintillation' x R. arboreum ssp. delavayi.
'Bibiani', 'Dame Nellie Melba', and 'Ivery's Scarlet' are excellent for me. These plants rapidly become small trees in our climate. They bloom in late March and their flowers seem to be frost resistant. They are, however, tender and all were lost during the cold winters of the early 1980's. Rhododendron arboreum ssp. delavayi, Broderick Castle form, grew well, but was also lost during the early 1980's. Rhododendron arboreum ssp. delavayi KW 2176 has not grown for me, however its hybrids grow well.
'Bibiani' x 'Belle Heller' produced a tall, vigorous, upright plant with pink flowers that will be named later. 'Bibiani' x R. hyperythrum produced low to tall growing plants that have just begun to flower. Many other crosses have been made with 'Bibiani'. It seems to be fertile only as the seed plant.
'Dame Nellie Melba' seems to be completely sterile, 'Ivery's Scarlet' is fertile, both as seed and pollen plant. I have only recently begun using it for breeding. Its hybrids grow well.
I do not have many plants from R. arboreum ssp. delavayi crosses. Rhododendron hyperythrum x R. arboreum ssp. delavayi is a low, open-growing, sparsely foliated plant with red flowers. Rhododendron hyperythrum x R. delavayi KW 21976 is a scraggly, low-growing plant with deep pink flowers and silvery indumented leaves. Rhododendron pseudochrysanthum x R. arboreum ssp. delavayi KW 21976 produced a dense, low-growing plant with leaves resembling R. pseudochrysanthum. Rhododendron metternichii Oki Island form x R. arboreum ssp. delavayi KW 21976 produced low-growing, small leaved plants with deep pink flowers and silvery indumented leaves. The plant and flowers closely resemble the pollen plant.
I have been unable to grow R. arboreum other than ssp. delavayi or any of the hardier R. arboreum hybrids.
I have only recently become interested in R. ponticum. Reports from Japan indicate seedlings of R. ponticum grow very well under hot conditions. It has been reported to be highly susceptible to Phytophthora root rot and die back, however other reports indicate selected forms of R. ponticum are resistant to these diseases.
During the early 1970's I grew seedlings of R. ponticum and found that some of them grew well, but I abandoned them in favor of other species. Except for 'Anah Kruschke', the R. ponticum hybrids have not been successful for me.
In 1985, I purchased a budded plant of R. ponticum from the Rhododendron Species Foundation. The plant was damaged in shipment and did not survive, however, pollen was obtained and several crosses were made, ('Blue Ensign' x R. hyperythrum) x R. ponticum have yielded vigorous, upright plants with dark green leaves and lavender flowers. Rhododendron metternichii 'Kyushu' x R. ponticum has produced vigorous, upright plants with dark green leaves. These R. metternichii x R. ponticum seedlings grow much better than R. metternichii crossed with R. catawbiense, R. maximum, R. decorum, R. fortunei or with the Dexter hybrids. 'Westbury' x R. ponticum has produced vigorous, tall-growing plants that have not bloomed.
R. chapmanii and R. minus
Rhododendron chapmanii has been crossed with R. racemosum, R. pubescens, R. spiciferum and R. scabrifolium. None of these crosses produce attractive flowers.
Rhododendron chapmanii crossed with R. ciliatum and R. bullatum has produced plants with white flowers. The plants lack vigor and seem to be disease susceptible.
Rhododendron chapmanii x R. mucronulatum are extremely vigorous, adaptable plants that often bloom in the fall and winter. The lavender flowers are not particularly attractive. Rhododendron chapmanii x R. dauricum has produced low-growing, twiggy plants with lavender flowers. These plants lack the vigor and adaptability of the R. chapmanii x R. mucronulatum. They seem to be quite disease susceptible.
Rhododendron chapmanii x R. formosanum var. iteophyllum has yielded an upright, medium-growing plant with pink flowers. This plant seems to be very disease resistant and adaptable. It has been named 'Willow Run'. Rhododendron minus, white form (Kolomiki) x R. formosanum has produced extremely vigorous willowy plants with white flowers and a yellow blotch. These plants seem to be very disease resistant. One has been named 'Will-O-the-Wisp'.
(Rhododendron minus x R. chapmanii) x R. mucronulatum, mahogany red form has yielded medium-growing plants with deep purple flowers of excellent quality., (Rhododendron minus x R. chapmanii) x R. mucronulatum var. taguetii has resulted in low-growing plants with attractive lavender flowers.
R. formosum and R. formosum var. iteophyllum
These plants come from India. They are much more disease resistant than are R. chapmanii and R. minus. Except for drainage requirements, they rival the southern Indica azaleas for ease of cultivation. They are, however, tender. Rhododendron formosum could perhaps be crossed with other hardier lepidote rhododendrons to obtain heat resistant hardy plants. They might be crossed with other Maddenii series plants to obtain heat tolerant greenhouse plants.
R. fortunei, R. decorum, R. diaprepes
Rhododendron fortunei grows and blooms very well for me. Early forms have attractive flowers. I have been unable to grow any of the R. fortunei hybrids except 'Van Nes Sensation' and 'Mrs. AT. de la Mare'.
Selected forms of R. decorum grow well for me. Most forms bloom late and have poor quality flowers. Some forms are shy bloomers. 'Caroline', the Gable hybrid, reportedly is a R. decorum open pollinated seedling. It grows very well and is very disease resistant, but, 'Caroline' has been a disappointing parent. One plant, however, 'Caroline' x (R. fortunei x R. ponticum) shows promise.
The hardy Dexter hybrids have been suggested to be mostly R. decorum x R. catawbiense hybrids. As a group perhaps one-half of them grow fairly well for me. The Dexters crossed among themselves and with 'Caroline' and 'Van Nes Sensation' have yielded disappointing results.
Rhododendron diaprepes 'Gargantua' recently has been used to make a number of crosses. The seedlings are growing very well.
'Albert Close' (R. maximum x R. macrophyllum) is the only R. maximum hybrid I have been able to grow. Rhododendron maximum as a parent has been disappointing. Its hybrids lack vigor and adaptability. Only plants of R. maximum x R. hyperythrum and R. maximum x R. pseudochrysanthum have produced worthwhile plants.
Rhododendron catawbiense var. insularis theoretically should be a good parent for heat resistant plants. These plants grew and flowered for me for several years before dying.
A number of crosses have been made with R. catawbiense var. insularis. The seedlings grew poorly and were subject to diseases. Only R. catawbiense var. insularis x R. hyperythrum and R. catawbiense var. insularis x R. pseudochrysanthum have produced satisfactory plants.
The old R. catawbiense hybrids, 'English Roseum' and 'Roseum Elegans' grow much better here than R. catawbiense var. insularis and they grow much better than the other R. catawbiense hybrids including those of Leach and Shammarello. Perhaps they have some R. ponticum blood in them.
'English Roseum' x R. adenopodum produced low-to-medium growing plants with lavender flowers that seem worthwhile. 'Roseum Elegans' x R. makinoi produced low-growing, sun tolerant plants with pink flowers.
I have tried many of the R. catawbiense hybrids crossed with R. fortunei and R. decorum from the ARS Seed Exchange. None of these have produced plants that grow well here.
The red R. catawbiense hybrids have been hard to work with as parents. They seldom have viable pollen in our climate, and their seed capsules usually die before maturing. 'Nova Zembla' x 'Grierosplendour' produced a low-growing extremely floriferous plant that will be named 'Nova Splendor'. The flowers are dark red.
R. adenopodum, R. makinoi, R. metternichii
Rhododendron adenopodum is an excellent plant. It flowers profusely from an early age and has very attractive flowers. Rhododendron makinoi and certain forms of R. metternichii likewise do well.
The hybrids of these three species seem to lack vigor and their flowers are often unattractive. The plants are not particularly drought resistant. Joe Gable made many crosses using these species, but seemed to produce little worthwhile results.
I have been unable to grow R. griersonianum, however, about one-half of its commercially available hybrids grow fairly well. These hybrids are all very susceptible to spider mite infestation. 'Damozel', 'Grierosplendour', 'Van Veen', and 'Anna Rose Whitney' seem to be the best of the R. griersonianum hybrids for us. 'Damozel' is fertile only as the seed plant. Its hybrids, except those with R. hyperythrum and R. formosanum have not grown well. 'Grierosplendour' hybrids grow considerably better than those of 'Damozel'. 'Grierosplendour' x R. adenopodum, R. metternichii and R. makinoi plants have grown fairly well, but the flowers are not attractive. 'Anna Rose Whitney' seems to be completely sterile in our climate.
Rhododendrons should be easily established, disease and pest resistant, attractive in plant and flower and drought resistant if they are to become popular plants with casual gardeners. The R. hyperythrum hybrids are easy to establish and disease and pest resistant. Second generation R. hyperythrum hybrids should give a good color range of attractive plants.
To be drought tolerant here, plants should be grown in built-up beds with adequate depth and volume so that a large root system can develop. Rhododendron arboreum (and possibly R. simiarum) seem to contribute greatly to drought resistance in rhododendron hybrids. 'Bibiani' x R. hyperythrum, 'Bibiani' x R. simiarum or R. hyperythrum x R. arboreum ssp. delavayi might be crossed with commercial hybrids containing R. arboreum blood to get attractive drought resistant plants.
'Pink Pearl' has R. arboreum in its ancestry. 'Antoon van Welie', 'Annie E. Endtz' and 'Professor J. H. Zaayer' have been suggested to be 'Pink Pearl' x 'John Walter'. 'John Walter' is R. catawbiense x R. arboreum. I have had difficulty getting seed from these Pink Pearl hybrids. Perhaps someone in the upper South or someone with an air-conditioned greenhouse could get seed.
Gardeners in central and southern Florida cannot grow many plants that thrive in northern Florida. Rhododendron hyperythrum reportedly grows from 500 to 5000 feet elevation in Taiwan. Since Taiwan has a climate similar to Cuba, perhaps forms of R. hyperythrum or some of its hybrids might do well in southern Florida. Rhododendron formosum and R. formosum var. iteophyllum might be good plants to try in Florida. Crossed with other Maddeniis they might produce large flowering fragrant plants that will grow outdoors. Likewise, R. championae and R. ellipticum of the Stamineum series might do well in southern Florida. They might be crossed with the other Stamineums to produce a unique class of rhododendrons for mild climates.
For further information see Dr. Thornton's article "Growing Rhododendrons in the Gulf South", ARS Journal Vol. 43:4 (Fall 1989).