JARS v53n3 - Rhododendron hyperythrum and Its Hybrids: A Hope for the South

Rhododendron hyperythrum and Its Hybrids: A Hope for the South
Dr. Robert Means
Winston-Salem, North Carolina

Several rhododendron hybridizers in the South are placing their hopes in the species Rhododendron hyperythrum to pass onto its progeny the ability to thrive in the extremes of our Southern climate. This Japanese species has proved to be heat and cold tolerant and resistant to drought, disease, and pests and is already proving to be a prize of a parent.
I have been using Rhododendron hyperythrum in crosses since 1986. Dr. John Thornton of Franklinton, Louisiana, has been using it even longer. Several others also have an interest in using R. hyperythrum and its existing crosses in their own hybridizing programs.
In late November 1996, Stan Southerland from Chapel Hill, North Carolina, came to spend the day. Reid Bahnson from Winston-Salem and I guided him through our home and nursery test gardens and enjoyed sharing knowledge and experience. Stan is a young neuro-molecular technical scientist at the University of North Carolina, Chapel Hill. He and his teacher bride of four years have been acquiring and installing rhododendrons on their one plus acre wooded lot. Some heat tolerant Rhododendron arboreum and R. hyperythrum hybrids had been obtained from John Thornton. He showed us pictures of many of his plants in bloom and demonstrated a keen sense of observation which reminded me of what there is to see. He obviously loved his plants and knew them intimately.
The plants of Rhododendron hyperythrum Stan has in his garden vary from a dark green, heavily re-curved leaf to a lighter green, flatter leaf. He has the pink form from John Thornton named 'Doctor John L. Creech'*, and seedlings raised from ARS Seed Exchange seed from the Japanese Rhododendron Society.

He had read in one of our Piedmont Chapter newsletters of my interest in using Rhododendron hyperythrum as a parent. (I have done so on 19 occasions since 1986.) He was shown the original plant, now 8 by 8 feet, with no record of its source. The flowers are large, slightly lax, and white with a small deep purple blotch. The leaves are medium green, coriaceous (leathery), shiny, slightly re-curved and show some quilting. The pale yellow midrib is recessed on the upper surface. The plant was probably a seedling from the ARS or Royal Horticultural Society seed exchange and, I suspect, came from the Taiwan Venture in the early 1970s. John Patrick (4, 5), who participated in that plant exploration trip, suggested that many R. hyperythrum seedlings later circulated on the West Coast were second generation and open-pollinated.
I showed Stan a group of 4-year-old Rhododendron hyperythrum seedlings (ARS 223-92) which are more compact with marked leaf re-curving. Many have very attractive red petioles and growth and bloom buds which contrast nicely with the green leaves. He was given one of these to use in his future hybridizing efforts. They were fully budded and bloomed for the first time in spring 1997. I also gave him a seedling of one of my second generation crosses made in 1991 [('Madame Masson' x 'Nestucca') x ( R. hyperythrum x R. fortunei , white flowered)].

I hope to share many of these seedlings with Stan for continued evaluation and use as parents. His youth is a definite plus. He has already demonstrated his purpose and passion and I'm inclined to believe he has the third requirement of a hybridizer, which is perseverance.
Stan Southerland's hybridizing goals are tough, low maintenance plants that easily adapt to conditions in the South. Rhododendrons with large red flowers in a tight truss and thick dark green leaves are also a goal. By "tough," Stan means that the plants can take extremes of heat and humidity and are resistant to disease and insect attack. He points out that the ability to take the wild temperature fluctuations of fall and late winter is important as is reasonable drought resistance. The ability to take extreme cold is less important. Tolerance of -10°F to -15°F would be fine. The record low in his area is -9°F. He has purchased from Dr. Thornton, the hybrids 'Charles Loomis'* ('English Roseum' x R. hyperythrum ) with clear, white flowers, 'Opal Thornton'* ('Antoon Van Welie' x R. hyperythrum ), 'Michele Smith'* ('Scintillation' x R. hyperythrum ) with large white flowers with a dorsal purple blotch in a nice truss, 'Pam Morris'* ('Trilby' x R. hyperythrum ), and 'Jim Lynch'* ('Trilby' x R. hyperythrum ) with pink flowers in a nice truss, holding its color well. It appears its pollen is sterile but as a seed parent it is fertile. Stan says the R. hyperythrum hybrids are generally very attractive plants with very dark green glossy leaves, some of which are heavily veined and re-curved. Stan's own crosses include 'Jim Lynch'* crossed with 'Kilimanjaro', 'Noyo Chief, and 'Peter Faulk'.

Wyatt LeFever of Kernersville, North Carolina, first became interested in Rhododendron hyperythrum when my son Rob brought a truss of my original species seedlings, white with a purple dot, to one of our chapter meetings years ago. I have observed many of his personal crosses made over the years, some extremely nice foliage plants with good bud set. Wyatt has the Windsor, Patrick, and Exbury forms of R. hyperythrum in his garden as well as numerous hybrids by Mike Stillwell of Dugspur, Virginia. Mike's crosses of the species with 'Albert Close', 'Dexter's Peppermint', and 'Ponticum Roseum' were all great foliage plants and fully budded. The two crosses by Mike Stillwell, ('Albert Close' x R. hyperythrum ) and ('Maxecat'* x R. hyperythrum ) are in almost full sun, proving sun and heat tolerance. They are floriferous and have no problems with dieback or lacebugs and have grown into well shaped shrubs without pruning or pinching in any way.

John Thornton (1) used the white form of Rhododendron hyperythrum to cross with 'Westbury', 'English Roseum', 'Van Nes Sensation', 'Trilby', 'Antoon van Welie', R. maximum , and R. degronianum ssp. heptamerum . He also used a deep pink form to cross with red hybrids yielding a high percentage of plants with red flowers. Many of his named hybrids are very promising. Rhododendron hyperythrum has proved to be an excellent parent for breeding for heat tolerance in his breeding program. He notes that the pure white forms give very little color in the first generation hybrids, and the deeper pink forms give more color in the first generation. He has named a long, narrow leaf form 'Doctor John L. Creech'*. It is pink in the bud opening white and is a vigorous grower and good parent.
Dr. Leonard Miller of Grove, Oklahoma, says that in his area Rhododendron hyperythrum is one of a few species that is not sensitive to root rot ( Phytophthora cinnamomi ). He also finds that it survives drought better than most garden plants. Although the hardiness varies from region to region, he has had 5-year-old plants bloom after -8°F.
Rhododendron hyperythrum was first described by Hayata in 1913 from a specimen collected on Mt. Shichisei, Taiwan. It was introduced by Lionel de Rothschild in the 1930s (2). It is endemic to Taiwan (Formosa) along with three other elepidote species: R. morii , R. pachysanthum , and R. pseudo-chrysanthum . Of the four only R. pachysanthum has indumentum and this on mature specimens (3). Dr. Chen-Chang Hsu, taxonomist and faculty member of the National Taiwan University, had located a population of R. hyperythrum at low elevations near Taipei (4, 5) in the tropical to subtropical climate similar to Cuba (1). There is no overlap in the geographical distribution of R. hyperythrum and the other three species (3).
Olin Holsombach of Ringgold, Georgia, accompanied the Taiwan Venture and collected seed of Rhododendron hyperythrum and other species. He reported several forms of R. hyperythrum in 1986: two Patrick forms, one pink, one white, and the white Windsor form. He also mentioned a form called Sunningdale. His seedlings had not bloomed at the time of this article, but he was enamored with the brilliant and colorful bronze new growth. He was also impressed with their hardiness - surviving -12°F in the North Georgia mountains.

Evidently there is considerable variation in the species stature and flowers according to published descriptions. Harold Greer (2) estimated a height of 3 feet in 10 years. The flowers were described as white occasionally with purple spotting. In Rhododendrons of China , the 1980 English translation of a Chinese book published in 1974, Rhododendron hyperythrum is described as an evergreen shrub to 3 meters high. The flowers are white and there is no mention of spotting. H. H. Davidian (7) described the height from 3 to 8 feet and the flowers white or pink with or without purple spots.

Koichiro Wada began using Rhododendron hyperythrum as a parent in 1960. When he died in 1981 he left behind about 2,000 seedling crosses with heat and sun tolerance transmitted as a dominant trait. His son (8) Tomoo in 1983 suggested that it would take several generations to dilute the dominance and improve on other qualities, such as flowers. At that time he had made two selections, both having white flowers with a dark red blotch.
My own brief experience using Rhododendron hyperythrum is too soon and brief to make any worthwhile conclusions. The plants are heat and sun tolerant and quite hardy in USDA Zone 7. They are disease and pest resistant with very attractive foliage and compact plant habit. The flowers from early crosses are large but somewhat lax. Therefore my recent efforts have concentrated on obtaining fuller, better shaped trusses.
Our best hope for the future is to disseminate and critically evaluate existing hybrids of Thornton, Stillwell, LeFever, Wada, and many others as they they are produced; and encourage youthful hybridizers like Stan Southerland.

Rhododendron hyperythrum Hybrids
A report of names with " hyperythrum " in either parent field by Clover Springs Computer Services, Rhododendron and Azalea Names, copyright 1999. Reprinted by permission.

Registered Names
'Blazing Star' ('Delp's Fireking' x R. hyperythrum )
'Cecil's Goal' [('Koichiro Wada' x 'Bow Bells') x R. hyperythrum ]
'Hypermestra' ( R. hyperythrum x 'Rosy Morn')
'Omo' (selection of R. hyperythrum )
'Peppermint Twist' ( R. hyperythrum x 'Scintillation')
Quaker Girl Group ( R. hyperythrum x 'Avalanche')
'Rhodsoc' ('Delp's Fireking' x R. hyperythrum )
'Ring's Early Splendor' ( R. hyperythrum x 'Catalgla')
'White Crest' ('Crest' x R. hyperythrum )
'Woodcock' ('Elizabeth' x R. hyperythrum )

Unregistered Names
'Aunt Clara' ('Scintillation' x R. hyperythrum )
'Charles Loomis' ('English Roseum' x R. hyperythrum )
'City Park' ( R. metternichii x R. hyperythrum )

'Doctor John L. Creech (form of R. hyperythrum )
'Elizabeth Ard' ('English Roseum' x R. hyperythrum )
'Hypcat' ( R. hyperythrum x 'Catalgla', green-eyed form)
'Hypermax' ( R. hyperythrum x R. maximum )
'Jim Lynch' ('Trilby' x R. hyperythrum )
'Lisenne Rockefeller' ('Van Nes Sensation' x R. hyperythrum )
'Michele Smith' ('Scintillation' x R. hyperythrum )

'Opal Thornton' ('Anton van Welie' x R. hyperythrum )
'Rhodworks' ('Delp's Fireking' x R. hyperythrum )
'Ruby Magee' ('Westbury' x R. hyperythrum )
'Starglider' ('Tols' x R. hyperythrum )

Registered Names of Second Generation R. hyperythrum crosses
'Dandylicious' ('Midsummer' x 'Hypermax')
'Mad about Max' ('Midsummer' x 'Hypermax')

References
1.  Thornton, John T. 1991. Breeding rhododendrons for the Gulf South. Journal ARS 44: (2) 91.
2.  Greer, H. E. 1982. Greer's Guidebook to Available Rhododendrons. Eugene, OR: Offshoot Publications.
3.  Cox, Peter A. 1983. Rhododendron pachysanthum . Journal ARS 37: (4) 188.
4.  Patrick, John. 1970. Taiwan Venture. Quarterly Bulletin ARS 24: (1) 43-44.
5.  Patrick, John. 1975. Field investigation of a new Taiwan rhododendron. Quarterly Bulletin ARS 29: (1) 20-25.
6.  Holsombach, Olin. 1986. Journal ARS 40: (3) 133.
7.  Davidian, H. H. 1992. The Rhododendron Species Vol. III . 162. Portland, OR: Timber Press.
8.  Wada, Tomoo. 1983. Journal ARS 37: (4) 191-196.

* Name is unregistered

Dr. Means is a member of the Piedmont Chapter.