JARS 43n2 - Breeding For Hardy Vireya Rhododendrons

Breeding For Hardy Vireya Rhododendrons
J. W. Gerdemann
Yachats, Oregon

Most Vireyas will tolerate a temperature of 32°F, or a degree or two lower, for brief periods. However, only a few will survive a prolonged freeze with temperatures in the mid-twenties. In mild climates even a slight increase in hardiness would be useful. At my location on the Central Oregon Coast only during one winter out of the past seven have temperatures dropped below 25°F. However, the temperatures here can remain below freezing considerably longer than is the case farther south. It is not uncommon for temperatures to remain below freezing for 12 or more hours when minimums are in the range of 25°F - 28°F. Such prolonged freezes will kill or severely injure most Vireyas.
For a Vireya species or hybrid to be of much use in breeding for increased hardiness I believe it should survive, uninjured, at least 25°F with the temperature below freezing for 12 or more hours. Few Vireyas meet these criteria.
The following have been tested and have failed to meet these minimum requirements: R. alticola SE 76-724 Kores C.W'. Bull Dog Road, New Guinea; R. beyerinckianum , pink and red forms; R. christi two sources, unknown and Wei 1500 P78; R. hellwigii (from seed labeled R. superbum - plant and red flowers fit the description of R. hellwigii ); R. macgregoriae Pratt; R. pauciflorum ; R. quadrasianum var. rosmarinifolium - survived but was severely damaged; R. rarum red flowered; R. retusum ; R. rugosum Mossman & Goheen Mt. Kinabalu 10,900 feet; R. womersleyi (?) Don Stanton; R. 354382, Peter Schick suggests this may be R. macgregoriae x R. phaeochitum .
A much more intensive search for hardiness should be made. Other collections of the above listed species need to be tested along with other Vireya species native to high elevations.

Sources of Hardiness
R. kawakamii : This is an attractive species with small yellow flowers produced in mid to late summer. It grows as an epiphyte in Taiwan and thrives on rotten logs in my garden. It has grown at my location without injury from cold for eight years, and is grown in the open ground at Willard Thompson's Nursery 3 miles east of Waldport, Oregon. It is probably hardy to 10 or 15°F. This is an attractive plant in itself, but it will probably be of limited use in breeding as all efforts to cross it with Malesian Vireyas have failed. It has been crossed with R. santapaui (5).
R. santapaui : This species was discovered by Cox and Hutchison in Arunachal Pradish in 1968. The small waxy, white flowers produced in late summer are quite attractive. It has grown in my garden for two years. Cox (1) suggests that its hardiness may be similar to R. kawakamii . I have crossed it with R. pauciflorum . Abundant seed with low germination was produced. Seedlings lacked vigor and all subsequently have died. Further attempts at hybridizing with the other Vireyas should be made.
R. saxifragoides : This is the R. yakushimanum of the Vireyas, low growing, compact, and probably the hardiest of all New Guinea rhododendrons. I have not as yet grown it in my garden, but feel certain it would meet the minimum criteria for hardiness. It grows at elevations up to 13,000 feet in New Guinea, where night temperatures frequently are below freezing. It is an extremely difficult species to obtain and to keep alive. I now have a few seedlings that are growing very slowly. This is a very attractive plant, but its usefulness in hybridizing may be limited by its slow growth and temperamental nature.
I obtained two seedlings of a cross made by Dr. Frank Mossman, R. lochiae x R. saxifragoides , at the plant sale during the ARS Convention in Portland in 1983. At first they showed great promise, growing into attractive, compact plants that were hardy in my garden. They now have nearly stopped growing and seem intent on suicide. Attempts at obtaining other hybrids should be made whenever pollen is available.
J.L. Rouse has described and named what is probably a natural hybrid between R. saxifragoides and R. womersleyi , 'Rogue Red'. It is larger and more free flowering than R. saxifragoides , and its high alpine origin suggests that it would meet or exceed my criteria for hardiness.
R. commonae : This is the most promising species I have found. The form collected by S. Pratt at Dolo Pass R.S.F. 73-035 has been growing in my garden for four years and has never been injured by low temperatures. It tends to be dormant during the winter and resume growth in the spring.
Halligan (2) reported that it was uninjured by 25°F. He also tested its leaves for hardiness in vitro and found them hardy at 20°F. (3). They were not included in his test at 15°F, but at 10°F the leaf tissue was killed (personal communication). Two other collections, one from Don Stanton and another from the garden at Laigan P.N.G. survived the past winter without injury (28°F - 12 hours below freezing). This species is easily grown, crosses readily with most other Vireyas and its hybrids are vigorous. Its only fault is small flower size.
R. lochiae x R. pseudonitens : ( R. pseudonitens is now considered to be synonymous with R. commonae ) (4). This is a hybrid from Peter Sullivan. Under my conditions it is dormant during the winter and flowers for a long period during the summer. It has survived without protection in my garden for the past three years. This is an excellent hybrid that should be named. It could also be very useful in hybridization as it is hardy to 25°F. By back crossing to R. commonae and selecting the progeny for hardiness it may be possible to obtain hybrids hardy to 20°F or lower. I believe that initially hardiness should be the priority. Quality of plant and flower can be improved by later hybridization and selection.

R. lochiae x R. pseudonitens
R. lochiae x R. pseudonitens , this hybrid makes a
wonderful display for at least two months in the late
summer, usually August and September.
Photo by J. W. Gerdemann

Breeding For Hardiness
I have made the following crosses and the progeny are being tested for hardiness:
( R. lochiae x R. zoelleri ) x ( R. commonae ): This is a cross made in 1985. The winter of 1985-86 was very mild, with a low of 30°F. This was not cold enough for selection of seedlings for hardiness, so in mid January I took three flats containing 330 seedlings to my son's backyard in Albany, Oregon, in the Willamette Valley. Unfortunately temperatures at the site were not recorded; however, data from nearby weather stations suggest a low in the high 20's on the 14th and mid to low 20's on the 15th. These were ideal conditions for selection for hardiness. Although no seedlings were killed, all were severely damaged and all but 30 of the least injured were discarded.
Further selection for vigor reduced the number to 16, which were tested again during the following winter. During the winter of 1987-88 flats containing the remaining 16 seedlings were placed on an open deck. A recording thermometer was placed near the plants. A minimum of 27°F was recorded, with temperatures below freezing for 12 hours, and the planting mix in the flats became frozen. There was, however, only slight injury to the new growth of a few plants.
Seed from this cross was also sent to Don Paden, Urbana, Illinois, who tested the seedlings for hardiness under various conditions. He found that a few seedlings were uninjured by exposure to 24°F; however, all seedlings exposed to 18°F were killed. The seedlings of ( R. lochiae x zoelleri ) x R. commonae are, as one might expect, highly variable. Those resembling R. zoelleri appear to be slightly hardier than those resembling R. lochiae . The flowers from this cross should be interesting, but I doubt if any of the plants will be hardy much below 25°F.

R. wrightianum var. cyclopense x R. commonae
R. wrightianum var. cyclopense x R. commonae ,
a seedling two years old illustrates the compact
growth habit of this hybrid.
Photo by J. W. Gerdemann

R. wrightianum var. cyclopense x R. commonae : This cross was made in 1986. During the winter of 1987-88 a flat containing sixteen seedlings was placed on an open deck. None of the plants was injured by exposure to 27°F with temperatures below freezing for 12 hours. These are beautiful plants, vigorous, profusely branched, with small, dark green leaves. Flowers should be small red bells. Hardiness should be at least 25°F, perhaps lower.
( R. lochiae x R. pseudonitens ) x R. commonae : This cross was made in 1986. During the winter of 1987-88 forty one seedlings in flats were placed on an open deck; none was injured by exposure to 27°F with temperatures below freezing for 12 hours. This is my most promising hybrid. The seedlings have attractive foliage. Flowers should be red, and will no doubt vary considerably in size among the seedlings. If subjected to just the right degree of freezing it might be possible to select plants hardy to a low of 20°F.
Seedlings from all crosses have now been planted in the ground and await a good test winter.

Literature Cited
1. Cox, Peter A., 1985. The Smaller Rhododendrons , Timber Press.
2. Halligan, Pat, 1985. "Experiences with growing tropical rhododendrons". American Rhododendron Society Journal , 39: 74-75.
3. Halligan, Pat, 1985. "In vitro determination of hardiness of rhododendrons". American Rhododendron Society Journal 39: 127-130.
4. Van Royen, P. & P. Kores, 1982. The Ericaceae of the High Mountains of New Guinea . J. Cramer.
5. Withers, Dr. R.M. (1987) "The non-Malesian Rhododendrons, Section Vireya". American Rhododendron Society Journal 41: 148-153, -166.

I wish to thank Donald W. Paden for testing some seedlings for hardiness and Thomas F. Tatum for helpful comments on the manuscript.

Postscript ( February 27, 1989)
This paper was submitted for publication prior to the extreme cold of the first eight days of February. In my garden night temperatures were below freezing for eight consecutive nights and daytime highs were below freezing for three days. The lowest temperature recorded was +15°F. Weather of this kind provides a good opportunity to learn more about the hardiness of rhododendrons and other ornamentals.
Here is a preliminary report on the survival of "hardy" Vireyas.
R. kawakamii : Except for a slight browning of foliage on one of five plants there is no apparent damage to plant parts either above or below the snow line.
R. santapaui : Killed to the snowline.
R. commonae : Survived below the snow. One plant of R.S.F. 73-035 exposed to 17°F prior to being moved into a garage was injured but will very likely survive. Plants without snow cover and subjected to the entire episode were killed. A hardiness rating of +20°F seems reasonable.
R. lochiae x R. pseudonitens : Survived below snow.
R. hellwigii nearby but under deeper snow than R. lochiae x R. pseudonitens was killed.
My hybrids: The seedlings were exposed to +24°F. Then a prediction of +10°F prompted me to move them into the garage for the duration. They were replanted when temperatures went above freezing. Some plants of the cross ( R. lochiae x R. zoelleri ) x R. commonae show signs of injury and can be discarded. All plants in the other two crosses survived and, except for new growth on a few, they appear uninjured. This was not a good test winter. Had the plants been left outside for the entire cold period all would have been killed to the snowline.
Events of this month support the idea that R. commonae hybrids can be produced that are hardy to the mid or low 20's.

Jim Gerdemann's garden is located in a naturally protected area of the Oregon coast. He grows many tender rhododendrons not normally seen in this area. The garden was a featured stop during the Coast Tour, 1987 ARS Convention, held at Eugene, Oregon.