Hybridizing Rhododendrons in the Midwest Climate
H. Roland Schroeder, M.D.
Reprinted from Massachusetts Chapter "Rosebay"
This is an update on my hybridizing program for producing hardy rhododendrons in the Midwest region of the United States. I must say at the outset that I am a neophyte at hybridizing, and that this program is only eleven years old. You soon learn that this amount of time is almost infinitesimal when it comes to hybridizing.
There are no native azaleas in this region, and the climate is a hostile one. The soil along the Ohio River is acid, and would be conducive to raising rhododendrons if it weren't for the climate.
Some climatological data about this area is worthy of note. The prevailing wind direction is from the south, but during the winter months we have strong cold winds blowing from the north and northwest, following cold frontal passages, and the arctic dome of frigid air settles in. But as soon as the high pressure ridge moves by, moderation usually begins, with the wind shifting to the west and again to the south.
Geographically, Evansville lies in the path of moisture-bearing low pressure formations that move from the western Gulf region northeastward over the Mississippi and Ohio Valleys to the Great Lakes and northern Atlantic coast. A great part of Evansville's precipitation is a result of these storms, especially during the cooler part of the year.
Both temperature and precipitation are closely related to the movement of the polar front and the storms which move along the front. This is especially true in the winter and spring months. These factors cause a considerable variation in seasonal and monthly temperatures and precipitation from year to year. The summer and early autumn changes are less severe, and periods of polar air invasions are less prolonged.
Convective thunderstorms in the maritime tropical air from the Gulf of Mexico and squall line activity seem to be the factors which combine to supply the summer rainfall.
Most wintertime precipitation occurs when storms move eastward to the south of Evansville. A comparatively few miles of difference in the distance south of Evansville of the paths of these storms often spell the difference between whether the precipitation here is snow, rain or freezing rain.
Snowfall varies greatly from season to season, as do rainfall and temperature. Approximately thirteen inches of snow falls in Evansville during an average year. Of note is the fact that snowfalls of two or more inches are very infrequent, and these amounts usually melt away in a day or two.
The temperature fluctuates rapidly at times, causing heaving of the soil. The soil temperature four inches below sodded ground averages in the low 40's F. in April, with a rapid heat-up to 81°F. by July.
The air temperatures during the winter of 1976-77 dropped to -26°F. in January, and the ground froze to a depth of 24 inches. During the winter of 1981-82 the air temperature dropped to -18°F., with a 60 MPH wind from the northwest and no snow cover. The wind chill factor was -80°F.
The average date for the last freezing temperature in the spring is April 7, and the earliest freezing temperature in autumn is on October 23.
There are colder regions where rhododendrons grow, but these areas are almost always covered with fairly deep snow for long periods of time, thus protecting the plants under the snow at +32°F. regardless of air temperature below -30°F.
It is my feeling that environment plays a great role, along with genetics, in the behavior of plant physiology. It has been observed time after time that plants raised here survived and flourished, while plants brought in from more congenial climates struggled until their deaths in two years or less.
In the beginning, I grew a large flat full of R. fortunei seedlings. These were planted in the open field without wind protection, and after two years there were only fifteen plants left. These fifteen plants flowered during their fifth year and have bloomed each year since. They withstood the winter of 1976-77 when temperatures in the nursery dropped to -26°F. I have used these R. fortunei plants extensively for hybridizing.
One hybrid seedling, R. fortunei x R. 'Chesterland', is being registered under the name R. 'Hillsdale'. It has deep olive green glossy leaves and an upright truss of 16 to 18 flowers, 2½ to 3 inches each, of Neyron rose color. It holds its leaves for three to four years. Another beautiful pink cross of R. fortunei x R. 'America' is very promising, along with others yet to bloom. This R. fortunei crossed with other rhododendrons has a very high survival rate regardless of the tenderness of the pollen parent.
The Bosley Dexter 1005 is an especially promising R. fortunei hybrid for hybridizing. Bosley Dexters 1020, 1016 and 1046A have also been used, and are hardy.
Another elepidote rhododendron that shows great promise for this area is R. maximum. Last year I bloomed a seedling of R. maximum x R. 'Polynesian Sunset' and got a Chinese yellow flower with picotee edge of Spanish orange. This seedling was in a four inch deep flat in a cold frame with screen wire top during the blizzard of 1978 when the temperature dropped to -26°F. There are several beautiful foliage plants of R. maximum crosses yet to bloom, with reddish and pinkish tinted new growth. One cross with R. decorum is outstanding and extremely hardy.
R. 'Russell Harmon' is a very cold hardy and heat tolerant plant. It is somewhat difficult to work with, but I find all R. maximum hybrids and species to be very "tricky" when pollinating them. Great care must be extended when preparing the flower as the female parent. Other R. maximum hybrids such as R. 'Edith', R. 'Maximum Roseum' and R. 'Summer Rose' are showing evidence of being promising parents for cold hardiness and heat tolerance.
R. 'Maximum Roseum' x R. 'Polynesian Sunset' has produced large flowers that are Spanish orange in color. The foliage is about the same as R. 'Maximum Roseum'. This bloomed in four years from seed. R. 'Maximum Roseum' hybrids seem to be disease resistant.
R. 'Edith' x R. 'Marinus Koster' are very hardy seedlings yet to bloom after five years. I hope to produce a larger flower in a nice clear pink color.
The seedlings of R. 'Summer Rose' crosses show a great deal of tomentum with silver and gold sheen on the under surface of the leaf, thus making a beautiful foliage plant.
The crosses with R. 'Purple Splendour' and R. 'Marinus Koster' are magnificent foliage plants, but have not bloomed after four years from seed.
Only a few R. catawbiense hybrids are useful for this region. The ones with some R. arboreum blood in them do best. R. catawbiense is certainly cold hardy, but will not take our high summer time soil temperatures. Most succumb to phytophthora.
R. 'John Walter', an old English hybrid, has transmitted hardiness to rhododendrons like R. haematodes. Some of these have bloomed in four years with nice pink frilled edges. Most of these plants are dwarf or semi-dwarf in stature.
R. 'Sham's Ruby' and R. 'Romeo' have given very nice red "varnish like" new growth. None of these hybrids crossed with the west coast hybrids have flowered as yet. The crosses with R. 'Tomeka' look outstanding as do R. 'Purple Splendour' and R. 'Harold Amateis' seedlings.
A word must be said for R. 'La Bar's White'. This is one R. catawbiense that is heat and cold tolerant. Even though it may transmit color, I feel it is the purest white I have. I have not had any seedlings bloom as yet, but R. 'La Bar's White' should transmit little if any color to its offspring. The buds are almost pure white, whereas R. 'Powell Glass' F6 and R. 'Catawbiense Virgin' have color in the bud. This to me means they are not as pure white as R. 'La Bar's White'.
R. 'Catawbiense Album' is also a good rhododendron for this area, but is not as pure as the old true 'La Bar's White'. There are many seedlings of R. 'La Bar's White' that have been distributed in the trade that do not have the buds without color.
R. 'Ice Cube' and other R. 'Catalgla' hybrids do well and are good parents. They set seed well and give strong seedlings.
R. smirnowii most years has bud blast or imperfect trusses with many frozen pips. It also fights our summers. The hybrid R. 'King Tut' performs about the same, but crossing it with R. hyperythrum makes for heat tolerance and should give a better showing. Seedlings of this cross have nice foliage with some indumentum.
R. yakushimanum cannot stand our hot summers with high soil temperatures. It continuously struggles until death, in spite of very tender loving care. R. yakushimanum x 'La-Bar's White' has produced seedlings that are hardy and heat tolerant. R. yakushimanum x R. hyperythrum produces seedlings that are too tender for our winters.
R. 'King Tut' [(R. smirnowii x R. 'America') x red Catawbiense seedling] x R. yakushimanum F.C.C. also are not tolerant of soil heat; they get phytophthora and struggle on to their demise. R. 'Yaku Prince' and R. 'Yaku Princess' however seem to be more heat tolerant, but our winter air temperatures blast the flower buds almost every year.
Another approach I have taken for hybridizing is to cross complicated hybrid elepidotes by other complicated ones. This surely "shakes" the genes, and you have all kinds of seedlings. The reason for making this type of cross is the thought "why go back to square one when that work has already been done?"
A few "good-doers" showing good combinations of genes and producing good strong seedlings are:
1. R. 'Professor Amateis' (R. 'Everestianum' x R. 'Van Nes Sensation'), x R. 'Everestianum' is probably a R. catawbiense hybrid x R. griffithianum, and R. 'Van Nes Sensation' is R. 'Sir Charles Butler' (same as R. 'Mrs. Butler', a variety of R. fortunei) x R. 'White Pearl' (a sport of R. 'Pink Pearl'). R. 'Pink Pearl' is R. 'George Hardy' x 'Broughtonii', R. 'George Hardy' being R. griffithianum x R. catawbiense and 'Broughtonii' probably R. arboreum x R. maximum.
2. R. 'Rochelle' (R. 'Dorothea' x R. 'Kettledrum'). R. 'Dorothea' is R. griffithianum x R. decorum, and R. 'Kettledrum' is a R. catawbiense hybrid.
3. R. 'Graf Zeppelin' (R. 'Pink Pearl' x R. 'Mrs. C.S. Sargent'). R. 'Mrs. C. S. Sargent' is an R. catawbiense hybrid which does not do well in this locality, but R. 'Years of Peace' (R. 'Mrs. C.S. Sargent' x R. 'Mrs. C.S. Sargent') is tolerant of all our conditions.
4. R. 'Bravo' (R. catawbiense var. album x (R. fortunei x (R. arboreum x R. griffithianum))).
5. R. 'Oregon Sunset' ((R. auriculatum x R. 'Fabia') x R. 'Tally Ho'), a Lem hybrid.
I have had very little experience with the lepidotes for hybridizing, except to try to make R. carolinianum heat tolerant. I crossed the "type" with R. chapmanii for heat tolerance, and have double flowers that are pink. These seedlings have been hardy here. Of course, this cross has been made many times before, in various regions.
Surprisingly, I find all the southeastern native azaleas hardy in this area. The only one with a little tenderness is R. prunifolium, which rarely bud blasts. The deciduous azaleas from England and Holland do not stand the hot summer sun and soil temperatures here. The foliage is nice only in the early spring; by June and into July it dries out and turns brown. Deep shade helps some, but as a foliage plant they are not satisfactory here. Therefore, the Exbury, Knaphill, Ghent, Ham and Slocock hybrids are being crossed with the southeastern native azaleas.
It seems that the crosses with R. austrinum and R. atlanticum are the most vigorous. R. occidentale is completely "out of the question", being extremely sensitive to "hot soil", and therefore very susceptible to phytophthora. Hybrids with R. occidentale blood also do very poorly.
One outstanding cross in 1974 was R. austrinum x R. 'Hugh Wormald'. The flowers are a clear yellow, three inches in diameter and tubular-campanulate in shape. The seedlings have bloomed every year since 1978. The foliage is a reddish purple in the fall of the year. The plants are heat and cold tolerant, free of disease. I have not noticed any mildew on crosses of deciduous hybrids by southeastern native azaleas; one reason is that they will grow in full sun, if crossed with R. austrinum or R. atlanticum.
Several evergreen azalea crosses have been made in the past ten years, with the object of getting dwarf, large-flowered, compact hardy shrubs. It is very difficult to improve on the azaleas we already have, because they offer all colors and shades, except yellow, plus large and small flowers and dwarf to large size shrubs. The Gable, Girard, Greenwood and Shammarello azaleas are the only reliable ones here.
After culling out and much "settling of the dust" over the years, I find the Shammarello hybrids crossed with the Satsuki azaleas and vice-versa give a fair number of hardy dwarf azaleas, which are twelve inches high or less in six years. These grow in full sun and exposure in the test plot.
The Gable, Glenn Dale, Back Acres, Greenwood, Linwood, Robin Hill and Girard azaleas have not produced much different azalea colors, foliage or flower shape than we already have. Work with these has been primarily to produce ironclad plants.
One cross using R. 'Elsie Lee' x R. 'Dream' has given three different shades of color with double flowers, namely: iridescent pink, salmon and lavender.
In conclusion, going back to the statement that environment plays a great part in hardiness, and that plants grown in your locality have much better chances of survival than those shipped in from temperate climates, I would like to cite this example: I made a cross, R. 'Stewartstonian' x R. 'Purple Splendour'. Both plants were shipped in from the south, but are supposed to be hardy azaleas. During the current winter (1981-82), seedlings of this cross, planted in the same location as the parent plants, did not bud blast, but the parent plants bud blasted completely.
Rosebay note: Dr. Schroeder, a gynecologist, lives and practices in southern Indiana. He is also the proprietor of Holly Hills. Inc., a nursery in Evansville, with an impressive list of rhododendrons, including some 400 evergreen azaleas. As if this were not enough to keep him occupied, he served for 2 years as president of the Indiana Chapter of the A.R.S., and currently is serving on the National Board of Directors and Executive Council of the A.R.S. He has written for the A.R.S. Quarterly Bulletin, and is the contributor of no less than 257 separate, hand-pollinated seed lots to this year's National Seed Exchange.
Editor's Note: Dr. Henry R. Schroeder (deceased August 30, 1984).