QBARS - v35n4 Some Thoughts on Breeding Rhododendrons for Cold Climates

William Fetterhoff, Gibsonia, PA

After retirement is a good time to go over my notes and the notes jotted down about other hybridizer's ideas that have accumulated over the years. Although most rhododendron areas along the Atlantic Coast have severe growing conditions compared to much of the Pacific Northwest, some eastern areas depend on selected forms of R. fortunei for hardiness. It is farther inland that temperature of -20° F, or sudden changes in temperature as much as 45° F in thirty-six hours in January and February, or winds up to 55 m.p.h., or sunshine on ice covered plants, are not uncommon. Obviously, both plant and buds have to be tough to survive such conditions.
Rhododendron hybridizing for Western Pennsylvania, Ohio, parts of Michigan and other Midwestern areas had an upsurge after the Great Lakes Chapter was formed in the spring of 1959. Being interested in rhododendron hybridizing, I decided to read the latest book on genetics in the Pittsburgh Carnegie Library. 1 think every hybridizer should have a basic knowledge of: 1) how the chromosomes and genes operate to influence the results of crosses, 2) the multitude of recombination's of characteristics the genes can produce, and 3) the more complicated. the parentage, the more astronomical the number of possible recombinations.
I started hybridizing in 1960 using the "Ironclads" as seed parents with pollen from the West Coast. It was my intent to produce clearer colors on hardy plants of smaller stature. Del James, Cecil Smith, Ben Lancaster, Dr. Carl Phetteplace, Mrs. Ken Janeck, Lester Brandt, Mrs. Virginia Jefferies (Pennsylvania) were most generous with pollen and with words of encouragement and suggestions about what might be most fruitful. Corresponding with these people was a great pleasure. Visits to Dave Leach's garden in Brookville, Pennsylvania, was a real education. Dave gave freely of both advice and plants.
I made all the mistakes in the book plus a few more. Maybe a person should learn to grow rhododendrons for three to five years before starting to hybridizing program. Over the years I did manage to flower a few plants from some of the crosses. As more successes were achieved, I have tried to spread the interest in hybridizing during the last twenty years by giving to rhododendron society members and others more than 100,000 seeds in over 800 packets. The following are a few of the crosses made in the early sixties which produced good rhododendrons: 'Mrs. J.G. Millias' x 'LaBar's White' (One of the plants won Best New Seedling Award and Best of Show Award at Great Lakes Show in 1972.), 'Boule de Neige' x 'Yellow Creek', 'Atrosanguineum' x 'Leaburg', 'America' x 'Britannia', 'Indian Chief' x 'Britannia', 'Blue Peter' x 'Roslyn', R. maximum x R. wardii , R. maximum x 'Azor' seedling, 'Kettledrum' x R. wardii , and unknown x R. yakushimanum .
Some of the hardy plants we have to work with in this cold climate are: R. catawbiense var. album - it is a very good parent that passes on to its off-spring a good degree of hardiness. There are many self-pollinated seedlings of R. catawbiense var. album in existence. Some are open growing plants. The best known clones are 'Catalgla', 'Catanea', and 'Powell Glass', sometimes called R. catawbiense var. album 'Glass'. Another R. catawbiense var. album from the wild is called 'La Bar's White'. Mostly, I used 'La Bar's White' and the hybrid 'Catawba Alba'. Edmund Amateis reported that R. catawbiense 'Rubrum' produced the poorest seedlings of any parent in his hybridizing program. My experience with it has not been much better. However, when crossed with 'Mars' by Dave Leach, a number of good hardy reds were produced; he named one plant, 'Blaze'. Dave also has a yellow plant, 'Good Hope' which is a cross of R. catawbiense 'Rubrum' x R. wardii . 'America' has been a good parent. 'America' crossed with 'Blaze' produced two clear reds. As a follow-up ('America' x 'Blaze') #12 was then crossed with Lanny Pride's Red Brave ('America' x 'Mars'). I sent seed of this cross to five members and to the ARS Seed Exchange. My seedlings dried up in the flats. Weldon Delp and Dr. Thomas Ring received seed from the Seed Exchange and both have beautiful reds out of this advanced cross. 'Mrs. C. S. Sargent' has also been a good parent. It produced beautiful pinks on nice foliage plants.
Both R. brachycarpum and R. aureum ( chrysanthum ) are useful in a hybridizing program if you can be satisfied with slow to bloom seedlings with small flowers, at least in the first generation. R. brachycarpum has many flowers (often 20 to 22) in a truss, a very desirable character. Dr. D. L. Hinerman, Ann Arbor, Michigan, has a R. brachycarpum ssp. tigerstedtii that bloomed early from seed. He crossed it with 'Crest' and gave some seed to Weldon Delp, Harrisville, Pennsylvania, who bloomed the resulting seedlings in his greenhouse in two years. R. aureum has a dwarf stature. I recently acquired pollen of B.C. Potter's "Serendipity" ( R. aureum x R. yakushimanum ) and it will be interesting to see what might be gained from using this compact hybrid as a parent. R. smirnowii , H2 (American Hardiness Rating, is good because of its indumentum and most of its seedlings are compact plants although some tend to bloom in the fall.
R. yakushimanum , an H2 plant, has been an excellent parent, passing on to most of its hybrid seedlings good, compact plant habit, leaves, trusses, and some indumentum if the other parent has indumentum in its ancestry. One fault of first generation hybrids is the fading of their flowers. 'Purple Splendour' x R. yakushimanum has bluish-pink flowers (I thought they would be light purple.) that soon fade to an off white. Other crosses hold their color quite long before fading. My best crosses are 'Pygmalion' x R. yakushimanum , 'Professor F. Bette x" x R. yakushimanum , ('Charles Dickens' x 'Atrosanguineum') x R. yakushimanum , and unknown x R. yakushimanum . This unknown was a compact plant with good green foliage but with rosy pink flowers similar to 'Roseum Elegans'. In the cross with R. yakushimanum there were three plants with heavy, glossy, dark green leaves, the darkest green I have seen. One plant won a silver trophy at the Great Lakes Chapter Show in 1979 because of its good foliage.
Edmund Amateis thought R. maximum was a good parent, transmitting the desirable quality of hardiness in quantity and not passing on to its offspring the plant habit or its very late blooming. Most of the early breeders said R. maximum crosses didn't produce anything in a primary cross and accordingly made few crosses with R. maximum . First generation catawbiense crosses are also only an intermediate goal. I see from going over the ARS Seed Exchange catalogs that there have been quite a number of crosses with R. maximum made in the last fifteen years. I have been waiting with anticipation to see the good plants that will come out of all of these crosses. When making crosses on R. maximum the results are much improved if the R. maximum is an outstanding form. R. maximum varies considerably in the wild as to hardiness, plant habit, plastered indumentum and flower color, ranging from normal pink and white, light pink, dark pink, pure white with cream buds, even to ivory, or you may call it ivory white. My cross of R. maximum x R. wardii produced four yellows out of five plants brought to blooming size. I thought it was pure luck. In discussions with fellow hybridizers, they thought there had to be a reason for the high percentage of yellows. When I started reviewing my notes of the early mid-sixties I found they were right; I had crossed R. wardii on an ivory form of R. maximum at the Treesdale Estate. I had not thought about it all those years. Three years ago I made three trips to Treesdale Estates during blooming season before I relocated the plant, struggling for survival from overgrown shrubs and trees. The new growth was only ¼" long. After fertilizing the plant each year, early this fall I took five cuttings (new growth ¾" long) to Weldon Delp with the hope that he will be able to root them.
One of the yellow-flowered plants of this R. maximum crossed with R. wardii , although slow growing, is straggly. The others are slow growing compact plants. Two have been named 'Adele's Yellow' and 'Susan Kay'. Some of the other R. maximum crosses are: R. maximum x 'America', R. maximum x 'Kettledrum', R. maximum x 'Britannia', R. maximum x 'Mrs. Furnival', 'Mrs. J.G. Millais' x R. maximum , and R. maximum x 'Azor' seedling. The last one has been named 'Adele's Pink'. When that super hardy of all rhododendrons is developed, I predict that R. maximum will be somewhere in its parentage.
It seems to me that much more should be done with R. carolinianum to expand the range of colors of its hybrids. Crossed with the right parents, its hybrids are nice, well-clothed dwarf and semi-dwarf plants that fit well near today's home. Some existing R. carolinianum hybrids might be improved by cleaning up the colors, intensifying the colors and, in some cases, improving the hardiness. Has anyone any ideas about how to produce a red or orange R. carolinianum hybrid by combining it with other lepidote rhododendrons? In making crosses with lepidotes, one should particularly try to acquire the best forms, be it the color, dwarfness, or more evergreen character. Some of my best R. carolinianum crosses are: R. carolinianum x R. yungningense ( glomerulatum ), R. carolinianum x 'Gable's Pioneer' (In my opinion this is an excellent cross, with good plants, good leaves and some with deep solid pink flowers., and R. carolinianum x R. dauricum (same cross as 'P.J.M.'. Most of these plants have different leaves than 'P.J.M.'. One plant is mauve pink. Several R. carolinianum hybrids have been named.
My R. lutescens hybrids are the result of help from Otto Prycl, New Stanton, Pennsylvania. He had a plant of R. lutescens planted against his house and he covered it with plastic in the winter. One year he collected open pollinated seed. I received a pinch of those seedlings from a seed flat. They produced some nice hardy plants with good foliage and yellow flowers. Two plants have been named. How they obtained so much hardiness in the first generation I don't know.
In addition to an understanding of the basics of genetics, it is always valuable to know what earlier rhododendron hybridizers have accomplished. The following, gleaned from letters and the literature should have been followed more closely instead of crossing everything in sight.
Guy Nearing - the ideal cross to shake Mendel's hat would be one between two primary hybrids, one example ('Catalgla' x R. wardii ) x ( R. aureum x R. yakushimanum ).
Cecil Smith - The greatest variation in seedling batches I have seen has been between two hybrids with one ancestor in common, one example ( R. maximum x R. wardii ) x ( R. yakushimanum x R. wardii ).
Ben Lancaster - The less complex the parentage the greater the possibility of reaching your goals.
Ben Lancaster on reciprocal crosses - In reciprocal crosses there is a marked difference in germination, vigor and seedling character. Reciprocal crosses give a clue to dominant traits of the plant which carry through to a marked degree in succeeding generations. Also, seedlings from reciprocal crosses generally develop plant habits resembling the seed parent; the greater the difference in plant size and type of plant used, the more evident these parental traits will be. Note: If we desire a dwarf hybrid from crossing R. maximum with 'Little Gem', we should use 'Little Gem' as the seed parent. But would there be as much hardiness in the seedlings?
Dave Leach - In reciprocal crosses of R. discolor and 'Catalgla' he saw appreciable differences. The superior cross was using 'Catalgla' as the seed parent. It has been my experience that leaf size, branching habit, foliage quality, hardiness, and number of flowers to a truss favor the maternal parent. It is reasonable to assume that the cytoplasm in the ovules interacts with the genes in the nucleus, thus affecting the characteristics which favor the female parent. The prudent hybridizer will be quick to note that the mathematical odds favor a predominant influence of the female parent in his crosses. The parent with the majority of good characteristics might well be chosen to bear the seed. Note: In this cold climate hardiness is our first priority so H1 hybrid or H1 species is nearly always the preferred maternal parent. In the second generation where you have an H 1 parent on each side of the cross, then the plant with the majority of good characteristics can be used as the seed parent.
Rudolph Henny - A good yellow will only come from crossing a red hybrid x R. wardii and crossing the results with a white; also, a good yellow will not come from an R. campylocarpum cross. (I have not seen a yellow hybrid by this method.) William Whitney produced some good yellow hybrids by crossing R. wardii with a red hybrid and then crossing the results with a R. campylocarpum hybrid. R. wardii produces good plants and its pigment is intense and clear. I believe that is successive generations the more R. wardii we put in the crosses with some R. campylocarpum or other yellow species, the more success we will have in producing a deep yellow hardy rhododendron. To date, most of the crosses of yellow with yellow either have not increased the intensity of the color, or increased it only slightly. Maybe someday a hybridizer will find the right combination. Now if we had a super hardy orange red, that would be another avenue to explore.
In this cold climate I don't recommend back crossing. We are looking for bushy plants with clear colors. What can be gained by back crossing on our "Ironclads"? There has been progress toward more hardy rhododendrons having clearer reds, whites, pinks, light yellows, light oranges, near blues and better purples. Leaving out the whites and pinks, some need better plant habit, some need deeper colors and some need better foliage; but hardly any have the hardiness or toughness to bloom year after year in the colder parts of our climate. In addition to disease resistance, super cold hardiness is the most elusive characteristic with which we must deal.
This covers only a few points on hybridizing, and for every rule there seem to be several exceptions. With the progress already made, the greatest potential for further advancement seems to be to improve what is already started by acquiring the plants and pollen available, and to be sure that an H1 parent is on each side of the cross of a well defined hybridizing program. How do we get the rest of the way toward a super tough, bushy, well-clothed, clear colored rhododendron? Perhaps our goals may be reached by crossing like colors and selecting the best, most hardy plants surviving high stress, for successive generations.
Ben Lancaster - To you who have experienced the thrill of anticipation and the sight of some rare hybrid coming into bloom for the first time in your garden, there waits a much greater thrill with a "tonic for living" effect second to none when (after careful thought) you select those parents and bring into bloom anew hybrid that approximates your expectations.