JARS v61n2 - Let's Talk Hybridizing: Northeast Yellow Hybridizing

Let's Talk Hybridizing: Northeast Yellow Hybridizing
Allan and Shirley Anderson
Franklin Lakes, New Jersey

In one sense Northeast rhododendron hybridizers have an advantage over their Northwest counterparts; they have many challenging objectives that are yet to be accomplished. Western breeders work hard to make incremental improvements in their already luxuriant, very showy elepidote hybrids. Easterners on the other hand can cite many gaps they would like to fill to extend the range of growable rhododendrons in their gardens. Perhaps the most compelling need is to breed good garden hybrids with flowers in the yellow to orange range of hues. This has been our own breeding objective for more than twenty years. Of course any new introduction to be successful must overcome the obstacles of the Northeast climate. This involves factors that are not easy to describe and much harder to quantify. In so large an area many conditions exist that impact on growability. There are, in fact, two kinds of eastern climate that can be described as either maritime or continental.

Gardens that are close to the shore, particularly those that protrude into the ocean like Cape Cod or Long Island, will benefit from the maritime effect. A large body of water cools more slowly in early winter and tends to humidify the air that passes over it. This effect along with its equally beneficial tendency to moderate summer high temperatures, can produce growing conditions quite favorable as compared to areas farther inland. Some coastal gardens are nearly as rhododendron friendly as those in the Northwest.

Much more severe are conditions in continental areas. While the ocean can influence weather for miles inland the general pattern for air movement in the northern hemisphere is from west to east. Such patterns frequently bring very cold winter temperatures from the middle of the continent and air that is dry and damaging to rhododendron buds and foliage. Eastern gardens are found anywhere along the Atlantic coast to many miles inland and this makes the hardiness of any individual plant in a particular garden difficult to predict. Therefore, gardening reports from various places in the Northeast give us contradictory impressions of what is growable. From the more continental areas we may hear that almost none of the showier hybrids from the Northwest or Europe will do well in outdoor gardens. Some of the same plants may be grown quite well, however, in coastal locations. Of course in many intermediate places the actual behavior of any hybrid may be highly variable.

For hybridizers working in eastern locations it is difficult indeed to predict where their new introductions may be grown. New plants are tested mostly in the breeder’s garden so that performance in other areas is not known. It is probably fair to suggest, however, that plants developed in colder inland areas have an advantage over those tested in milder regions. They have been exposed to more challenging conditions such as low winter temperatures, high summer temperatures, dry air or even drought. Whatever the specific details, it is clear that new introductions should be distributed and evaluated in a variety of places if they are to be useful in more than a few site-specific places.

The Yellow Problem

It is not surprising that in much of the Northeast relatively few of the popular Northwest elepidote hybrids can be grown with any degree of success. From the earliest days rhododendron enthusiasts have struggled to produce some of the showier features missing from hardy eastern varieties. Conspicuously missing have been flowers in the yellow to orange color range. This has been especially disappointing to those of us who admire the dazzling yellows we have seen in the Northwest.

Efforts to create hardy yellows by the pioneering breeders were especially intense. They recognized that to transfer color from yellow species to new hybrids they must select parents that were already adapted to the Northeast. They hoped thereby to compensate for the weak growing, cold tender, and heat sensitive character of the yellow and orange species such as Rhododendron wardii or R. dichroanthum that they depended on for color (see Fig. 1).

For many years the process yielded rather poor results. Nearly all of the seedlings resulting from the early struggles had flowers so pale that they could hardly be called yellow. There were many such plants that we called "eastern yellow." Such a very pale ivory color occurred so regularly that we thought it might be linked with the hardiness we were trying to produce. It was a very discouraging period.

Perhaps it is cause to regret that the early breeders used mostly species that were native to the region to introduce winter hardiness. Rhododendron catawbiense in its white forms and R. maximum were used over and over with similar results. We believe, after studying the parentage of crosses from our own program and that of others, that a genetic component in species from the subsection Pontica has a strong tendency to suppress color expression in yellow flowers. Crosses made with other Pontica species such as R. brachycarpum or the Japanese indumented group yielded similar results.

A New Direction

More recently Northeast hybridizers have discovered that using the hardier members of the subsection Fortunea has led to much better results. Some of the seedlings from these yellow crosses bloom with emphatically yellow flowers. Donald Hardgrove may have been the first to recognize the advantage of Rhododendron fortunei in yellow breeding more than forty years ago. He introduced ‘Golden Star’ and ‘Donna Hardgrove’ after crossing R. wardii and R. dichroanthum with R. fortunei . Both plants are rather leggy with loose trusses, but they are marginally hardy and have better color than anything seen prior to his time.

During the last few decades many more yellow and golden flowering hybrids have appeared and the majority have resulted from crosses with a few of the best and hardiest Dexter rhododendrons. ‘Janet Blair’ and ‘Scintillation’ have been extensively crossed with favorite Northwest yellow and the results have been impressive. The fact that Dexter hybrids are derived primarily from the Fortunea subsection supports our thesis that this group of species is far more likely to express yellow color than those in the Pontica subsection.

Our Own Experience

We are working in a somewhat continental area in northern New Jersey about 25 miles northwest of New York City. Our program has been in progress for nearly thirty-five years. Casual at first, then more focused in later years, the last twenty have been primarily directed toward producing hardy yellows.

Each year we make 100 or more hand pollinated crosses and select 30 to 40 seed lots for starting in a small heated greenhouse. Seedlings are transplanted into flats in January where they grow through the first winter. Several hundred of these are potted individually in the late summer and then spend the following winter in an unheated hoophouse. In the fall of their second year 400 to 500 survivors are planted out in seedling beds for growing on until they can bloom and can be evaluated or die. We should comment that young rhododendron seedlings are quite vulnerable when growing in small pots. Such containers dry out quickly, particularly when exposed to midday sun. Some degree of protection is helpful; we use 30% knitted shade cloth. We also use automatic overhead watering which relieves us of some of the oversight needed to prevent water stress on small potted plants.

When 2-year-old seedlings are planted in the ground in one of a dozen small beds we find it extremely helpful to set them in straight rows so that they can be irrigated by drip tapes. Our sandy loam soil dries quickly at the surface which threatens the shallow rooted plants when it doesn’t rain for a week or more. Automatic irrigation applied through drip tapes applied three times weekly does much to protect them particularly when in active growth. It also helps to mulch such beds with shredded oak leaves (see Fig. 2).

One of our early hybrids resulted from a cross of 'Vulcan’ by an unregistered plant sold to us as a Dexter hybrid seedling. It had no special attributes other than large pale flowers with a strong fragrance. We registered the best of thirty or more seedlings as ‘Amanda Joan Young’. It is a small plant with bright peachy pink flowers that blooms heavily with a restrained growth habit. ‘Amanda Joan Young’ was later crossed with ‘Golden Star’ and from this group we selected and registered ‘Sea Gold’. The interesting feature of this result is that in the first generation a good yellow can occur with only one-quarter of the genome coming from a yellow species. We believe results like this are only possible in the absence of yellow color diluting factors from the parents. It suggests that yellow may actually be a dominant trait in such cases (see Fig. 3).

To gain some insight about yellow inheritance we studied the parentage of some yellows developed by eastern hybridizers like ‘Bud’s Yellow’, ‘Vinecrest’, ‘Crystal Yellow’ and ‘Lehigh Gold’ as well as a dozen or more others. We learned that every one included a significant contribution from a subsection Fortunea species. We also studied the heritage of many important yellows from Northwest gardens and found that they also derive from subsection Fortunea crosses with yellow species. By far the largest contribution comes from Rhododendron fortunei ssp. discolor and some from a generous admixture of R. fortunei , R. hemsleyanum , R. griffithianum and R. decorum .

It is unfortunate that, except for some forms of Rhododendron fortunei , the rest of the subsection is not easily growable in the Northeast. We can now recognize the importance of some of the Dexter hybrids in eastern yellow hybridizing. They are much hardier than any of the subsection Fortunea species and have better trusses and plant habit; yet they obviously are descended from that group.

In the early period of our breeding program we also found many seedlings with pale ivory colored flowers much like the white Rhododendron catawbiense hybrids from some of the early pioneers. This color invariably resulted from yellow crosses with R. maximum , R. degronianum ssp. yakushimanum and R. brachycarpum . While the evidence is anecdotal we are convinced that the color diluting effect is common to most of the subsection Pontica species.

We believe that most of the Pontica group will share this drawback in yellow crosses, but our experience with Rhododendron aureum and R. caucasicum suggests that these two may be exceptions. The fact that these species frequently produce yellows in the first generation leads us to believe that they may belong in a group separate from the rest of subsection Pontica. We were charmed by these dainty yellow seedlings but found them to be quite vulnerable to summer heat and have reluctantly stopped using them in crosses.

We do try to use yellows that are likewise known to be good parents like ‘Nancy Evans’ and several other unnamed hybrids from our western friends. We also continue to make crosses with Dexter hybrids and to use the offspring in subsequent generations (see Fig. 4). One might ask why a group of plants descended from several marginally hardy species could produce such excellent garden subjects as ‘Scintillation’, ‘Parker’s Pink’ or ‘Janet Blair’, especially since Mr. Dexter grew them in a maritime climate more benign than the rest of the region. There is no obvious conclusion, but remember that he grew many thousands of seedlings which were tested ultimately in hundreds of eastern gardens. It seems likely that after many years a few plants have proven to be exceptionally good. These are the fittest of a very large original population. It is difficult to otherwise explain their superiority and so further speculation seems pointless. Let’s enjoy and make use of them in crosses!

From hundreds of seedlings we have selected many that are hardy and acceptably yellow. Most have resulted from yellow by yellow pairings. Various combinations of hardy yellows and half-hardy yellows produce populations that yield high percentages of yellow flowers. It is exciting at times to find occasional seedlings that are hardier and generally superior to either parent. This is a strong incentive to grow as large a population as time and space permits.

Where Now?

While our progress in the early days was slow and often disappointing our later generations are beginning to show more promise and some of the seedlings may be candidates for registration. With the best of this group and the growing availability of plants from other breeders the future "lovers of the rhododendron" may have the luxury of growing and selecting the best from among many. For all of us it will now be a matter of comparing the available plants to determine the relative adaptability and quality of each.

At one time not so long ago we thought that any yellow that survived and bloomed after most winters was a garden treasure. Now, of course, having yellows that merely survive is not enough. We want them to thrive in most of the Northeast area. We would also like them to bloom freely, hold their foliage at least two years, restrain their growth to 6-inch internodes or less and have a naturally branching habit (see Figs. 5, 6). Finally, if it would not be too much to ask, the plant should be resistant to spotting fungi and chewing insects.

Do we have such a wonderful creation? Of course not. But we do have more than a few acceptable hardy plants that provide yellow to rhododendron gardens. And somewhere among the hundreds of seedlings in our test plots there will appear new and better ones, better in incrementally small ways perhaps but that is how progress is made. Experienced breeders will have their own strategy and will need no special advice, but for the benefit of any newcomer to hybridizing who might gain from our experience, we offer the following comments. We hope that they can help to avoid mistakes. Preventing errors will save time and time is a vital resource in hybridizing.

• Cross western yellows with very hardy eastern parents. Avoid where possible hybrids derived from subsection Pontica species. Cross these offspring with each other or other similar seedlings to recover a second generation both yellow and hardy.

• As a source of color use the best of available western yellow hybrids. They confer color as well as any species and they are stronger growers with larger flowers.

• Introducing red flowered species can sometimes be a benefit. Many red by yellow crosses result in yellow flowered seedlings.

• Some species from subsection Pontica may be used for particular effect. Rhododendron brachycarpum produces seedlings with splendid dark foliage. Rhododendron degronianum ssp. yakushimanum can improve compact plant habit. Some eastern yellows already contain these species. Just consider that such parents will need to be out-crossed to separate them from the dilution factor.

• Make crosses between hardy eastern yellows. This will produce many hardy yellow seedlings and some may be hardier hopefully and better than either parent.

• Study the heritage of intended parents where possible. Try to include R. dichroanthum to extend the range of hues and to improve intensity.

• Do make a few Dexter hybrid crosses. They have been so successful in the past that more good results are a virtual certainty.

This long discussion has given us the opportunity to express thoughts resulting from thirty-five years of growing seed. We have enjoyed the experience and with luck may continue a while longer. We feel especially in retirement that everyone should have an intense interest in something they feel is worthwhile. Rhododendron breeding has met that need for us. Whatever small contribution we might make to gardening has been more than compensated by the pleasure we have received from doing it. What a wonderful thing it is to savor the moment in spring when a new seedling blooms - and it is beautiful!

Allan and Shirley Anderson are members of the Tappan Zee Chapter.