Notes on Rhododendron Breeding in the Eastern U. S.
Joseph B. Gable
The problem of breeding new varieties of broadleaf rhododendron of better color that will prove hardy in the Eastern U. S. is a real one. However the need is at least as real as the problem and we may therefore harbor the hope (if necessity yet mothers invention) that progress along these lines will soon begin to evidence itself in our Eastern gardens.
The words of certain prophets of yesteryear that seemed to me, at the time to be but the mumblings of pessimistic "wet blanketeers," that it would be many years before we would produce any new varieties and see them commonly grown' have proven basically true. With the single exception of "Dr. Dresselhuys" very few large Eastern nurseries have entered a single new rhododendron name on their lists in the last thirty years.
In sharp contrast the azalea section of the genus, especially the evergreen group, is in danger of being overworked as hundreds of new varieties have appeared and are constantly being added to. Many of these are so very little 'different' from each other that they must sooner or later be eliminated. In many cases there seems to have been little or no trial for hardiness before introduction and it is obvious that free flowering habit has been sacrificed to beauty of the single floret in others. Nevertheless we have a world of these to choose from and it now requires quite an estate for a hobbyist to grow them all. And when those have weeded themselves from the lists that prove undesirable - or ungrowable - we shall have the true value of our work left to us.
But, perhaps because we are unwilling to face the more difficult task, the broadleaved group remains in comparative neglect in the East. For many years in Britain, on the continent and latterly in our Pacific Northwest much work has been done and is being done but it is only accidental when some variety results from the breeding done in these areas that will prove of value in itself as a garden plant in our part of the world.
But this does happen and I can mention four varieties not generally known here or conceded to be hardy that have proven quite satisfactory over enough years to warrant unqualified endorsement. They are R. 'Madame de Bruin', R. 'Goldsworth Yellow', R. 'Cynthia' and R. 'Essex Scarlet'. Something over a hundred of other varieties selected from English lists according to their hardiness ratings have been tried and quite a few exist and flower on occasion but the above four are the only ones I can conscientiously recommend. In passing just let me say this,-that while the British Society's method of rating hardiness may mean a little to us comparatively, do not regard it as an absolute criterion by any means, for many plants rate 'A' may he killed outright the first winter while others listed as low as 'C' or 'D' may take quite an effort to get along with us. Obviously there are other limiting factors in climatic conditions affecting hardiness aside from minimum temperatures.
What does all this mean? As I see it we must go right hack to E. H. Wilson's advice in his 'Aristocrats of The Garden' "that if we wish new hardy types here we must build them here and build them upon the hardy species and hybrids that we have here." In a little over a quarter century of breeding I have found no single exotic species sufficiently hardy that when crossed with a tender species (or variety, possessing no genes of our hardy American species) it would produce progeny of fully satisfactory hardiness.
True we can get growable hybrids, beautiful things that once we see them we would never part with willingly, from the use of such exotic species as R. brachycarpum, R. caucasicum (?), R. smirnowii, R. discolor, R. fortunei, etc., but these species hardy enough in themselves, do not possess the necessary margin of hardiness when crossed with tender sorts, to produce a satisfactory degree of resistance to cold in their progeny.
What then do we have left to us as a potential source of hardiness in our hybrids? Only R. maximum and R. catawbiense? Primarily - and from personal experience only - yes, including of course those hybrids already produced that contain the blood of these species. But the use of R. maximum tends to small flowers and the purple of R. catawbiense is murderous to anything of color? Only too true it is.
Selective hybridization is the only way out and perhaps through several generations. It is helpful to select good forms of these species to start with. There is not too much variation obvious in R. maximum but careful examination will disclose superior plants and there are both red and white forms of R. catawbiense to be found. The white form found by the late Mr. Powell Glass of Lynchburg, Va. comes true from seed which should add greatly to its value as a parent and the forms which approach red in color should yield much finer flowered seedlings than the type in my own experience they have already proven this.
When it comes to hybrids they vary exceedingly in value as parents. The old and now little grown, R. 'Atrosanguineum' has proven to give good results both in color and hardiness. R. 'Essex Scarlet', which I had presumed would be much infected with tender genes shows no sign of them so far, and carries a good share of its color in practically all its seedlings. Its drawback of course is its poor plant habit. R. 'Lady Mitford' is a good parent if in the desired color class.
Yellow is the will-o-the-wisp in truly hardy broadleaves. R. 'Goldsworth Yellow' is yet the only thing in this color class that can be recommended for general planting. It is hardy, of good plant habit and its color is fair if one does not investigate too closely. Other somewhat yellow hybrids have flowered for me, mostly of R. campylocarpum or related species but none of real worth. It might be well to note that the whole Thomsonii series together with its hybrids seem prone to a blight, though many hybrids are apparently hardy.
In all of the above remarks 'rhododendron' refers to the broadleaved section. In the lepidote class there is perhaps more to be done - or at least, less has been done - than in the broadleaves. In this section we have perhaps our most valuable species as regards margin of hardiness in the exotic R. mucronulatum. It has crossed with almost anything in the lepidotes and also some of the Tsutsusi azaleas. And it will improve the hardiness of anything it is crossed with including our native carolinianum, etc.
Very few of the great multitude of exotic lepidotes are satisfactory plants in themselves in this area. Some forms of R. racemosum do well enough and are very pretty but should be left alone once established. They resent moving more than any rhododendron I know. A nearly white form of R. yunnanense is surpassingly lovely but not that hardy. Even so the plant, is never injured, only the buds.
So we really have little to build on in the lepidotes unless we flower the more desirable species under glass to mate them with our hardier sorts. This is being done by some enthusiasts and no doubt they will be heard from in due time.
Interest in the production of new varieties of rhododendron has increased greatly in the Eastern U. S. in late years and no doubt worthwhile things are being produced. It require, years to produce a new hybrid and then if it has outstanding flowers. years more to prove its hardiness and worth in other ways. Then unless it is a sort that roots readily from cuttings it is a slow process to build up a sufficient stock to offer it on the market.
Nevertheless the work grows increasingly interesting with each passing year and the period of our being satisfied with the old "Centennial" hybrids will soon he passing.