QBARS - v19n2 The Rosebay Rhododendron: Historical Oddities, Unusual Forms, Its Value as a Parent

The Rosebay Rhododendron: Historical Oddities,
Unusual Forms, Its Value As A Parent
David G. Leach, Brookville, Pa.

The Rosebay Rhododendron of the northeastern United States is probably known to more people, misused by more gardeners, cursed by more game hunters and has baffled more breeders than any other species in existence. Its bole has been used for making pipes. Its leaves produce a soft grey dye for Blue Ridge mountaineers. A deadly poison in its nectar, concentrated into honey, has been producing occasional violent illnesses and deaths which have been reported by American doctors since 1794. The same active agent, extracted and purified, is a potent remedy for high blood pressure in controlled doses.
In the early years of American medicine, doctors and college professors jousted in print over remedies for human ills, poisonous or otherwise, and sometimes with comic heroism. In Professor B. S. Barton's "Collections", published in 1794, he observed that "This (the Rhododendron maximum , or Pennsylvania's Mountain Laurel) is certainly a poison. It is a species of the same genus as Rhododendron chrysanthum , which has lately acquired much reputation in the cure of chronic rheumatism." A few years later Dr. Jacob Bigelow, professor of materia medica at Harvard, published his "American Medical Botany" in which he bravely described how, "under the conviction that the plant was not particularly dangerous, I have swallowed a green leaf of the middle size, so large that it required some resolution to masticate so unpalatable a morsel, but have found no ill effect whatever to result from it." The account of the determined, black garbed New England professor browsing on a Rhododendron leaf to prove his firm belief evokes an entertaining mental image.
Dr. Bigelow did not know it, but he was safe enough. Sixty-six pounds of R. maximum leaves produce only a little more than seven grams of crude andromedotoxin, the unidentified poison which, concentrated in honey, has plagued mankind in Asia Minor by written record for more than two thousand, four hundred years. There, of course, the source of the deadly nectar is R. luteum.
R. maximum appears to be a constant, stable species but when unusual forms have been found they have been altogether extraordinary. There is a strange colony of red and variegated flowered plants on Mt. Mitchell, in North Carolina, which appear to have blood instead of sap. The under tissues of the stems are similarly colored and the new leaves, held up to the light, have red pools outlined in their centers. In 1956 I cut scions from the plant with the reddest flowers and grafted them on maximum seedling under stocks but when they bloomed the blossoms were a pallid pink. The following year I took cuttings and rooted them. Again, the flowers turned out to be a wan, ordinary pink.
It has been suggested that the red color intensity is maintained on Mt. Mitchell by the large difference between the day and the night temperature which is characteristic of mountain altitudes; at lower elevations the drop in temperature at sundown is much less, and the pigment producing process is slowed.
The flower truss pictured on the cover of this issue of the Bulletin was produced on a plant grown from open pollinated seed collected from a red flowered specimen in the original colony on the mountain. It was a gift to me from the well known Rhododendron specialist, Warren Baldsiefen. Strongly pigmented flowers were produced at his New Jersey nursery, near sea level, and it has bloomed similarly at an elevation of eighteen hundred feet at my trial grounds in northwestern Pennsylvania. The heavily pigmented areas of the flowers change radically from year to year, however, producing entirely different patterns. This form has been described and registered as a clone under the name 'Mt. Mitchell.'
A red flowered maximum which I have not seen is reported to be owned by a New England nurseryman but the late flowering Rhododendron sparingly distributed under the name 'Nassau Red' is a hybrid, contrary to its description, and of little ornamental value.

Another uncommon form of this common species is maximum var. leachii , illustrated in the accompanying photograph. It was found by collectors for LaBars' Rhododendron Nursery in West Virginia. It is a dwarf, with globular instead of cone-shaped flower buds. The emphatically irregular, undulant leaves give it an oriental appearance of rather startling distinctiveness. This author was curious about the inheritance of its unique characteristics so several seedling lots were grown. They duplicated the parent, thus proving that the form was a true botanical variety and Bernard Harkness, the botanist who described it, named it for the writer. Hand pollinated seeds which will reproduce the dwarf, wavy leafed original specimen are being distributed through the American Rhododendron Society's seed exchange this spring.
Rhododendron maximum has been in cultivation since 1736, much longer than its southeastern relative, the Catawba Rhododendron. Rosebay is native to the Northeast, too, continuing its distribution up through New England into Canada so it might be presumed to be the hardier of the two. Why, then, do the records show myriads of hybrids with R. catawbiense in their heritage and only a handful with the Rosebay Rhododendron in their pedigrees? In a hundred and fifty years R. (maximum x arboreum) 'Lady Eleanor Cathcart', 'Lady Clementine Mitford', and R. (maximum x griffithianum) 'Halopeanum', which has also been distributed under the name 'White Pearl', are the only maximum hybrids which come readily to mind as having competed successfully for places of any permanence in the nurserymen's lists.
In a realm where few categorical statements are safe, it can be said with some confidence that R. maximum from its northern distribution is more bud tender than is R. catawbiense. No form of the Catawba Rhododendron has ever lost its buds in my U. S. D. A. zone 5a climate, but twice in fourteen years the native R. maximum has failed to bloom. And it seems certain that R. maximum has a different complement of the genes which determine flower color than does the white variant of the Catawba Rhododendron. R. maximum x 'Theresa', maximum x 'Radium', both crosses of mine; maximum x 'Essex Scarlet' (Gable) and maximum x strigillosum (Amateis), together with other such matings of the Rosebay Rhododendron with red flowered pollen parents have produced red flowered progeny. There is no record of 'Catalgla' or 'La Bars' White', both forms of catawbiense var. album , ever having done so in similar crosses.
R. maximum has a poor reputation as a parent among contemporary breeders, despite the goading of landscape architects for the production of later blooming hybrids. Why? Does it deserve its disfavor, or does the attitude of our hybridists come from unproved hearsay and a tradition which never had factual support?
Breeders who are edging over into the group described as "veteran" are often questioned by newcomers in hybridizing about the value of R. maximum as a parent. An effort should be made to give an answer with respectable evidence to support it, and the following account of my results to date is hopefully offered to provide some help.

Reviewing this tabulation and comparing it with other experience, there is strong evidence that:

1. Primary crosses between R. maximum and other Rhododendrons exhibit various degrees of sterility to a much greater extent than do crosses of R. catawbiense . Fewer seeds are produced and those that germinate produce far fewer viable seedlings. A high percentage of the survivors lack normal vigor. A high percentage of normal appearance grow very slowly.
2. F2 hybrids of maximum are notably lacking in adaptability to U. S. D. A. plant hardiness zone 5a, as shown by the sharp attrition in the number of survivors when counts were made in early 1965.
3. As a parent, R. maximum produces an extraordinarily large number of seedlings lacking evidence of hybridity. These are presumably apomicts since the percentage of accidental self pollinations should be approximately equal with all seed parents.
4. Maximum first generation hybrids are much slower to come into bloom than are hybrids of the Catawba Rhododendron. The average age of plants at first bloom is more than ten years.
5. First generation maximum hybrids are notably lacking in ornamental quality. Out of an estimated 400 seedlings involving many different species and combinations only one plant was judged to be of sufficient quality to be worthy even of further observation.
6. Second generation maximum hybrids are usually almost totally lacking in adaptability to U. S. D. A. plant hardiness zone 5a unless both parents incorporate maximum or some other hardy species in their pedigrees.
7. Second generation maximum hybrids in which both parents do incorporate a hardy species yield an exceptionally small percentage of seedlings sufficiently adaptable to survive.
8. Sterilities of varying degree continue into the next generation to produce a high percentage of plants lacking normal vigor in second generation maximum hybrids.
9. Second generation maximum , hybrids tend to bloom several years younger than do first generation maximum hybrids.
10. Apomixis is not common in second generation hybrids as it is in first generation hybrids.

Too few different maximum hybrids have been tested as seed parents to draw conclusions for the second generation with the same conviction that seems justified for a jaundiced anticipation of first generation results. Because of the small number of seedlings surviving to maturity there is yet almost no evidence as to the ornamental quality to be expected in second generation maximum hybrids. But the dominance of maximum floral characters in the first generation, and the usual failure to obtain segregates embodying high flower quality in combination with hardiness in all Rhododendron breeding does not seem to offer a particularly promising prospect.
In assembling statistics it is nearly impossible to distinguish between lack of vigor or outright mortality due to dissimilar genetic constitutions of the parents and the same occurrences caused by plants struggling in an environment too harsh for normal growth. But maximum is not noted for its adaptability. It is a plant of shade and shelter, found in ravines, on the north slopes of our mountains or beneath an overhead tree canopy. Its ability to transmit a general adaptability to the variety of planting sites required for commercially practical Rhododendron hybrids might reasonably be questioned.
Of equal or greater moment is the large stature of maximum . Unless a dwarf species is included in the heritage, maximum hybrids of normal vigor are almost certain to be far too big for the single story houses and small gardens which dictate modern landscaping needs.
After having grown seventy first and second generation maximum hybrid crosses it seems to me that the newcomer to Rhododendron breeding can employ his time and talents more productively in other directions than the creation of maximum hybrids. Now that I have completed the tabulation and studied it, the evidence appears to me to be much more unfavorable than I had previously thought. It is no longer necessary to rely on vague generalities and sweeping, unsupported assertions of old-time breeders that maximum is "no good" as a parent. The veterans are right but the skeptical novice need no longer feel compelled to prove it to his own satisfaction.
I feel that the landscape architects are justified in urging breeders to produce more late flowering hybrids which will ornament patios and terraces after they come into intensive use with the arrival of the season of outdoor living. But the late flowering characteristics should probably be contributed by Asian summer blooming species such as auriculatum , serotinum and kyawii rather than R. maximum .