JARS v50n4 - Rhododendron catawbiense f. insularis

Rhododendron catawbiense f. insularis
Robert L. Schwind
Atlanta, Georgia

At the end of the 18th century, the English plant explorer John Frazer discovered the species Rhododendron catawbiense on the peak of Roan Mountain, Carter County, Tenn. He sent plant material back to England in the early 19th century where it was used in the development of several robust hybrids, many of which are still in the nursery trade. Over a century passed before anyone undertook the re-examination of the species. In a study appearing in the October 1919 issue of the Journal of the Elisha Mitchell Society (2), Professor W.C. Coker of the University of North Carolina described the species and its various habitats and distribution. He acknowledged that the normal form of the species was found on the mountain peaks of the Southern Appalachian Mountains at elevations in excess of 1,200 meters but described also a variant of that form found at various locations in the lower Piedmont and upper coastal plain of North Carolina at elevations under 90 meters. Professor Coker noted differences not only in habitat but also in the morphology between the normal and variant forms of the species, and on the basis of their differences he gave the varietal form the name of R. catawbiense f. insularis .

In May 1971 the author accompanied by his young son and Professor Herbert Hechenbleikner of the University of North Carolina inspected three colonies of the variant form at three locations in North Carolina. They were: 1) in Chapel Hill, Orange County, N.C, at an elevation of 82 meters; 2) in Johnson County, N.C, at an elevation of 67 meters; and 3) near Lillington, Harnett County, N.C, at an elevation of 83 meters (2). The plants in all three locations were in full bloom at the time of the visit at the middle of April. Normally the blooming reaches its high point around the first of May, many weeks before the mountain form of the species begins to bloom.

Casual observation readily disclosed the more obvious similarities between the normal and variant forms, among which were truss size and color range. The author recalls the color range of the variant form to be from light pink to dark pink and mauve and lavender. All three colonies thrive on precipitous bluffs overlooking rivers or streams under the cover of deciduous trees.

In addition to the three colonies in North Carolina, the author, accompanied by E.E. Cline, foremost lay botanist in the South, inspected a similar variant colony in Cherokee County, Ga., at an elevation of 320 meters. The only difference between the North Carolina and Georgia variants was the apparent smaller size of the plants of the latter, most of which were under 2 meters.

Since the author's examination of the North Carolina colonies in 1971, further observations have been conducted by Dr. E.R. Bahnson, M.D., of Winston-Salem, N.C. He and his colleagues have found many interesting new forms including a pure white cultivar in Johnson County, N.C, subsequently named 'Ken's Find' by its finder, Ken Moore, superintendent of the North Carolina Botanical Garden in Chapel Hill, N.C. (1).

It is significant to note that the mountain form and low elevation forms of R. catawbiense have evolved in response to entirely different environmental conditions. The peaks of the Southern Appalachian Mountains at elevations of 1,200 meters and reaching upwards to 1,800 meters experience weather conditions which are severe. At such elevations there are on the average only about 120 or fewer frost free days annually. Even in mid summer the temperature rarely exceeds 27°C , and in the winter the temperature can fall to -40°C. In addition, the plants are exposed to the full sun all year long and to the cold, drying winds of winter. In contrast, the low elevation variant experiences about 260 or more frost free days annually, rarely minimum winter temperature of -12°C, maximum summer temperature of 38°C, in what is known in the South as the "long, hot summer." These vastly different conditions limit the adaptability of the two forms. The author and others have experimented with growing the mountain form at lower elevations in Atlanta and elsewhere and have noted that the mountain form invariably dies after several months. The low elevation variant, on the contrary, thrives at the lower elevations of Atlanta and elsewhere.

The above differences raise the question about the origin of R. catawbiense f. insularis . In 1971 the author posed the question to Professor F.R. Bell of the University of North Carolina in Chapel Hill, N.C. Professor Bell offered the opinion that the insularis variant was probably the result of the effects of the later Ice Ages on the evolution of the flora of the South. The glaciers did not reach the South, but the advancing cold had a severe impact upon the flora of this region. As the climate cooled, the species may have "wandered" along the river valleys southwards and eastwards where it survived under protected conditions of milder climate at lower elevations. When the Ice Ages abated, the low elevation variant simply evolved in response to the gradually warming temperatures of the post-Ice Age period. Professor Bell's explanation remains the most plausible to date. Until the usual morphological studies can be supplemented by modern genetic testing, the origins of R. catawbiense f. insularis and its relationship to the mountain form must remain speculative. The author has made several crosses of R. catawbiense f. insularis with other species and hybrids in an attempt to develop more heat resistant sorts of rhododendrons better suited to the environmental conditions of the South, more specifically to the long, hot summers and high temperatures of the region. Most of the resulting hybrids, while heat resistant, were inferior from an aesthetic viewpoint. The best hybrids resulted from crossing R. catawbiense f. insularis with R. fortunei , but most of these crosses contain the bluish and mauve tints characteristic of R. catawbiense which are not in favor with many rhododendron friends. Presently further experiments are being conducted by crossing dark red rhododendrons with lighter colored R. catawbiense f. insularis plants including the pure white form, but it is too early to predict how these crosses may turn out. Continued experimentation by hybridizers more knowledgeable than the author are needed to test the potential of R. catawbiense f. insularis in the quest for better rhododendrons suitable to the conditions of the South.

References
1.  Bahnson, E.R. White catawbas - a recounting, new forms. J. Amer. Rhod. Soc.42:90-93; 1988.
2.  Coker, W.C. Distribution of Rhododendron catawbiense . J. of Elisha Mitchell Soc.; Oct. 1919.
3.  Schwind, R.L. Rhododendron catawbiense at low elevations. Quart. Bull. ARS. 25:180-181; 1971.

Mr. Schwind is a member of the Azalea Chapter.