Observations on Heat Tolerance
Ben Lancaster, Camas, Wash.
Our letter to the Editor, published in the April 1964 issue of the A.R.S. Bulletin, regarding the listing of rhododendrons and azaleas with natural heat resistant qualities seems to have drawn a lively interest from widely separated places where high summer temperatures have been a problem in the successful culture of these plants. The many interesting reports and variety lists, most of them with complete weather and climatological data, have been very enlightening. Many excellent cultural tips concerning sun exposure, shade and wind protection requirements of different varieties have come to light in the correspondence elicited by the Editor's footnote to our published letter.
Although living in one of the prime rhododendron growing areas of the that might produce plants that could U.S. where very few serious drawbacks be expected to grow well in various exist in the growing of these plants, we became concerned with heat tolerance, along with hardiness, as amateur hybridists in our search for parent lines climates or conditions. The several heat tolerant variety and species lists we have received to date seem to indicate that the Ponticum series, together with the hybrids containing the blood lines from these, have more heat resistance than any other single group. The hardy Fortunei series appears to he next in order with the newly identified species, R. chlorops, and R. fortunei itself being the most reliable in this classification. Strangely enough the tender Thompsonii group, when crossed with members of the Ponticum series, have produced some outstanding sun-tolerant hybrids for us with R. williamsianum parentage heading the list!
Naturally rhododendron 'buffs' in our ideal Northwest climate are not as concerned with heat tolerance as growers in the east, mid-west, lower, or central California where summer heat is unkind to many rhododendrons. However we like to grow them in full sun, without protection of any kind, in our permanent plantings as they develop into sturdier plants that flower more prolifically and generally at an earlier age.
With our present heat tolerance survey only a scant year in progress, our conclusions are necessarily on the tentative side. However with the many detailed reports we have received in general agreement with the survey conducted recently by the Pacific Coast Nurseryman publication, a general pattern is beginning to fall into place, with only one or two exceptions, where any one of the several difficulties which plague rhododendron growers might easily have been responsible. High or low humidity (luring high temperatures have not seemed to make as much difference as one would expect according to several lists. Incidentally, with only a few exceptions, this general pattern seems to indicate a definite relationship between heat and cold tolerance.
If the American Rhododendron Society feels that this heat tolerance project has any merit, or might be of aid in extending the knowledge of rhododendrons to any advantage, I would like to offer a suggestion (also voiced by others) that a committee of members from "heat problem" areas be appointed to pursue the subject further by soliciting and coordinating reports etc. Since this would be a long range project, we believe that younger members than ourselves would be more able to carry the project forward successfully. We will be happy to assist such a committee since we will no doubt continue to receive correspondence pertaining to the subject.
The splendid co-operation we have received from the collaborating members has been deeply appreciated by yours truly. Their detailed reports indicate much thought, time and effort spent on the subject. We are preparing a tentative variety list, condensed from these reports, that will be available for publication at the discretion of the editorial staff.
The success of such a project will depend largely upon member co-operation from the various areas plagued by high temperatures and low humilities. Known characteristics of plants has ever been the foundation for selecting parentage lines in all plant breeding. Heat resistant qualities could very well play an important part in future rhododendron hybridizing.
Does some member in a relatively hot, dry climate want to work on this? - Ed.