Herbert A. Spady
On Oct. 29, 1993, the Board of Directors of the American Rhododendron Society adopted this new definition of hardiness for rhododendron hybrids and clones of rhododendron species.
Cold hardiness is defined as the temperature range through which damage to flower bud, leaf or plant structure can be expected to occur, in a plant at least five years of age and in good health. Flower bud damage is defined as that which detracts from a normal floral display. The range is given by two figures, expressed in both Fahrenheit and Celsius. The lower figure is the lowest temperature after which the plant has been observed to perform normally; the upper figure is the highest temperature after which cold damage has been observed.
Example: Hardiness range Rhododendron 'Some name' -15F (-26°C) to -5°F (-21C). [This would mean that this rhododendron had been observed to perform normally after a temperature of -15°F. This may have been when temperatures cooled gradually and the cold period was brief. On the other hand and under different circumstances the same clone was observed to have sustained damage at a temperature of -5°F. Perhaps at that time the duration of cold was longer or the cold weather came earlier or later in the season.]
This new definition has arisen out of the experience of many of our members that the past methods of defining hardiness have not provided adequate information. The hardiness of a plant has been found to not exclusively depend upon the survival and performance of a plant in an ideal location with ideal gradual seasonal decline in temperature and short duration of cold periods. In the past the rating has often been reported under these circumstances. This new definition expresses hardiness as a temperature range through which damage may be experienced depending on various factors such as unseasonable cold weather, duration of cold, exposure, site, snow cover, unseasonable warmth prior to cold and other variable factors. It provides a more honest appraisal of the possible performance and risks associated with a particular clone.
Since the hardiness of species varies with each individual clone the hardiness for species must be expressed in terms of only individual clones.
There are at present at least three methods of expressing hardiness. It is hoped that this confusing array will ultimately be replaced by this method which simply expresses hardiness in terms of commonly used temperature scales.
Providing this data to our members and the non-member public will require time and effort by the Society. As an ancillary to the adoption of this new definition the Board has established a committee of the Society to gather this information and ultimately publish it. Publication can only occur after the range of hardiness of various clones becomes defined by observation. The gathering of this data will require the cooperation of many of our members in all parts of the world.
Herbert Spady, ARS Western Vice-President and member of the Willamette Chapter, authored the article "Defining Hardiness" in the Summer 1991 issue of the Journal.