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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com

Volume 28, Number 1
January 1974

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Standards for Evaluation of Rhododendron Hybrids in Test Gardens
John E. Ford, Curator, Secrest Arboretum
H. A. J. Hoitink, Associate Professor, Dept. of Plant Pathology

        After several years of experimentation in the Rhododendron Test Garden of the Great Lakes Chapter (American Rhododendron Society) at the Ohio Agricultural Research and Development center, Wooster, Ohio 44691, a procedure for evaluating new hybrids has been developed. The system basically is designed after that described in "Rhododendrons of the World" by David G. Leach. The evaluation check list includes criteria discussed by Ted Van Veen in a recent ARS bulletin, except for ease of rooting. It is felt that this characteristic should be judged by the hybridizer. The information used in evaluating is not intended to be a complete description of a plant but is supplementary to information on the ARS Plant Registration Form previously prepared by Edwin K. Parker. During the past four years several minor problems and peculiarities were encountered in the acceptance of plants for testing and rating of characters. For this reason a series of policies were adopted.

General Policies:

  1. Only plants with flower buds and well-developed healthy root systems are acceptable. Some plants that were received as rooted cuttings did not set flower buds after four years, even though normal bud set occurred each year on standard varieties.
  2. At least two plants are rated of each type. If possible, the overall rating is compared with sister plants in a display garden which has a somewhat different exposure at this location.
  3. Rating does not start until one year after planting so that the root system has had adequate time to become established. Winter injury during the first season after planting is recorded but not used to compute overall hardiness unless it reoccurs during another winter.
  4. Each plant is rated for three years on blooms. This period should include one winter with the normal average temperatures in the area so that proper hardiness data can be obtained.
  5. All plants are maintained under optimum nutritional conditions. No attempt is made to force more than one flush of growth per season. Supplemental water is applied only during prolonged periods of drought.

Peculiarities: Some plants were received with sub-clinically-infected root systems. In some cases, one of two plants under evaluation died from Phytophthora root rot. Furthermore, sub-clinical infections reduced the overall rating of a hybrid considerably. It is important, therefore, that plants be disease-free when received. A procedure for production of such plants has been published previously in ARS Bulletin 26(2):94-97.
Since rhododendrons are relatively free of diseases other than root rot, the use of fungicides is not desirable in a test garden. Only Botryosphaeria die-back has been encountered so far (other than root rot). The pathogen of this disease usually only infects dead or dying plant parts. In case a virulent pathogen enters the test garden, e. g. Phytophthora, Phytophthora cactorum (die-back) or Cercospora spp. (leaf spots) a spray with Dithane M-45, Manzate 200 or Fore is used combined with sanitation procedures to prevent an epidemic. Some notes on susceptibility to various diseases have been taken so far.
Several insecticides are used to control various insect pests. The black vine weevil is the most damaging of all. Significant differences in resistance to this insect pest have not been encountered. This includes R. yakushimanum and hybrids thereof.

Explanation of Check List


Remarks in this section include such observations as unusual weather, winter kill of twigs or branches, bark splitting, rodent damage and items of a similar nature.


Remarks in this section include any items about flowers not included above, such as frost burn, length of time that flowers are in bloom, etc.


Severe disease and insect problems, spray injury, windburn, mechanical damage and other special features should be described here.

Root growth: It is determined when plants are transplanted or moved out of the test garden after completion of the test period. Root quality, quantity, depth and spread are rated on a scale of 1-5.

Tentative rating: It is the rating for the current year and follows the ARS system which is a fraction where the numerator represents the rating for the flower and the denominator the rating for the shrub.

Final rating: Determined when the plant is removed from the test plot and, to a certain extent, based on an average of the yearly ratings. Consideration is given to the trend of the ratings from year to year since in many instances rating will improve as the plants become established and grow older. Some plants received a lower rating with age.

    Not all characters are weighed on an equal basis and, all characters are not assigned a numerical rating. The final rating, therefore, cannot be based on a mathematical formula. This evaluation system does not lend itself to a computer analysis of a large number of hybrids. The evaluation check list serves mainly to remind the evaluator of shortcomings and strong features of a given hybrid. An overall impression of a hybrid still is the key factor used for final rating.
A critique of this rating system is welcomed by both authors. Suggested changes could perhaps be handled best in the column "Experts and Answers" that should come out in future ARS bulletins.

Plant Evalaution Check List

Volume 28, Number 1
January 1974

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals