QBARS - v28n1 Standards for Evaluation of Hybrids in Test Gardens

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


  • Number of breeder: Each breeder is assigned a number. This number is entered on the check list so the evaluator will not be influenced by knowing the breeder's name.
  • Rhododendron test number (Rt. No.) for plant: Each plant is assigned a number for identification purposes. This number is put on a tag fastened to the plant.
  • Location of plant: Each location in the test plot is identified by a letter representing the row and a number representing the location in the row. The plant can thus be identified by its location in the plot in the event the tag with the Rhododendron Test Number is removed or lost.
  • Breeder's name for plant: Sometimes the plant has been named; most often the plant has the breeder's accession number on it. This name or number is entered on the check sheet to enable the breeder to identify his plant.
  • Years in present location: Assists the evaluator in determining whether or not the plant has had time to become established.
  • Present age of plant: Assists the evaluator in determining whether the plant is early blooming and if the plant is mature enough to receive a final rating.
  • height and breadth of plant: Measured every year at the time of the first inspection about mid-April and before the current seasons growth has started. All plants are measured on the same day. Measurements have been made in inches but in the future may be in the metric system.
  • Lowest temperature previous winter: Helps the evaluator to determine the hardiness rating.

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.


  • Flower color: Determined with the Nickerson Color Fan (Munsell Color Company, Inc., Baltimore, Maryland). Color name and number is listed, the basic colors being matched with the chart. If wide variations exist in colors on the same flower or in flowers from different parts of the plant it can be noted under remarks. Very often there are a number of different shades of a given color in a flower and it is impractical to try and list all of them.
  • Number of lobes: Most plants have five but occasionally a plant has a different number; it may or may not be a factor in overall judging of a flower.
  • Number of flowers in a truss: Since the number varies from truss to truss on the same plant, the highest and lowest numbers are noted. The number in a truss may vary from year to year depending upon size and age of the plant. It can also be influenced by the weather.
  • Diameter of flower: It is measured in millimeters which is not consistent with measuring height and breadth of the plant in inches.
  • Fragrance: Fragrance of rhododendron flowers in the test plot has been very slight in the few plants that have any fragrance at all. Some azaleas do have a pronounced fragrance. A pleasant fragrance is an asset to any plant.
  • Shape of truss: We have changed our original nomenclature to that used on the ARS Plant Registration Card: Conical, shaped like a cone; tall, cylindrical or oval; ball, rounded or globose; flat is topped flat with florets across the truss; lax or bald, a truss that hangs or droops with a hole or bald spot with no flowers in the center. A lax truss, generally, is not a desirable feature.
  • Substance of flowers: Refers to the texture of the petals and plant tissue going into the overall makeup of the flower. Some plants have such flimsy flowers that they break or tear in a light moderate wind. The ARS rating of 1 = poor, 2 = below average, 3 = average, 4 = above average, and 5 = superior on this and other ratings of individual characteristics is used.
  • Blooming season: In the Arboretum at Wooster the earliest rhododendron ( R. dauricum ) normally begins blooming about mid April while the latest types bloom in early July ( R. maximum ). Blooming seasons may vary from year to year depending upon the weather. Usually at Wooster, plants listed as early blooming flower during the second half of April; early mid-season during the first half of May; mid-season the second half of May; late-blooming the first half of June; very late blooming after the middle of June.
  • Bud hardiness: Takes a number of years to be determined and the rating "of a single year is only a very small part of the whole picture.

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.


  • Quality of foliage: Includes such items as color of foliage, freedom from spots, dead areas and excessive insect feeding, shape of leaves and the way leaves hang.
  • Quantity of foliage: Density or total number of leaves.
  • Habit of growth: Includes the general overall plant shape, compactness of growth, brittleness of branches, leggyness, holding of branches down to the ground and general symmetry.

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