Rating Southeastern Rhododendrons
In February, 1987, Bruno Kaelin, the newly appointed national plant rating chairman, sent a letter to ARS chapter presidents. In general, he stated that ARS publications, rhododendron books and pamphlets and nursery catalogs include ratings. These ratings are subject to questions such as who did the ratings, to what geographical area do the ratings pertain and how wide a base of experience is included. These questions would be unnecessary if the ratings were carefully performed by the chapters of ARS and made available on a chapter, regional and national level. Ratings probably make sense only on a chapter or regional basis, on established plants and if derived from several growers. Bruno included in his letter many ideas and suggestions as to the form and content of a rating sheet. He requested that a chapter rating chairperson be appointed, an exchange of ideas be established and ratings be initiated. I contacted several members of our chapter, Southeastern, but couldn't get a commitment from anyone. Since I thought the concept was very worthwhile and shouldn't be abandoned, I appointed myself, chapter rating chairperson.
Based on Bruno's suggestions and ideas of my own I devised a rating sheet and instructions. In 1987, twenty members of the Southeastern Chapter rated 80 rhododendrons. A list of about 220 rhododendrons was sent to chapter members who were asked to circle the rhododendrons on the list that they were growing and that had bloomed in the past or would bloom in the coming season. When the lists were returned to me, only those rhododendrons being grown by five or more people were selected for rating. That is why only 80 varieties were rated.
It was apparent that more ARS members would have to be involved if more rhododendrons than that first group of 80 were to be rated. The next list of 157 varieties was sent in 1989 to members of the Southeastern, Piedmont, William Bartram and Azalea Chapters. Forty-four members responded and there were 110 rhododendrons on the list of 157 being grown by five or more members. Rating sheets and instructions for rating were sent to each of these members. The 1989 rating sheet and instructions were modified from those of 1987 based on the experience gained from the 1987 rating. These changes prevent a direct comparison of the 1987 and 1989 results. There were 32 returns of completed ratings. Considering that there were five pages of instructions, a 73% return was pretty good. The original goal of only reporting those varieties being grown by five or more was relaxed to three or more. This allowed ratings of 100 rhododendron varieties to be determined.
Before we tackle the ratings you should be brought up to date on the status of the national ratings committee. I was advised by Bruno Kaelin in 1988 that he had resigned his post in December of 1987. He had received only four replies to his request for assistance and had found little support from members he spoke to at the national convention. Subsequently, the then ARS president, Harold Greer, named Pat Halligan as national ratings committee chairman. You have read in the ARS Journal Vol. 43:3, Summer 1989, about the rating system, involving a test garden, proposed and described by Pat Halligan.
I will be the first to admit that the Halligan system is more comprehensive, precise and scientific. However, sometimes practically dictates that something less than the best be used.
For example, the area represented by this rating ranges 200 miles from north to south and 350 miles from east to west. Record minimum winter temperatures range from -22°F. to +10°F. Record maximum summer temperatures range from 90°F. to 108°F. Annual average rainfall ranges from 30 to 75 inches. Mountains, foothills and plains are represented. In order to determine which rhododendron varieties could adapt to this range of conditions, more than one test garden would be required. How a rhododendron performs is just as important as how it looks. A test garden or gardens and someone to maintain it would be great but locating those willing to take on the responsibility poses a real problem. I believe it is important for different sections of the country to have rhododendron ratings that apply specifically to them. New ARS members will especially benefit from this kind of information. The Halligan rating system is ideal if you can set up a test garden. If not, you can devise an alternative system for your chapter or region incorporating the features from the Halligan system that strike your fancy. Since the system being reported here was initiated before the Halligan system was published, the concept is different. This system is not suitable for evaluating new and untried hybrids. However, maybe it will also give you a few ideas.
Ratings of the top 20 rhododendrons out of the 100 that were rated are shown in the accompanying table. These varieties had the highest sum of bloom rating plus plant rating plus overall performance rating. The other 80 rhododendrons that were tested are as follows: 'Accomac', 'Albert Close', 'Album Novum', 'Amethyst', 'Antoon van Welie', 'Atroflo', 'Betty Arrington', 'Blue Ridge', 'Brandy Green', 'Brookville', R. chapmanii, 'Cherry Red', 'Conewago', 'Count Vitetti', 'Daphnoides', R. dauricum, 'Dexter's Harlequin', 'Dexter's Orchid', 'Dr. Rutgers', 'Donna's Day', 'Dorothy Amateis', 'Dorothy Russell', 'Evening Glow', 'Everestianum', 'Faggetter's Favourite', 'Flaming Snow', 'Furnivall's Daughter', 'Gloxineum', 'Goldsworth Yellow', 'Grace Seabrook', 'Graf Zeppelin', 'Great Eastern', 'Haag's Choice', 'Halfdan Lem', 'Hallelujah', 'Halsite', 'Helen Everitt', 'Helene Schiffner', 'Ignatius Sargent', R. impeditum, 'John Walter', 'Kelley', 'Ken Janeck', 'Lavender Queen', 'Lem's Monarch', 'Macopin', 'Merley Creme', 'Mist Maiden', 'Mrs. A.T. de la Mare', 'Mrs. Betty Robertson', 'Mrs. Charles R. Pearson', 'Mrs. E.C. Stirling', 'Odee Wright', 'Pink Pearl', 'Powder Puff', 'Princess Elizbeth', 'Purple Lace', 'Purpureum Elegans', R. racemosum, 'Red Eye', 'Red House', 'Red Velvet', 'Robert Allison', 'Rochelle', 'Roseum Superbum', 'Roslyn', 'Shaazam', 'Sham's Ruby', 'Shawme Lake', 'Taurus', 'The General', 'Tony', 'Unique', 'Van Veen', 'Vivacious', 'Warwick', 'Wyanokie', 'Yaku Prince', 'Yaku Princess' and 'Yaku Queen'.
Some explanations and comments concerning the rating procedure and the data in the ratings table follow. Some growers were evidently more lenient than others in their various ratings. Conversely, some were stricter. If the lenient and stricter raters had rated the same varieties, or the total number rating a particular variety was large, these differences would have tended to average out. Since this wasn't true in many cases, we will have to accept the fact that ratings on some varieties will be biased up or down. The effect of a bias, if any, will be less the larger the number of raters for a particular variety. In any case, for any of the average ratings, the effect would probably amount to a rating difference of less than one. I wouldn't have much confidence that rating differences less than one were significantly different. Ratings should realistically have been rounded off to the nearest whole number. I vacillated whether I should do so in the table, but finally decided to show the decimal fraction. Should anyone use these data for whatever future purpose, please round off the average ratings to the nearest whole number.
The ten year height and width number was determined by plotting plant size verses age from all growers of a variety. An average value was interpolated from the plot. There could be differences of 1½ feet from the average among growers. Climate, site, soil and maintenance are evidently as important as genetics in determining growth rate.
Ratings are based on 10 being best. An interesting observation concerning the 1-10 rating is the following. For bloom, plant, plant condition and overall performance ratings, 10 was reported 302 times, 9 reported 392 times, 8 reported 674 times, 7 reported 505 times, 6 reported 304 times, 5 reported 175 times, 4 reported 67 times, 3 reported 46 times, 2 reported 11 times and 1 reported 13 times. I believe a 1-10 rating rather than the normal 1-5 rating allows more opportunity to differentiate between varieties.
Overall performance rating is based on the current plant condition (health) of the rhododendron and its blooming performance. The average condition rating is a measure of how well the plant has adapted to its environment over the region. Briefly, the condition rating is based on the following observations. A condition of 10 indicates that there is nothing wrong with the plant's health. A condition of 7-9 indicates that the plant is in good health except for some minor items. A condition of 6 indicates more serious difficulties but you expect the plant's health to remain the same or improve. A condition of 5 indicates improvement in health questionable. A condition of 4 indicates a decline in health expected. A condition of 2-3 indicates an expected loss of plant. A condition of 1 indicates the plant died. Zero to 3 points are subtracted from the plant condition rating, depending on the severity of blooming deficiency(ies), to get the overall performance rating.
Blooming deficiencies are designated by the letters A-I:
A—Slow to start flowering
C—Tends to fall bloom
D—Flowers often damaged by spring freezes
E—Petal blight especially objectionable
F—Not reliably bud hardy below 0°F.
G—Not reliably bud hardy below -5°F.
H—Not reliably bud hardy below -10°F.
I—Not reliably bud hardy below -15°F.
The factors determining or explaining why the plant is not in peak condition are designated by the numbers 1-18:
4—Winter temperature below -5°F.
5—Winter temperature below -10°F.
6—Winter temperature below -15°F.
7—Winter temperature below -20°F.
8—Early autumn freeze
9—Early spring freeze
10—Late spring freeze
11—Fungi such as dieback, etc.
15—Rodents or other mammals
16—Sun scald and/or wind burn
17—Soil and nutrient deficiencies
Concerning the factors adversely affecting blooming: A was reported 86 times; B, 92 times; C, 17 times; D, 43 times; E, 10 times; F, 25 times; G, 20 times; H, 18 times; and I, 20 times.
The health or condition of plants was reported to be adversely affected by 1, 72 times; 2, 4 times; 3, 75 times; 4, 18 times; 5, 9 times; 6, 12 times; 7, 9 times; 8, 0 times; 9, 8 times; 10, 16 times; 11, 89 times; 12, 17 times; 13, 2 times; 14, 27 times; 15, 0 times; 16, 18 times; 17, 5 times; and 18, 42 times.
Some raters put a lot of data and some hardly any on their rating sheets in the columns labeled, blooming deficiencies and factors determining plant condition. Undoubtedly, the missing data for the majority was a case of just don't know or remember. At any rate, slow to bloom, shy bloomer and flowers damaged by spring freezes were the most common complaints about blooming deficiencies. Drought, hot weather, fungi (dieback) and unknown causes were most responsible for contributing to less than peak condition of the rhododendron in this rating. One would expect low temperatures to have been reported more often as an important factor. However, 18 of the 32 reporting had not experienced a minimum temperature below zero Fahrenheit.
There are all sorts of ways of treating the data in the ratings table. If your main interest is in the bloom, you will look for those rhododendron with the highest bloom rating: likewise for plant or performance. Another way is to add together the bloom, plant and performance rating of each variety. Those varieties in this list of 100 having the highest sum should be the best all around rhododendron for the southeastern region. Of course, in addition to using the ratings, you should go to other sources and look for the type and color of truss that appeals to you and the time of bloom that suits your needs. You might want to check the columns in the rating table labeled blooming deficiencies and factors determining plant condition to see if they apply to your situation. Pay particular attention to those items which were reported by two or more raters. For instance, if the variety is reported as a shy bloomer, it certainly will be more so in a fairly shady location.
Let me emphasize that these ratings and comments apply mainly to the southeastern region. Plant performance will be different for different climates. One might think that a regional average bloom and plant rating for a large number of raters would be essentially the same as the nationwide average. However, the number of varieties that can be successfully grown will be smaller in some regions than others. The highly rated varieties in one region might be only average in a region where a much larger number of varieties can be grown. Also, even though the rating instructions ask for bloom, plant and performance ratings independent of each other, that goal is difficult to achieve. When a plant is not performing well, it is hard to be objective in rating the bloom and plant independently. Therefore, it follows that bloom and plant ratings will vary somewhat from region to region.
In response to questions on the rating sheet, the following information was also obtained. All but three growers use mulch. Most apply from two to four inches of it. All but four growers fertilize. Seventy percent spray for insects. Fifty percent spray for fungi. All but one grower uses supplemental watering. After five summers of much drier than normal conditions, the absence of watering for many would have resulted in the absence of many of their plants. No correlation could be found between the overall average condition of plants calculated for a set of growers and such items as spraying, fertilizing, hot weather, cold weather, average plant age or rainfall. Perhaps site, soil, source of plants and planting methods are the important factors.
Some growers reported that their plants had experienced in some years 60-70 days with temperatures greater than 90°F., others only 3 days. The other extreme was 11 days in some years with a temperature less than 0°F. If your climate is in this range, these southeastern ratings might be suitable approximations for your region. Otherwise, maybe you will get some ideas for devising a rating system for your section of the country if establishing a test garden is not feasible.
Dr. Gilkey is an active member of the Southeastern Chapter and serves on the ARS Research Committee.