Ratings for Rhododendron Varieties
By J. Harold Clarke, Long Beach, Washington
The writer would like to take this opportunity of thanking the many rhododendron growers who gave generously of their time in filling out the questionnaires. Those replies which were not acknowledged by letter hereby acknowledged with thanks.
Ratings to indicate the probable garden value of varieties have proven useful with several different groups of ornamental plants, especially where the number of varieties is very large. The Royal Horticultural Society ratings of Rhododendron species and hybrids have been especially valuable in the British Isles and have been of considerable help to many American growers. The R. H. S. quality ratings provide four stars for the best varieties, with fewer stars in order of decreasing merit. The poorest varieties have no stars, which is sometimes confusing as new varieties not yet rated are necessarily listed with no stars.
To the beginner it might be hard to tell whether a particular variety is an old one unworthy of any stars, or a new one not yet rated. The Royal Horticultural Society is well fitted to make such ratings as it includes many Rhododendron specialists among its members, and also runs extensive garden trials at the RHS Gardens at Wisley.
Dr. Clement Bowers of Maine, New York, in his book "Rhododendrons and Azaleas" has adapted the British duality ratings for use in this country and has added ratings on a number of hardy hybrids commonly grown in the East.
The British ratings for hardiness are a little hard to interpret for American conditions until one has had enough personal experience to be able to correlate performance with ratings. The British hardiness ratings are as follows:
(A) Hardy anywhere in the British Isles and may be planted in full exposure if desired. (B) Hardy anywhere in the British Isles but requires some shade to obtain the best results. (C) Hardy along the seaboard and in warm gardens inland. (D) Hardy in south and west but requires shelter even in warm gardens inland. (E) Requires shelter in most favored gardens. (F) Usually a greenhouse shrub.
American Ratings Needed
American growers have felt the need of ratings, in addition to those given by Bowers, which would he more directly applicable than those of the RHS to conditions in this country. The British ratings, while useful to experienced growers, are not as specific as could he wished for American conditions. In order to meet this need, the Committee on Nomenclature of the American Rhododendron Society set up a procedure for establishing ARS ratings which was published in the Bulletin for October 15, 1919, and additional material on April 15, 1950.
The ARS duality ratings are distinguished from the British by the use of a lower case x instead of the stars or asterisks but are similar to those of the RHS in that the best varieties are indicated by xxxx. Furthermore, it seems desirable to indicate those varieties which are not worthy of at least one "x" by rating them "o". This is necessary to distinguish them from other varieties too new to be rated.
The ARS hardiness ratings are based on the minimum temperatures which the varieties may be expected to stand with no appreciable injury to leaf, stem or bud. These ratings, briefly, are as follows:
- H-1--Varieties hardy to -25°F
These varieties should be hardy in southern New York and southern New England
- H-2--Hardy to -15° F
- H -3--Hardy to -5° F
These varieties should be fairly hardy in the southern part of Long Island and around Philadelphia
- H-4--Hardy to +5°F
- H-5--Hardy to +15°F
These varieties should be hardy during most winters in the Portland and Seattle areas
- H-6--Hardy to +25°F
These varieties should be hardy close to the Coast in the Pacific Northwest or in sheltered areas throughout Western Washington and Oregon
- H-7-Hardy to +32° F
Test Winter 1919 - 1950
The above ARS rating system was established particularly to be used in connection with the ARS Test Gardens where new varieties will be grown and rated and considered for special award. However, the winter of 1919-1950 was especially severe in the Pacific Northwest where many rhododendron growers are located. Temperatures to -18°, and lower, were recorded in certain gardens of Western Oregon and Washington, whereas other areas in that general locality reached only +5° to +7°. As could he expected, there was considerable injury to plants in the low temperature areas. This seemed to present a valuable opportunity to secure data which could he used for establishing ARS hardiness ratings for many of the standard varieties and so the writer was asked to make a survey for publication in the Bulletin. It seemed an opportune time to secure information which would enable the ARS to set up quality ratings for the same varieties.
A questionnaire was sent to 40 rhododendron growers who were selected because they have, or have access to, fairly large variety collections. Thirty returns from this questionnaire have been received to date. Not all of them contained all of the information requested but there was enough to give a rather clear idea as to the observations and opinions of these growers with respect to the varieties listed. The growers were asked to record injury observed on 137 listed varieties and to write in additional varieties or species with which they might be familiar. They were also asked to suggest both hardiness and quality ratings based on all their observations and experiences. Several of the varieties were apparently not widely grown and so only 113 have been considered to be suitable for listing at the present time. At least five growers have commented on each of these varieties although in most cases at least ten to twenty growers have suggested the ratings.
Hardiness Difficult To Rate Accurately
Hardiness to cold injury is a baffling problem as was brought out by comments from many of the correspondents. In addition to minimum temperature, the following factors, plus some not mentioned, also affect hardiness-length of exposure to low temperature, rate of temperature fall, the time of year the low temperature occurs, protection from wind, presence of snow, exposure to the sun, and of course the growth condition of the plant as affected by age, previous flowering, fertilizers used, the condition of the graft union and the kind of stock if the plant is grafted, and other factors. Furthermore, types of injury vary a great deal, occurring on different parts of the plants and in different degrees.
Quality ratings also involve a number of variables including the personal opinion of the grower as well as the precociousness of the variety, the size, color, fragrance and shape of the flowers, plant and foliage characteristic. vigor, disease resistance, and adaptability to local conditions. The novelty of the variety, or its unusualness, would also be a factor; that is, a variety blooming extremely early might be rated higher than the same thing would be if it bloomed at midseason
As might be expected, the range of replies was rather wide for any one variety. In most cases, however, when the information was tabulated it was fairly easy to fit the variety into the classification which represented most accurately the opinion of the majority of the growers. It is realized that these, or any other ratings, must be considered as tentative and subject to change at a later date if more adequate information indicates the need for such a change. It is felt, however, that the presently suggested ratings represent reasonably well the experiences and opinions of a majority of the growers in the Pacific Northwest. The writer would be very glad to have comments or criticisms which might re useful for later revisions.
Other Varieties Should be Rated
It would seem to be desirable to establish and publish ratings on many additional old varieties and on new varieties as they appear. It would be helpful, also, if we could get rating on the same basis for many of the hardy Eastern varieties as well as for species and for the Azalea group. A considerable volume of information of this type was received in footnotes and additional sheets in the replies to the questionnaire. Ratings as to some of the species might be worked out from the material available but this information should be increased as much as possible before such ratings are finally established. Additional questionnaires may be required.
ARS And RHS Ratings Presented Together
The accompanying table gives a summary of the results for the 112 Rhododendron varieties and one Azaleodendron, namely Broughtonii aureum. It seemed desirable to mark those varieties which are group varieties rather than clones with the symbol gr. This information was secured from the 1947 Rhododendron Handbook of the RHS. Probably many of the older varieties which are so listed, are actually in the trade only in one clonal form. However, according to the practice of the English growers, additional crosses between the original parents may be made, and the resulting seedlings entitled to the use of the group name. We are sure. at least, that many of the more recently introduced group varieties are present in this country in several forms. Since these forms are separate seedlings, they necessarily vary from each other in greater or less degree. Such variation might well be enough to affect the quality or hardiness rating. In other words, the best form of a particular group variety might rate H-2 and xxx, whereas an inferior seedling of the same group might rate H-4 and x. It is highly desirable, of course, that ratings eventually he given to all important selected and named forms. This has been done with varieties such as the King George form of Loderi. Information is not available at the present time, however, for ratings on all of the forms of even the more important new varieties. The inclusion of the symbol gr. after the variety name will at least warn the reader that it is a group and that variation may be expected.
It is interesting to note that the American and the British ratings agree rather closely both as to hardiness and quality. There are enough differences, however, to indicate the effort necessary to secure the information and work out the ratings was justified.
Some Varieties Unexpectedly Hardy
On the questionnaire, growers were asked to record their minimum temperature and also to list varieties which were uninjured as well as those which had varying degrees of injury. It is interesting to note that 57 of the varieties listed on the questionnaire were recorded by one or more growers as being uninjured at temperatures of -15° F. or lower. However, it is general practice to plant rhododendrons in rather protected places. Furthermore, there was considerable snow on the ground in some areas although correspondents were requested to record the condition of plants not protected by snow. Other factors such as unreliability of thermometers, of course make it undesirable to give too much emphasis to single reports of unusual hardiness. When the figures were tabulated, as will be observed, most of the varieties were considered to be hardy to +5° or in many cases to -5°, and in quite a few cases to +15°. It is of importance that only a very few varieties were reported to have been actually killed by temperatures as low as -15° to -18°. Comments were frequently made that the plants were thought to be killed but were making new growth from the base. This, of course, points up the importance of plants being on their own roots where frequent freezing damage may he expected.
It is not possible, of course, to say that varieties surviving -15° in Oregon will also be able to survive -15° in the East. Summer and Fall conditions undoubtedly have a good deal to do with the hardiness of plants during the Winter. There results, however, do indicate in a rather definite way which varieties are relatively more hardy than the others. It would seem that quite a number of those rating H-2 or H-3 should be able to survive most winters in the milder parts of the Middle Atlantic States. It is hoped that the ARS Test Garden at The Morris Arboretum near Philadelphia, as well as other Eastern Rhododendron collections, may be expanded sufficiently to test many of the newer varieties which have proven so unexpectedly hardy in the Northwest.
R. H. S. RATINGS A. R. S. RATINGS Variety Hardiness Quality Hardiness Quality 'Alice' B ** H-3 xx 'Arthur J. Ivens' gr. B H-2 xxx 'Arthur Osborn' gr. B *** H-3 xxx 'Azma gr. H-4 xxx 'Azor' gr. B *** H-4 xxx 'Bagshot Ruby' B * H-4 x 'Beauty of Littleworth' B *** H-3 xxx 'Betty Wormald' B *** H-3 xxx 'Blue Peter' A *** H-2 xxx 'Blue Tit' gr. B **** H-3 xxx 'Bowbells' gr . B ** H-3 xxx 'Bric-a-brac' gr. H-3 xxx 'Britannia' B *** H-3 xxx 'Broughtonii aureum' B *** H-3 xxx 'Bulstrode Park' B * H-3 xx 'Butterfly' B *** H-3 xx 'C. B. Van Nes' C ** H-4 xx 'Cilpinense' gr. C *** H-4 xxx 'Cornish Cross' gr. C *** H-4 xxx 'Cornubia' gr. D *** H-5 xx 'Corona' C *** H-3 xx 'Corry Koster' B * H-4 xx 'Countess of Athlone' B H-3 x 'Countess of Derby' gr. B ** H-3 xx 'Countess of Haddington' gr. F ** H-5 xx 'Cynthia' B * H-3 xx
R. H. S. RATINGS A. R. S. RATINGS Variety Hardiness Quality Hardiness Quality 'Dairy Maid' B *** H-3 xx 'Day Dream' gr. H-4 xxx 'Diane' H-3 xxx 'Dido' gr. B ** H-4 x 'Doncaster' B ** H-3 x 'Dr. O. Blok' B ** H-3 xxx 'Dr. Stocker' gr. C *** H-4 xxx 'Earl of Athlone' C **** H-4 xxxx 'Essex Scarlet' B * H-3 xx 'Fabia' gr. B *** H-4 xxx 'Faggetter's Favorite' B *** H-3 xxx 'Fastuosum flore pleno' A ** H-2 xx 'F. C. Puddle' gr. H-4 xx 'Fragrantissimum' gr. F *** H-5 xxx 'Fusilier' gr. H-5 xxx 'Gill's Crimson' C **** H-5 xxx 'Golden Horn' gr. H-4 xxx 'Goldsworth Yellow' gr. B H-3 xx 'Gomer Waterer' A ** H-2 xx 'Helen Fox' (Barclayi) E **** H-5 xxx 'Helene Schiffner' B ** H-4 xx 'Hugh Koster' C ** H-4 xx 'Humming Bird' gr. C ** H-4 xxx 'Ivery's Scarlet' D * H-4 x
R. H. S. RATINGS A. R. S. RATINGS Variety Hardiness Quality Hardiness Quality 'Jan Dekens' B * H-3 xx 'J. G. Millais' B *** H-4 xxx 'J. H. Van Nes' C ** H-3 xxx 'King George' (Loderi) C **** H-4 xxxx 'Lady Alice Fitzwilliam' F *** H-5 xxx 'Lady Bessborough' gr. B **** H-3 xxxx 'Ladybird' gr. H-4 xx 'Lady Bligh' C *** H-3 xxx 'Lady Chamberlain' gr. C **** H-4 xxxx 'Lady Clementine Mitford' A ** H-3 xx 'Lady Primrose' B *** H-4 xxx 'Lady Roseberry' gr. C **** H-4 xxxx 'Lady Stuart of Wortley' C * H-3 xx 'Langley Park' B ** H-3 xxx 'Loder's White' C **** H-4 xxxx 'Lord Robert' A H-3 x
R. H. S. RATINGS A. R. S. RATINGS Variety Hardiness Quality Hardiness Quality 'Madame de Bruin' B ** H-3 xx 'Madame Fr. J. Chauvin' C *** H-3 xxx 'Margaret Dunn' gr. H-3 xxx 'Marinus Koster' C ** H-3 xxx 'Mars' B **** H-2 xxxx 'May Day' gr. C **** H-3 xxxx 'Moonstone' gr. H-3 xxx 'Moser's Maroon' B *** H-3 xx 'Mother of Pearl' B *** H-4 xxx 'Mrs. A. T. de la Mare' C ** H-3 xx 'Mrs. C. B. Van Nes' C * H-4 xxx 'Mrs. Donald Graham' H-3 xxxx 'Mrs. Furnival' B *** H-3 xxx 'Mrs. G. W. Leak' C *** H-4 xxx 'Mrs. J. G. Millais' A ** H-2 xx 'Mrs. Lindsay Smith' C ** H-3 xxx 'Mrs. Mary Ashley' B ** H-3 xx 'Mrs. P. D. Williams' A *** H-2 xxx 'Mrs. Philip Martineau' B *** H-3 xxx 'Mrs. R. S. Holford' B * H-3 xx 'Mrs. W. C. Slocock' gr. B *** H-3 xxx 'Naomi' gr. B **** H-2 xxxx 'Nobleanum venustum' B ** H-3 x 'Pink Pearl' B ** H-3 xx 'Praecox' gr. B *** H-3 xx 'Princess Elizabeth' B ** H-2 xxx 'Purple Splendor' A *** H-3 xxx 'Pygmalion' B ** H-3 xx 'Robert Fox' (Barclayi) E **** H-5 xxxx
R. H. S. RATINGS A. R. S. RATINGS Variety Hardiness Quality Hardiness Quality 'Romany Chai' gr. B *** H-3 xxx 'Romany Chat' gr. C *** H-3 xx 'Rosa Mundi' A H-3 x 'Rosamund Millais' C ** H-3 xx 'Royal Flush' gr. E **** H-4 xxx 'Rubina' gr. H-5 xx 'Sappho' A * H-3 xx 'Sarita Lotter' gr . C ** H-l xx 'Snow Queen' gr. B *** H-3 xxx 'Souvenir of W. C. Slocock' B *** H-3 xxx 'Tally Ho' gr. D **** H-5 xxxx 'The Hon. Jean Marie de Montague' H-3 xxxx 'Unique' B **** H-3 xxx 'Unknown Warrior' C * H-4 xx 'Van Nes Sensation' C H-4 xx 'Vulcan' gr. B ** H-3 xxx 'White Pearl' B H-4 x 'Zuyder Zee' gr. B ** H-4 xx