Cold Hardiness Ranking of Rhododendrons By Means of Flower Bud Damage
A cold wave in Russell Gilkey's area (Kingsport, Tenn.) in mid January in 1994 was responsible for a low of -7°F with a wind chill of -20°F. Rhododendron flower buds in his garden sustained damage ranging from 0 to 100 percent. He measured the damage by counting the number of blackened pips in the flower bud. Rhododendrons in his garden are ranked from hardiest to least hardy according to percent blackened pips. Because of the limited number of buds examined and variation among buds on a plant, it is estimated that a difference of about 30 percent blackened pips between plants is necessary in order to conclude that a difference in hardiness exists. It is suggested that an extension of this kind of observation by a number of ARS members in different areas with different abnormal weather conditions would lead to a reliable cold hardiness ranking for rhododendrons. Conflicting and exaggerated hardiness data reported in various publications for rhododendrons makes it difficult for gardeners to choose those which will satisfy their requirements. These requirements may range from bud hardy every year to most years to some years depending on individual desires, available space and zeal.
Winter temperature hardiness ratings for flower buds of a particular rhododendron range so much in nursery catalogs and in the literature that you don't know what to believe.1 Some of the confusion may come from the many factors that affect hardiness. Three primary aspects of winter hardiness were presented in the article by Pellet and Moe in the Journal (Vol. 40, No. 4, 1986): timing of acclimation, maximum cold hardiness capability and ability to resist rapid deacclimation or ability to reacclimate rapidly following unseasonable warm periods in late winter. The table, Midwinter Hardiness (°F) of Flower Buds of Rhododendron Taxa, in their article lists the lowest approximate temperature that will result in 50 percent or greater bloom. The data were obtained by exposing rhododendron flower buds to progressively lower temperatures and examining bud damage. This data would be excellent for determining comparable maximum cold hardiness capability. However, the other aspects of winter hardiness would not be covered. Also, laboratories capable of doing the experiments they carried out don't have the manpower available and the material at hand to investigate the large number of rhododendrons grown by ARS members. Comparative rankings by a number of rhododendron hobbyists might be the answer if enough willing participants could be found and someone would be responsible for compiling the data. The sample would have to be large enough to include a wide variety of weather conditions so that the various aspects of winter hardiness would be taken into account. Comparing the bud damage of a group of rhododendrons would allow ranking from hardiest to least hardy. If a rhododendron in the ranking blooms satisfactorily most or all the time in a given area, then those rhododendrons with approximately equal or with less bud damage should be suitable for growing in that area.
This project would require several years since an abnormal winter would be necessary in order to obtain the data. Such a winter occurred in my area in 1994. In mid January the day's highs had been in the mid forties and the previous lowest temperature had been only 12°F. Then a cold wave produced a low temperature of -7°F. The wind blew day and night and created a minimum wind chill (according to NOAA) of -20°F. There have been other years with a comparable minimum temperature but with far less overall flower bud damage. There have also been a couple of years with similar damage. I don't recall the differences in circumstances but suspect wind chill. Apparently, maximum cold hardiness capability must be related to wind chill rather than just minimum temperature. Flower bud damage in 1994 was obtained by picking buds, firm to the touch, from various positions on the plant, removing bud scales one by one and counting the number of blackened pips and the total number of pips. Pip, as used here, refers to the latent undeveloped flower in the flower bud. If the buds aren't firm, you can suspect that they had started swelling in the fall. They are consequently more susceptible to cold damage. The bud damage results for 1994 and 1995 are shown in Table 1. The 1995 data are included for comparison. The minimum temperature in 1995 was +1°F in mid February. There was very little wind. It is evident from the limited damage that a mild winter, for my area, is not suitable for separating rhododendrons according to their relative hardiness. The rhododendrons in Table 1 are listed mainly in order of percent blackened pips in 1994. The order was changed in a few instances based on 1995 results and previous blooming experience over the years. In those cases where more than one bud was examined, there wasn't much difference in the highest and lowest value for percent blackened pips for rhododendron buds with an average of up to 30% or more than 80% blackened pips. However, for those in the 30-80% range, there could be a difference of 20-80% between the highest and lowest value. Perhaps this was due to the location of the bud on the plant and whether or not it was somewhat sheltered from the wind. Considering the variation in percent blackened pips between buds and the fact that for many plants only one bud was examined, a difference of a few percent blackened pips in the table is not significant. There is not enough data for a statistical treatment, but a difference of about 30% blackened pips between plants seems to be necessary (all observations taken into account) in order to say there is a difference in hardiness. Ideally, ten or more buds per plant should have been examined. Practically and esthetically this was not possible. In the first place there were not that many flower buds on many of the rhododendrons. In the second place I grow rhododendrons to see the bloom.
Table 1. Susceptibility of Rhododendron Buds to Winter's Coldest Weather as Measured by Percentage of Blackened Pips in Buds Rhododendrons No. Buds
Buds and Blooms
on Plant (1994)
R. minus Carolinianum Group (pink) 2/1 0/0 8/8V R. catawbiense 'Catalgla' 1/- 0/- 2/2V R. catawbiense, RSF 77/b20 1/- 0/- 4/4V 'Chapmanii Wonder'* 3/1 0/0 286/allV 'Dora Amateis' 3/1 0/0 200/allV 'DuPont's Apricot'* 1/1 0/0 2/2V R. keiskei, Ebino form 3/- 0/- 0/10V R. maximum 'Mt. Mitchell' 1/- 0/- 4/4V "Ponticum Roseum' (syn. 'Maximum Roseum') 3/1 0/0 172/allV 'Merley Cream' 3/1 0/0 55/55V R. minus (pink) 3/1 0/0 216/allV R. minus (white) 2/1 0/0 9/9V 'Molly Fordham'* 1/1 0/0 9/9V R. mucronulatum' Mahogany Red'* 4/1 0/0 104/allV R. mucronulatum (pink) 3/1 0/0 133/allV PJM Group 2/1 0/0 7/7V 'Roslyn' 1/1 0/0 4/4V 'Caroline' (Gable) 3/4 3/2 30/30V 'America' 3/1 4/0 95/allV 'Mrs. Charles E. Pearson' 2/2 4/0 34/26V 'Macopin' 3/1 5/0 129/allV 'Windbeam' 3/1 5/0 52/52V R. minus Carolinianum Group, white, compact form 3/1 6/0 38/38V 'Francesca' 2/- 6/- 18/18V 'English Roseum' 1/1 8/0 7/7V 'Haag's Choice' 2/1 8/0 37/37V 'Maxecat'* l/4 8/3 41/41V 'Olga Mezzit' 3/1 8/0 183/allV 'Cyprus' 2/1 9/0 47/47V R. degronianum, Mt. Mitsumata form (formerly R. metternichii, Mt. Mitsumata form) 2/1 13/0 19/19V 'Ruby Bowman' 1/1 13/0 2/2V 'Wheatley' 1/1 13/0 3/3V 'Cloxineum' 1/1 27/0 6/6V 'Sphinx' 3/1 29/0 28/28V 'Chionoides' 1/1 14/0 14/14V 'Joe Brooks'* 1/- 14/- 6/6V 'Scintillation' 3/1 14/0 413/allV 'Dorothy Amateis' 1/1 20/0 14/14V 'Wyanokie' 3/1 17/0 57/mostV 'Edith Bosley' 1/1 20/33 3/3V 'Mary Fleming' 5/1 28/0 500/mostV 'Shanghai' 1/1 32/0 17/10V/5P 'Besse Howells' 3/1 32/0 96/12V/80P 'PartyPink' 2/1 13/0 16/4V/11P/1B 'Flaming Snow' 1/2 23/0 23/5V/14P/4B 'Percy Wiseman' 1/1 25/0 8/6P/2B 'Gigi' 1/1 29/0 9/8P/1B 'Cadis' 1/1 30/0 - 'Janet Blair' 3/1 30/0 - 'Edmond Amateis' 2/1 16/0 26/17V/4P PJM Group 3/6 31/17 - R. keiskei 'Yaku Fairy' 3/1 33/0 - 'David Forsythe' l/- 38/- 17/5V/7P/5B 'Parker's Pink' 3/3 41/13 98/20V/65P/13B 'Wynterset White' (syn. 'Kordus White') 1/- 43/- 3/3P 'Boule de Neige' 3/3 44/0 104/21V/39P/44B 'Mrs. Powell Glass' 1/- 47/- - R. yakushimanum, Exbury form 1/1 17/18 - 'Cloud Nine' 1/- 50/- - R. fortunei seedling 1/1 50/0 - 'Queen Anne's' 1/2 50/14 8/8P 'Vulcan's Flame' 2/3 52/11 26/11V/13P/2B 'Todmorden' 2/3 55/11 30/26V&P/4B 'Brown Eyes' 1/- 56/- - 'Governor's Mansion' 1/- 57/- 3/2P/1B 'Marchioness of Lanesdowne' 1/1 59/0 - 'Purple Splendor' 3/3 61/50 34/4V/6P/24B R. degronianum ssp. heptamerum Micranthum Group (known formerly as R. metternichii var. micranthum) 1/1 60/0 - R. degronianum ssp. pentamerum (known formerly as R. metternichii var. kyomaruense) 1/1 67/0 - 'Mrs. Furnival' 2/1 67/0 26/21P/5B R. yakushimanum 'Koichiro Wada' 3/3 67/32 117/18V/24P/75B 'Cunningham's White' 2/3 68/3 - 'Mars' 3/1 71/0 28/4V/3P/21B 'Blue Ensign' 6/1 72/0 134/3V/76P/55B R. degronianum (as yakushimanum form); known formerly as R. metternichii (tsukushianum form) 1/1 75/100 - 'Purple Lace' 1/2 75/64 5/4P/1B R. degronianum ssp. heptamerum Hondoense Group (known formerly as R. metternichii var. hondoense) 1/- 78/- - 'Madame Masson' 3/3 79/7 362/52V/134P/176B 'Robert Allison' 3/1 79/0 - 'Tom Everett' 1/1 79/71 - 'Bellringer' l/3 82/3 9/5P/4B 'Nestucca' 1/- 82/- 7/5P/2B 'Arthur Bedford' (syn. 'A. Bedford') 2/1 83/0 - R. racemosum, dwarf 4/6 83/33 - 'The Honourable Jean Marie' 3/1 85/0 - 'Mrs. Tom H. Lowinsky' 1/1 87/0 3/3P 'Souldis' 1/1 87/0 20/19P/1B 'Damozel' 3/1 89/0 - 'Red Eye'* 3/1 90/0 46/9V/23P/14B R. minus var. chapmanii, RSF 76/16 3/1 92/0 - 'David Gable' 3/3 95/3 - 'Anah Kruschke' 1/3 86/8 - 'Dexter's Orchid'* 3/4 91/11 62/18V/26P/18B R. racemosum seedling 4/7 94/26 - 'Furnivall's Daughter' 1/1 100/0 19/2P/17B 'Gomer Waterer' 1/1 100/0 6/6B 'Pink Pearl' 2/3 100/0 30/30B 'Taurus' 1/1 100/0 3/3B 'Cosmopolitan' 1/1 100/0 12/12B 'Mrs. W.R. Coe' 1/3 100/3 24/24B 'Tripoli'* 1/1 100/6 24/24B 'Helen Everitt' 1/3 100/8 11/11B 'Mezitt's Apricot Pink' 1/1 100/9 6/3P/3B R. makinoi seedling 3/1 100/9 33/33B 'Golden Jubilee'(syn. 'Pearce's Golden Jubilee) 1/4 100/12 14/14B 'Ladybird' 1/1 100/18 8/8B 'Sappho' 14 100/32 36/36B 'Mary Belle' 1/2 100/32 6/6B 'Antoon van Welie' 2/3 100/33 26/26B 'Royal Pink' 1/1 100/33 3/3B 'Wissahickon'* 1/3 100/36 10/1P/9B 'Ann Skinner'* 1/3 100/44 11/11B 'Direktör E. Hjelm' 1/- 100/- 3/3B 'Trude Webster' l/- 100/- 4/4B 1 Pip refers to the latent, undeveloped flower in the flower bud. 2 The number followed by V represents the number of buds whose trusses or blooms were visually okay. Close inspection might show missing flowers in some trusses. The number followed by P represents the number of buds with obviously partial trusses. The number followed by B represents the number of completely blasted buds (no bloom). * Name is unregistered.
The data in column three of Table 1 came from counting the buds in the fall, observing the plant in bloom and counting the partial blooms and blasted buds during dead heading. The missing data are the consequence of lack of time to make the observations because of the press of higher priority projects. The appearance of the plant in bloom corresponded very well with percent blackened pips. When blackened pips got above 40%, the blooming plant generally looked rather ratty.
One item that concerned me, because it seemed to be entirely out of line, was the result for Rhododendron yakushimanum. Not only do I grow the FCC and Exbury forms, but also seedlings from ARS Seed Exchange seed from all over the world. Nine different yaks planted in different locations ranged from 8-100% blackened pips in 1994 and 10-1 00% in 1 995. In general, their flower buds tend to be not very well developed and some appear to be off color (outer bud scales grayish green instead of greenish). Based on the assumption that bud blast fungus was the culprit, several fungicide sprayings starting in August were tried. It did not help. I would appreciate any ideas that might explain their relatively poor hardiness. A comparative ranking, which can be derived from Table 1, does not tell you what minimum temperature the flower buds on a rhododendron will stand. What it tells you is how one plant compares with another given a particular set of winter conditions. For example, you could expect the same flower bud damage or lack of damage for the pairs: 'English Roseum' and 'Roslyn' or 'Parker's Pink' and 'Boule de Neige' or 'Dexter's Orchid'* and 'Anah Kruschke'. If you know that one fares well in your area, then you could feel fairly confident in growing the other. It is not my intention to discourage you from growing those rhododendrons that have a high percentage of blackened pips in Table 1. For example, in my experience, 'The Honourable Jean Marie de Montague' and 'Vulcan's Flame' had less than satisfactory bloom in only 4 out of 18 years. The score for 'Scintillation' is satisfactory bloom in 20 out of 22 years. 'Sappho' can only claim 9 out of 18 years. These results compare favorably with the ranking in Table 1. But, for instance, if your neighbors have been completely disappointed with 'Vulcan's Flame' because of flower bud hardiness, don't try to grow "Jean Marie." Incidentally, 'The Honourable Jean Marie de Montague', 'Vulcan's Flame' and 'Sappho', among others, sustained extensive plant damage in 1985 because of a record low temperature of-21 °F and a wind chill that must have been at least -40°F. No rhododendron in my garden bloomed that year.
An article by Pellet in the Journal (Vol. 41, No. 4, 1987) describes the effect of an early fall freeze (-3°F in November) on several rhododendron flower buds. Interestingly enough 'Chionoides', 'Francesca' and 'Maximum Roseum', which are hardy according to Table 1, were found to be badly damaged by the fall freeze. One of the aspects of cold hardiness, acclimation, is a likely explanation for this difference. Rhododendron catawbiense, R. minus Carolinianum Group (pink), 'Olga Mezitt' and 'Windbeam' showed little damage to pips in both studies.
If enough data could be collected covering weather conditions involving acclimation, deacclimation, reacclimation, tendency to fall bloom promotion and maximum cold hardiness capability, a reliable comparative cold hardiness ranking could be developed for rhododendron flower buds. On further contemplation, this would be a monumental task. However, nothing started, nothing accomplished. If you should record flower bud damage via blackened pips, send me your data and weather conditions and I will either compile the results or find someone who will. Or better yet make it a chapter project for rhododendrons grown in your area.
Russell Gilkey, a member of the Southeastern Chapter, authored the article "Correlation Between Weather Conditions and Rhododendron Flower Bud Formation" in the Winter 1996 issue of the Journal.
1 Hardiness rating: refers to the "H" ratings frequently found in catalogues. The new ARS cold hardiness definition is discussed in the Fall 1995 Journal.