JARS v37n3 - Winter Damage to Rhododendrons at Winterthur

Winter Damage to Rhododendrons at Winterthur
After the Record-breaking Low Temperatures of January, 1982
Hal Bruce, Winterthur Museum, Delaware

Winterthur lies in a fairly mild climate for the East Coast, in the northern part of USDA Zone 7a. All the ironclad rhododendrons of course thrive here, as well as the hardier Asiatic species like R. fortunei , discolor , vernicosum , degronianum , makinoi , yakushimanum , mucronulatum , and keiskei . The Gable hybrids of both rhododendrons and azaleas seem tailor-made for the region. Dexter hybrids also do well for the most part. Kurume and Kaempferi azaleas have thrived for many years, as well as many Satsukis and Glenn Dales. R. mucronatum in its various forms also does very well. A few more delicate species hold their own, failing to bloom some years because of extremely low temperatures ( litiense , auriculatum , amagianum , weyrichii , serrulatum ) or late spring freezes ( fargesii , sutchuenense ), but by and large they are counted as permanent plants here. A very few English hybrids have proven themselves to be good plants here ('Blue Peter', 'Goldfort', 'China', 'Lady Cathcart'), and a few more survive but do not always give so good an account of themselves ('Citronella', 'Princess Elizabeth', 'Carita'). A great many more have succumbed over the years to our climate. Among these have been 'Unique', 'Countess of Derby', 'Naomi', 'Jalisco', 'Rickshaw'' 'Harvest Moon', and 'Moonstone'.

Temperatures usually fall to 0° F. for short periods during midwinter, but seldom go below this. Snow cover is variable, some years more abundant than others, but never lasting through the whole winter. Probably the most harm to rhododendrons occurs during the cold snaps when strong winds blow down from Canada causing wind-chill factors of sometimes -70F. Perhaps just as detrimental are the thaws which occur between cold snaps. For example, this year the thermometer registered 73° F. on Saturday December 4th. By the following Thursday night temperatures were falling into the 'teens,' and on Saturday night of the 11th seven inches of snow fell.

The winter of 1981-82 started in a milder fashion. After January 1 the ground was still unfrozen, and I recall writing to a friend that, the winter equinox being behind us, perhaps we were due for a mild winter. Within a week arctic winds howled out of Canada and temperatures plunged to a low one night of -16° F which broke records. Three days or so after the cold snap began, considerable snow fell, which certainly helped dwarf and herbaceous plants. The taller shrubs, however, suffered one of the worst years in my experience. This was no doubt complicated by a scarcity of ground water, the result of several years' drought (bad enough to cause water restrictions a couple of years ago in nearby Pennsylvania).

The result, when spring finally came, was that many plants had no bloom at all, a good many had very sparse bloom, and some had bloom that was strangely out of season. One Dexter hybrid, for example (a Winterthur numbered clone that is probably a hybrid of caucasicum with fortunei or decorum ) bloomed sparsely a month later than normal.

Perhaps the most conspicuous effects were seen in the large Kurume azaleas (plants 50 years old or more). One clone, 'Debutante', a tall-growing salmon, was for the most part killed almost back to snowline - trunks and branches as much as 4 inches in diameter killed outright. A plantation of younger (20 years old) specimens of this cultivar, in a lower and possibly more protected part of the garden, suffered considerable bud and twig kill, but much less damage.

To complicate matters, freezing temperatures and snow in April ruined buds and flowers of precocious bloomers like mucronulatum and keiskei . There was some speculation as to whether this late freeze might have been more injurious to rhododendrons in general than the January low, but in May the extraordinary appearance of nearly every Kurume cultivar - a mound of browned leaves punctuated by an occasional flower, but with a foot-wide skirt of solid color at the base, where the flower buds had been beneath the snow - indicated that the January low was indeed the culprit.

Blooming data collected in spring of 1982 resulted in the following:

Remarks:
The generally poor performance of the Kurumes is to be expected, since they have a history of bud damage in colder winters. The variability in susceptibility of various clones is rather surprising, however, and would indicate that the group is genetically more diverse than supposed. (It might be noted that two of the commonest cultivars in the trade, 'Coral Bells' and 'Snow' suffered 50% or more bud loss.) Since three of the putative parents of the Kurumes, kiusianum , kaempferi , and obtusum , suffered little damage here (in the case of kaempferi the sampling was extremely large, and plants grew in close proximity to many of the damaged Kurumes), it seems obvious that more tender plants are involved in the parentage of most of the Kurumes.

The native azaleas put on a surprising performance, since they all bloomed profusely. This is especially true in view of the fact that the southern lowland species austrinum , canescens , alabamense , speciosum , and serrulatum were cut to snowline by the long, hard winter of five years ago. R. prunifolium , which I once rated very tender, seems to get hardier as it matures. Our plants are some 12 feet now, and would be higher had they not been pruned back a few years ago.

Among the Tsutsusi species, poukhanense gave a good performance, as might be expected of this hardiest of the group. Mucronatum and its forms also had little damage, which is more surprising. The Gable azaleas fared well in general. 'Springtime',a hybrid of kaempferi x poukhanense , bloomed on schedule as the earliest of Tsutsutsi azaleas with no bud damage at all. The two fine whites, 'Rose Greeley' and 'White Star', also had little damage. The Glenn Dales varied, as might be expected from their varying parentages. Generally speaking, those blooming at midseason with large flowers suffered most, while the early bloomers with smaller flowers suffered least.

The Winterthur kaempferi hybrids, a group of midseason to late bloomers which I believe are primary hybrids of kaempferi and indicum were unhurt. Some of these plants are 15 ft. tall and more than 50 years old. In general the Satsukis performed poorly, but we have come to expect considerable winter damage from this group. Surely their tenderness must come from the sea-level R. eriocarpum in their background, since 'J.T. Lovett', which most authorities rate as pure indicum , is extremely hardy and performed well this year.

Among the Asiatic deciduous azaleas, schlippenbachii bloomed magnificently. By contrast, every bud on both amagianum and weyrichii was killed. The performance of both these species is very erratic, large numbers of buds being killed in milder winters than this. Location in the garden may be a factor here: our plants are situated on an exposed south-facing hillside. Our original plants of amagianum came from Joe Gable, who certainly bloomed the plant in the colder climate of Stewartstown, PA. I remember his telling me in the 60's that amagianum in his woods looked like a June-blooming, salmon-flowered dogwood.

The small percentage of lepidotes hardy enough to take our conditions no doubt accounts for the comparatively good performance of this group after the winter. A real surprise is chapmanii , which is much more cold tolerant than rated. All the species with the possible exception of keiskei are extremely bud hardy, as are their hybrids. The only difficulty with some of the species is that they are so early that spring frosts freeze the open flowers as well as the expanding buds. The April freeze finished off mucronulatum , dauricum , keiskei and their hybrids. The same applies to the elepidote species fargesii , sutchuenense and x. 'Geraldii'. The surprise in this last group is that as far as resistance to winter cold is concerned they seem hardier than the other members of ss Fortunea here.

Elepidotes:
As might be expected, catawbiense , maximum and their hybrids bloomed beautifully. The Japanese members of ss Pontica showed intermediate hardiness; all but brachycarpum showed some bud damage in the form of aborted buds in the truss. Members of ss Fortunei and vernicosum Rock aff. 18139 (both clones) suffered about 60% kill. By contrast an old plant of fortunei (Mrs. Butler') had twice as many flowers as the straight species. Both auriculatum and litiense (from Gable) were a total loss.

Gable hybrids performed as variably as their parentages; those with a strong dose of catawbiense ('Ann Glass', 'David Gable' and another clone from this same cross ('Atrosanguineum' x fortunei ), 'Beaufort', 'County of York', all performed well. Those with less 'Catawba' blood did not do well. The buds of 'Annie Dalton', 'Mary Belle', the Madforts, 'Red Head', were totally destroyed. Surprisingly, 6-64, another clone of the same cross as 'Red Head' ('Atro' x griersonianum ) showed much more hardiness than its sibling. (Unfortunately it is not nearly as attractive; such is always the way.) 5-64, a rather late hybrid of [( decorum x griffithianum ) x 'Purpuneum Elegans'] with good-sized attractive flowers of lavender bloomed well. 'Caroline' was a winner again. The more I observe this plant the more convinced I am that there is a strong shot of catawbiense genes in its makeup. 'Disca', ('Caroline' x discolor ), was a total loss, while 'Robert Allison', the reverse cross, had much less damage.

Among the English hybrids surviving here only three performed well: 'Blue Peter', 'Lady Eleanor Cathcart', and 'Goldfort'. The last had almost no damage at all, while 'China', growing right alongside it and a half sister, had no bloom at all. This would seem to indicate the hardiness from 'Jacksonii', one of the 'Goldfort's' grandparents, as the decisive factor as opposed to the tenderness of wightii , the mother of 'China'.

Among the Winterthur Dexter hybrids the rule of thumb observed in the foregoing cases was emphatically demonstrated; those with the early-appearing, large 7-lobed flowers indicative of ss Fortunea suffered the most damage. Later numbers, mostly with the tighter trusses, 5-lobed corollas and mauve or mauve-pink flowers showing 'Catawba' blood bloomed well. Rather dramatic examples of parentage choices occurred in two cases where parentage can be safely inferred. #6 ('Tan') and #54 are both large-flowered, 7-lobed apricots, the flowers of both very similar, the leaves olive green. #54 blooms a few days later and the plant is taller: similar in many ways to the Dexter clone 'Skyglow'. Bud loss on #6 was total above snowline, while #54 made a fair show. Examination of the flowers of each under the microscope shows that #6 has the hairy filaments of R. decorum while #54 has glabrous filaments like R. fortunei . The difference in hardiness between these two related species seems to have been the deciding factor when the thermometer fell far below zero. In another case I was puzzled because the eyed mauve #30 and the rather similar #59 showed such a difference in bud hardiness this year. #30 bloomed with no injury, while #59 showed close to total loss of buds. Now #59 has a very ponticum look to it, and I speculated that the difference here between ponticum and catawbiense was the deciding factor. I was able to unearth the parentage of #59, which is decorum x Sefton Special (sic). The tenderness here comes either from decorum on the one side, ponticum on the other, or both. In any case it makes all the difference between a good and a worthless plant for cold climates.