Leaf, Stem And Flower Bud Cold Hardiness Of Rhododendron Cultivars
Norman Pellett and Mary Holt
University of Vermont
Department of Plant and Soil Science
Reprinted from HRI, 230 Southern Bldg., Wash. D. C.
Winter injury to rhododendrons is a limiting factor for most areas where this plant is produced. Foliage desiccation (4, 9), low temperatures (2, 3, 6), and rapid temperature drop (10) are causes of winter injury to evergreen plants.
Relative injury of rhododendron cultivars may differ from year to year (6). The American Rhododendron Society published the coldest temperature which each rhododendron cultivar will normally withstand (1), but they did not distinguish between the cold hardiness of flower buds and other plant parts. We assume their cold hardiness ratings refer to midwinter hardiness.
Freezing injury may occur during autumn when some cultivars are slow to develop cold hardiness. We wanted to know when freezing injury occurs to different cultivars so we could determine their landscape value for regions where autumn temperatures damage plants.
The cold hardiness of leaf, stem and flower bud organs of "hardy" rhododendron cultivars was determined during autumn and early winter for several years. These cultivars, among the more cold hardy, as reported by the American Rhododendron Society (1) were growing at the University of Vermont Horticultural Research Center in South Burlington, Vermont. A more detailed description of this research is reported elsewhere (5, 6).
Study 1 - leaves and stems. We wanted to answer the following questions. Which stem and leaf organs are most sensitive to winter injury? Do leaves and stems acclimate rapidly enough in autumn to avoid injury by expected low temperatures? Leaves and stems, collected from the field, were frozen periodically in the laboratory from September to January during 2 years. The laboratory temperature causing injury to leaf and stem organs of 7 cultivars was determined.
We made extensive studies comparing the hardiness of 'Boule de Neige' and 'Nova Zembla'. Also, we compared the cold acclimation of 'America', 'Caractacus', 'Lee's Dark Purple', 'Catawbiense Boursault' and 'Roseum Elegans'. After freezing, leaf and stem samples were kept in polyethylene bags for 1 week at 28°C (82°F), then observed for injury. We compared the injury of leaf midrib, petiole, secondary veins and area between veins. Also the hardiness of the stem xylem and bark tissues were compared. The bark tissues refer to the cambium, phloem, and bark.
Leaf and stem hardiness
The leaf midrib and petiole of all cultivars were generally less cold hardy throughout autumn than other leaf organs. The stem bark tissues were less cold hardy than xylem throughout autumn. These less hardy tissues contain the major conducting elements of the plant and are crucial to survival of leaves. Their loss results in unsalable plants in nursery fields.
Laboratory and field temperatures causing winter injury were similar. Field injury to 'Boule de Neige' and 'Nova Zembla' occurred before December 22. The minimum outdoor temperature of -26°C (-15°F) on December 20 was similar to laboratory temperatures causing injury. During the second winter of the study field injury of 'Nova Zembla' and 'Boule de Neige' leaves occurred between November 22 and December 7 when the minimum outdoor temperature was -21°C (-6°F) on December 3. The other 5 cultivars tested showed field injury to the more tender stem and leaf organs on December 14 and 15. Field injury resulted in the second year when outdoor temperatures reached -21°C (-6°F) on December 3.
'Boule de Neige' was more cold hardy than 'Nova Zembla'. The leaf midrib of both cultivars had a similar pattern of cold acclimation, but there was greater hardiness difference between them during the first winter than the second, when temperatures were generally colder. The greatest difference in hardiness was between bark of these two cultivars. 'Boule de Neige' not only hardened more rapidly than 'Nova Zembla', but the maximum hardiness was greater by December 1 of both years.
Leaf and stem organs of 'Catawbiense Boursault', 'Lee's Dark Purple' and 'Roseum Elegans' were generally more cold hardy in laboratory freezing than 'America' and 'Caractacus'. These observations agree with our observations at outdoor test sites in Vermont (7).
Record low temperatures which may injure plants occur periodically. We compared laboratory temperatures causing injury with the lowest temperature ever recorded to date for Burlington, Vermont. The record minimum temperature was lower than the killing temperature for 'Boule de Neige' and 'Nova Zembla' in mid- to late November. Therefore, low temperatures may cause injury to these rhododendrons in November or early December, as temperatures approach record minimums in climates like Vermont.
Study 2 - flower buds. Evergreen rhododendrons bloom from overwintering flower buds. The term flower bud in this paper refers to the inflorescence bud. Floret refers to an unexpanded flower in the bud. We compared the flower bud cold hardiness of 5 cultivars during the later stages of cold hardiness development.
Five evergreen rhododendron cultivars were compared for flower bud hardiness on six dates in December and January from 1977 to 1980. The freezing dates were selected because leaves and stems of the same cultivars were injured by Vermont temperatures in 2 years during December or early January (Study 1). Flower buds with about 1 cm of stem were sampled from the top third of 1 to 1.5 m high plants. The buds were frozen in the laboratory at several test temperatures.
The buds were dissected and the florets evaluated as injured (black) or uninjured (white).
Flower bud hardiness. Florets of 'Roseum Elegans', 'Catawbiense Boursault' and 'Boule de Neige' were more cold hardy on most samplings than those of 'America' and 'Lee's Dark Purple'.
When flower buds of the cultivars were ranked from hardiest to least hardy, they generally followed the same order. 'Roseum Elegans' was the hardiest followed in order by 'Catawbiense Boursault', 'Boule de Neige', 'America', and 'Lee's Dark Purple'. 'Lee's Dark Purple' on January 5, 1978 and 'America' on January 9, 1979 showed field injury to more than 10% of the florets. The results were generally consistent with our field observations where 'Boule de Neige', 'Catawbiense Boursault' and 'Roseum Elegans' were apparently about equal in winter flower bud hardiness and more hardy than 'America' (7).
Cultivars with the more hardy leaves or stems did not always have the most hardy flower buds. 'Lee's Dark Purple' had leaf and stem tissues comparable in hardiness to other cultivars but had less cold hardy florets. 'America' generally had less cold hardy florets as well as less hardy leaf and stem tissues than other cultivars. 'Boule de Neige' and 'Catawbiense Boursault' had florets, leaves and stems among the most cold hardy in both studies.
Laboratory freezing can be useful in comparing the flower bud cold-hardiness of rhododendrons. Flower bud injury was easily determined because injured florets were either black (injured) or white (uninjured) after freezing in the laboratory or freezing injury in the field. Nurserymen can determine flower bud injury in early spring by dissecting the florets of azaleas and rhododendrons.
The hardiest evergreen rhododendrons may be injured where minimum temperatures are below -15°C (5°F) in late November or early December. The selection of hardy rhododendrons should not be based solely on mid-winter cold hardiness because the rate of hardiness development is an important consideration.
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