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Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 38, Number 4
Fall 1984

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Freezing Resistance of Rhododendron and Azalea
A. Sakai, Corvallis, OR

Editor's Note:
The following tabulations were received from Mr. Sakai shortly before his return to Japan. These lists should be of interest not only to hybridizers but also to almost anyone who grows rhododendrons or azaleas in borderline hardy areas.
        It is interesting to note that not all plants are similar in their cold resistance as applied to all of their plant parts. In Table 1 R. discolor and R. barbatum show a remarkable uniformity to cold resistance for all their plant parts. Whereas in Table 2 R. intricatum has a very wide ranging sensitivity to cold, depending on which plant parts are being studied. In Table 3 the flower bud of R. mucronatum is even slightly more resistant to cold than the leaf.

Table 1. Freezing Resistance of Rhododendron
  Freezing Resistance (°C)  
Species (Series) Flower Bud Leaf Vegetative
Bud
Cortex Xylem Distribution
R. brachycarpum (?) -30 -60 -60 -60 -35 C.N. Japan
R. catawbiense (?) -30 -60 -60 -60 -40 N.E. USA
R. dauricum (P) -30 -50 -60 -60 -40 N.E. Asia, E. Siberia
R. maximum (?) -25 -60 -60 -60 -35 N.E. USA
R. metternichii var.
   hondoensis (P)
-25 -60 -60 -60 -35 C. Japan
R. carolinianum -25 -40 -50 -50 -35 S.E. USA
R. metternichii var.
   pentamerum (P)
-23 -50 -50 -60 -30 C. Japan
R. metternichii (P) -23 -40 -50 -50 -27 C.W. Japan
R. metternichii var.
   yakushimanum (?)
-23 -40 -50 -50 -27 Yakushima Island
R. metternichii var.
   augustifolium (P)
__ -40 -50 -50 -27 C. Japan
R. ferrugineum (F) -23 -30 -25 -35 -30 Europe
R. smirnowii (?) -23 -30 -40 -40 -30 Caucasus, Asia Minor
R. micranthum -23 -25 -30 -30 -25 C. China
R. ponticum (?) -20 -35 -30 -30 -30 Caucasus, Asia Minor
R. campylocarpum -20 -25 -30 -25 -25 Nepal, Tibet
R. hodgsonii -25 -30 -25 -25 Nepal (3900m)*
R. wallichii (C) -20 -28 -30 -30 -25 Nepal (3900m)*
R. campanulatum (C) -20 -25 -30 -30 -25 Nepal (3900m)*
R. macrophyllum (P) -20 -20 -25 -25 -20 British Columbia
R. discolor (F) -18 -20 -20 -20 -20 Szechwan
R. barbatum -18 -18 -18 -18 -18 Nepal (3000m)*
R. fortunei (F) -15 -18 -25 -23 -23 C. China
R. griffithianum (F) -15 -18 -20 -18 -18 Nepal
R. rubropunctatum -15 -20 -20 -20 -20 Formosa (2000m)*
R. arboreum var. cinnamomeum (A) -13 -13 -13 -13 -13 Nepal (2800m)*
R. arboreum var. campbelliae (A) -10 -10 -10 -10 -10 Nepal (2200m)*
*Altitude of collection site.
(P) Ponticum, (F) Fortunei, (C) Campanulatum, (A) Arboreum, (F) Ferrugineum

  

Table 2. Freezing Resistance of Dwarf Rhododendron
  Freezing Resistance (°C)  
Species (Series) Flower Bud Leaf Vegetative
Bud
Cortex Xylem Distribution
R. parvifolium (Lapponicum) -35 -60 -60 -60 -50 N.E. Asia, E. Siberia
R. aureum -25 -60 -60 -60 -50 C.N. Japan
R. impeditum (Lapponicum) -25 -50 -50 -50 Yunnan (33-3900m)
R. dasypetalum (Lapponicum) -25 -50 -50 -50 -35 Yunnan (32-4000m)
R. anthopogon (Anthopogon)* -23 -50 -50 -50 -50 E. Nepal (27-4800m)
R. setosum (Lapponicum)* -23 -50 -50 -50 -50 E. Nepal (33-4800m)
R. fastigiatum (Lapponicum) -24 40 -40 -40 -30 Yunnan (31-4100m)
R. intricatum (Lapponicum) -23 -35 -40 -50 -35 Szechwan (36-4500m)
R. keiskei (Triflorum) -23 -25 -30 -30 -25 Japan
R. scintillans (Lapponicum) -20 -40 -40 -40 -35 Yunnan (33-4-200m)
R. russatum (Lapponicum) -20 -40 -40 -40 -30 Yunnan (33-4000m)
R. lepidotum (Lapponicum)* -20 -40 -30 -60 -30 E. Nepal (24-4500m)
R. russatum (Lapponicum) -20 -40 -40 -40 -30 Yunnan (33-4000m)
R. fimbriatum (Lapponicum) -20 -35 -35 -25 -25 Szechwan (3600m)
R. keiskei var.
   hypoglaucum (Triflorum)
-20 -25 -25 -35 -35 Japan
R. radicans (Saluenense) -15 -25 -25 -25 -25 S.E. Tibet (42-4500m)
R. racemosum (Scabrifolium) -17 -15 -18 -18 -18 Yunnan (17-3700m)
R. hanceanum (Triflorum) -15 -20 -20 -20 -20 Szechwan (5-3000m)
R. yunnanense (Triflorum) -15 -18 -20 -25 -25 S.E. Tibet (22-3000m)
R. augustinii (Triflorum) -15 -15 -20 -20 -20 Yunnan (18-2700m)
R. hippophaeoides (Lapponicum) -15 -15 -25 -25 -25 Yunnan (33-3400m)
R. amamiosimense -13 -15 -13 -17 -17 Japan (Amami Island) 800m
* Collected at East Nepal (3900m alt.)        

  

Table 3. Freezing Resistance of Azalea
  Freezing Resistance (°C)  
Species Flower
Bud
Leaf Vegetative
Bud
Cortex Xylem Distribution
R. mucronulatum Turcz. -30 -60 -60 -40 China, Korea, Japan
R. viscosum -30 -50 -60 -40 USA (East)
R. arborescens -30 -40 -60 -40 USA (East)
R. schlippenbachii -28 -30 -30 -30 Korea, Japan
R. japonicum (A. Gray) Suringer -25 -40 -50 -40 Japan
R. mucronulatum var. ciliatum Nakai -25 -50 -50 -35 Korea, Japan
R. tschonoskii Maxim. -25 -40 -40 -40 Korea, Japan
R. albrechtii (Maxim.) -25 -40 -40 -30 Japan
R. yedoense var. poukhanense Nakai -25 -30 -30 -30 -30 Korea, Japan
R. obtusum var. japonicum (Maxim.) Kitamura -25 -30 -30 -25 Japan
R. obtusum (Planchon) (Maxim.) -23 -30 -30 -30 -30 Japan
R. pentaphyllum var. nikoense -23 -25 -30 -25 Japan
R. reticulatum D. Don -23 -30 -30 -30 Japan
R. mucronatum (Blume) G. Don -23 -17 -25 -25 -25 Japan
R. occidentale -20 -25 -25 -23 USA (West)
R. macrosepalum Maxim. -20 -20 -25 -25 -27 Japan
R. nudipes Nakai -20 -27 -27 -25 Japan
R. serpyllifolium (A. Gray) Micquel* -20 -25 -25 -25 -25 Japan
R. simsii PI. -18 -30 -30 -30 China
R. ripense Makino* -15 -20 -20 -20 -20 Japan
R. scabrum G. Don* -13 -15 -15 -15 -15 S. Japan
R. tashiroi Maxim.* -13 -15 -15 -15 -15 S. Japan
R. eriocarpum (Hayata) Nakai* -13 -15 -15 -15 -15 S. Japan

  

Table 4. Freezing Resistance of Cultivars of Rhododendron
  Freezing Resistance (°C)
Cultivar Flower
Bud
Leaf Vegetative
Bud
Cortex Xylem
Red Catawbiense Hybrid* -27 -60 -60 -60 -35
America* -27 -50 -60 -60 -30
P.J.M.** -27 -50 -50 -50 -40
Album Elegans* -27 -40 -50 -50 -35
English Roseum* -27 -40 -50 -50 -35
Dora Amateis** -25 -40 -40 -40 -35
President Lincoln* -25 -40 -40 -40 -30
Fastuosum Plenum* -25 -35 -40 -40 -35
Ramapo* -25 -40 -40 -50 -35
Nova Zembla -25 -35 -40 -40 -25
Cheer* -25 -25 -30 -30 -25
Catawbiense Grandiflorum* -24 -25 -30 -30 -25
Lee's Dark Purple* -23 -35 -35 -35 -25
Christmas Cheer* -23 -23 -25 -25 -20
Prince Camille de Rohan -20 -25 -35 -35 -25
Scarlet Wonder -23 -23 -25 -25 -23
Blue Diamond -20 -25 -30 -30 -25
Pink Pearl -20 -25 -25 -25 -20
Elizabeth (forrestii hybrid) -20 -20 -25 -25 -20
Unique (campylocarpum hybrid) -14 -15 -25 -25 -25
Vulcan (griersonianum hybrid) -17 -20 -20 -20 -20
President Roosevelt - -15 -20 -20 -17
Thor -13 -15 -25 -25 -20
Gwen -13 -13 -17 -17 -17
*Catawbiense hybrid, **Carolinianum hybrid


Volume 38, Number 4
Fall 1984

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