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 | |||||