Frost Damage to Rhododendrons: January And February 1950
Harry R. Madison, Seattle, Washington
Frost damage to rhododendrons was severe during the winter of 1949-1950 at Ridgecrest, located just north of the Seattle City limits. The severe freezing weather occurred during the months of January and February in 1950 when the minimum temperature recorded was 4° F below zero. Temperatures below 12° F above zero occurred on thirteen days out of twenty-one days. (See daily minimum temperature records below.) This shows the extent of the prolonged cold snap.
The critical temperature at which the largest group of rhododendrons are frost damaged appears to be 12° F. This, of course, eliminates the rock bottom hardy group such as
catawbiense
,
maximum
, etc., and also some of the tender
maddenii
series. Other factors. besides the temperature to be taken into consideration in determining hardiness are: (1) the length of the freeze, (2) the amount of snow protecting the plants, and (3) the exposure to drying or desiccating winds. All of the above factors played a part in the frost damage to the rhododendrons at Ridgecrest in the winter of 1949-50.
| Minimum Temperatures at Ridgecrest North of Seattle City Limits | |||||
|
Date
1950 Jan. |
Min. Temp.
Degrees F. |
No. inches snow
on ground |
Date
1950 Jan. |
Min. Temp.
Degrees F. |
No. inches snow
on ground |
| 1 | +26 | ½ | 25 | - 4 | 6 |
| 2 | +17 | 2¼ | 26 | +21 | 10½ |
| 3 | +14 | 2¼ | 27 | +20 | 10 |
| 4 | +18 | 2½ | 28 | + 2 | 9½ |
| 5 | +23 | 2¼ Thawing | 29 | 0 | 9 |
| 6 | +32 | Patches thawing | 30 | - 2 | 8½ |
| 7 | +33 | None | 31 | -3 | 8½ |
| 8 | +32 | None | Feb. | ||
| 9 | +30 | None | 1 | -3 | 8½ |
| 10 | +33 | None | 2 | 0 | 8½ |
| 11 | +31 | None | 3 | +3 | 8 |
| 12 | +29 | None | 4 | +30 | 8 |
| 13 | +14 | None | 5 | +34 | 6 Thawing |
| 14 | + 5 | 12 | 6 | +34 | Thin patches |
| 15 | +13 | 13 | 7 | +33 | Thin patches |
| 16 | + 9 | 13 | 8 | +33 | Thin patches |
| 17 | +11 | 10 | 9 | +36 | Thin patches |
| 18 | + 2 | 10 | 10 | +33 | Thin patches |
| 19 | +23 | 15 | 11 | +36 | None |
| 20 | +37 | 10 Thawing | 12 | +37 | None |
| 21 | +42 | 3 Thawing | 13 | +41 | None |
| 22 | +39 | None | 14 | +42 | None |
| 23 | +19 | 4 | 15 | +46 | None |
| 24 | + 9 | 6 | |||
|
Summary of Frost Damage to Rhododendrons at Ridgecrest
During Winter of 1949-1950 |
||
| Variety | Plant Ht. in Inches | Remarks On Frost Damage |
| 'Azor' | 12 | No damage |
| 'Ayah' | 12 | Main stem killed. New growth 6" above ground following spring. |
| 'Blue Tit' | 10 | Terminal buds slightly damaged. |
| 'Bow Bells' | 18 | None |
| 'Britannia' | 30 | Slight Flower buds injured. |
| 'Butterfly' | 18 | Slight. Flower buds injured. |
| 'Day Dream' | 24 | All above snow killed. New shoots from below snow following spring. |
| 'Diva' | 20 | Same as 'Day Dream'. |
| 'Fabia' | 36 | Main stems killed above snow. New growth at base following spring. |
| 'Flame' | 48 | Top growth killed. New growth from main stem following spring. |
| 'Loderi King George' | 24 | Same as 'Fabia'. |
| 'Margaret Dunn' | 16 | None |
| 'Marinus Koster' | 24 | Terminal buds slightly damaged. |
| 'Mrs. E. C. Sterling' | 20 | Terminal buds slightly damaged. |
| 'Mrs. W. C. Slocock' | 30 | Terminal buds slightly damaged. |
| 'Nobleanum Venustum' | 14 | Main stems killed. New growth at base following spring. |
| 'Pink Pearl' | 16 | Terminal buds slightly damaged. |
| 'Rosabel' | 20 | Top growth killed. New growth from base following spring. |
| 'Rosy Morn' | 30 | Top growth killed. New growth from 15', above base following spring. |
| 'Sarita Loder' | 30 | Main stems killed. New growth at base following spring. |
| 'Spinulosum' | 48 | Same as 'Sarita Loder'. |
| 'Taranto' | 36 | Plant killed. |
| 'Unknown Warrior' | 24 | About half of the stems above snow killed. New growth from below in spring. |
| 'Vulcan' | 36 | Terminal growth slightly damaged, including buds. |
| 'White Swan' | 24 | Main stem killed. New growth from base. |
| arizelum | 14 | Slight damage to terminal growth. |
| auriculatum | 90 | Terminal growths partially damaged. Flower buds damaged. |
| californicum | 24-72 | Slight terminal damage. |
| calophytum | 36 | Slight terminal damage. |
| ciliatum | 4 | None. Under snow. |
| crassum | 10 | Main stem killed. New growth from base following spring. |
| croceum | 24 | None. |
| didymum | 13 | Plant killed. |
| discolor | 18 | None. |
| euchaites | 12 | Main stem killed. New growth from base following spring. |
| facetum | 18 | Same as euchaites. |
| falconeri | 10 | Plant killed. |
| fictolacteum | 8 | Slight damage to terminal growth. |
| fulvum | 16 | Main stem killed. New growth 6" above base following spring. |
| galactinum | 24 | None. |
| griersonianum | 30 | Plant killed |
| haematodes | 24 | Top growth killed to 15" above ground. |
| impeditum | 8 | None. |
| johnstoneanum | 10 | Top growth killed to 6" above ground. |
| maddenii | 10 | Main stem killed. New growth from base. |
| mallotum | 24 | Plant killed. |
| maximum | 36 | None. |
| minus | 36 | None. |
| moupinense | 6 | Main growth killed. New growth from base following spring. |
| neriiflorum | 12 | Same as moupinense. |
| ponticum | 18-48 | Slight damage to terminal growth. |
| protistum | 30 | Plant killed. |
| pubescens | 16 | Terminal growths damaged. |
| sinogrande | 42 | Plant killed. |
|
sinogrande
small seedlings |
7 | None. Under snow. |
| sperabile | 8 | Main growth killed. New growth following spring. |
| sperabiloides | 8 | Plant killed. |
| sutchuenense var. geraldii | 24 | None. |
| trichostomum v ar. ledoides | 6 | None. |
|
|
This photo of
R. discolor
taken when the
temperature was two degrees above zero shows the drooping, tightly rolled foliage. R. discolor was unharmed in the winter of 1950 when the following night saw -15° F. |
In studying frost damage data, the conclusion arrived at is that the Genus Rhododendron could well be divided in three groups. The first group would include the hardy and the rock bottom hardy varieties and species of those that are hardy from 11° F. above zero down to 25° F. below zero. The second group would be 12° F. to 28° F. The third group includes those that are tender below 29° F. By far the greatest number of rhododendrons fall in the second group. Because of the variables involved in frost damage appraisal, such as hardening off for winter, time of planting, snow protection, duration of freeze, extent of drying winds, protection from sun during frozen condition, air drainage, etc, it is difficult to pin point too closely the temperatures at which a plant will be damaged. In the Pacific Northwest, it is fairly safe to say that plants which are hardy to 12° F. are satisfactory except in a few localities, or during very unusual winters.