QBARS - v9n1 Frost Damage to Rhododendrons: January and February 1950

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.

R. discolor at two degrees above zero temperature.

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.