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.