QBARS - v27n2 Rhododendron Species Foundation Freeze Damage

Rhododendron Species Foundation
A Preliminary Report on Freeze Damage

P. H. Brydon, Salem, Ore.

Beginning December 7th, 1972 and continuing for four successive nights, the official low temperatures at the Salem, Ore., Airport were -4°, -12°, -5°, -2° and -3° F. Our last severe cold spell was in 1950 when -10°F was recorded and this recent -12° F is a record for the Salem area. Fortunately for the Rhododendron Species Foundation collection, my garden is some 200 feet above the airport in altitude and slopes sharply to the East and North with excellent air drainage. This, combined with an eleven inch snowfall, somewhat moderated the intensity of the cold. Our lowest temperature in the garden was -5° F and, due to variations in topography, tree cover etc., our temperature range during the cold period varied from -5° to +10° F. It is still too soon to give an accurate account of the damage, since many of the species which appear to be dead have green wood below the snowline. Occasionally latent buds will break into growth after the plant has been cut back and, in some instances, adventitious buds will appear on older wood.
This report is primarily concerned with the larger plants out of doors. The smaller plants under lath were completely covered with snow and many otherwise tender species survived where their older counterparts succumbed in the open. Speaking of the larger leaved species, the probability for loss is greatest in the Maddenii, Grande, Falconeri, Arboreum, and Irroratum Series, in that order. The highest survival percentage seems to be in the Taliense, Fortunei, Ponticum and Thomsonii Series. The smaller leaved and dwarf species in the Lapponicum, Saluenense, Uniflorum, and Campylogynum Series were practically untouched due to the snow cover. How they would have fared without a snowfall is another question.

R. uniflorum
Fig.22. R. uniflorum is a slowly mounding shrub with
purple flowers.  It is close to R. pemakoense , but blooms
several weeks later.
Photo by Cecil Smith

All of us have a tendency to try border-line plants and push the limits of their cold endurance beyond their capacity to survive. At times we succeed for a few years and enjoy some of the more exotic species and then the "unusual" happens. Such species in the Maddenii Series as carneum , parryae , veitchianum , etc. are in the H-1 and H-2 category for hardiness and the inevitable happened; but, even if they get frozen once in awhile, many of the Maddenii group have the propensity to grow again from the base and regain their former size in a short time.
This spring will tell the tale. In this Series, ciliatum , fletcherianum , and valentinianum appear to have weathered the storm but will need some light pruning to remove frozen wood.
In the Falconeri Series, fictolacteum , galactinum , and hodgsonii are untouched but falconeri , basilicum , arizelum and eximium look quite dead. (The small plants which were under snow in the lath house are all right.) Although I know that the chance of these large-leaved species pushing from the base is slight, I have not given up hope, not while there are latent buds on the lower 12 inches of growth. In the Grande Series all of the species out of doors have lost their leaves and the upper branches are completely dead. However, as in the Falconeri Series, I shall hold on until spring before discarding them.
Species in the Arboreum Subseries are apparently more tender than those in the Argyrophyllum Subseries. In the former, arboreum , lanigerum , niveum , and zeylanicum will have to be cut back quite severely, whereas, argyrophyllum , floribundum , hunnewellianum , hypoglaucum , insigne , ririei and thayerianum are relatively unblemished.
Members of the Irroratum Subseries as hardingii , laxiflorum , irroratum , pankimense , ramsdenianum , and shepherdii show extensive leaf injury but, aberconwayi and anthosphaerum are not harmed too badly. R. venator is the only member of the Subseries Parishii which seems to have come through, although elliottii may break from the base.
R. camelliiflorum , an H-1-2, is dead which is expected but I was agreeably surprised to note that wilsonae , also an H-1-2, is completely untouched. This member of the Stamineum Series was sent to us from Wakehurst and flowered last year. It is a low spreading plant, ultimately reaching six feet, with bright shining green leaves and 2-inch lavender-pink flowers which are delightfully fragrant.
We expected R. campanulatum and its var. aeruginosum to survive and they did. Another of this Series, fulgens , also came through without a blemish but tsariense was burned above the snow line. The Carolinianum, Dauricum, and Ferrugineum Series were not affected, but all the species in the Cinnabarinum Series were damaged although the wood looks green and fresh and there is a strong possibility that they will break again in the spring.. In the Subseries Barbatum, both barbatum and smithii are completely defoliated, perhaps dead. Other species in the Barbatum Series such as bainbridgeanum , crinigerum , glischrum , habrotrichum , spilotum , vesiculiferum , pachytrichum , and strigillosum were badly leaf scorched. The "toughies" in this group are anwheiense , maculiferum , pseudochrysanthum , and morii . They lost neither leaves nor buds.
The Fortunei Series came through in fine shape - diaprepes and griffithianum were defoliated and possibly bark-split. All the others seem to be in good shape, particularly orbiculare , fargesii , calophytum and praevernum .
Those species in the Neriiflorum Series such as chamae-thomsonii and other low-growing types were quite safe from injury under the snow but the larger leaved and taller growing species as mallotum , beanianum , and catacosmum were partly defoliated. The only species showing damage in the Ponticum Series is our native macrophyllum and I am certain it will recover. None of the Taliense Series were affected. However, in this Series, the Foundation has but 15 species out of a possible total of some 50. While many of them are not in cultivation and some merely botanical curiosities, there are a number of excellent foliage plants and we are continuing our efforts to introduce them from various gardens both here and abroad.
There were few casualties in the Thomsonii Series. All forms of wardii did not seem to mind the cold. By contrast campylocarpum was damaged in flower buds and foliage. The upper leaves of stewartianum were quite black whereas stewartianum var. aiolosalpinx was unhurt. All other species in the Thomsonii Series seem to be in good shape with the exception of cyanocarpum and viscidifolium , both of which have lost their upper leaves.

R. augustinii
FIG.21. R. augustinii reaches six to eight feet and its
upright habit makes excellent background for lower
rhododendrons. Very floriferous and blooming in April,
clones vary from lavender pink through violet and even
approach a true blue in the best forms.
Photo by Cecil Smith

Surprisingly enough, the larger plants of augustinii and its many forms survived remarkably well. The only weak sister was augustinii 'Electra' which lost its leaves to the snow line but should recover rapidly. The various forms of davidsonianum did not fare so well and are completely leafless now but should come back from the base. With the exception of concinnum , keiskei , hanceanum nanum and lutescens which were unharmed, most of the species in the Triflorum Series lost their leaves to the snow line.
A more specific account of the December freeze damage is being prepared in tabulated form for the next issue of the Bulletin and by that time we should be more fully informed on such details as flower bud survival, bark split and regeneration from the older wood.