QBARS - v19n2 Winter Damage at University of Washington Arboretum

Winter Damage at the University of Washington Arboretum
J. A. Witt, Seattle, Wash.

In assessing winter injury to plants it is always well to establish the conditions that caused the damage. The cold that we experienced in mid-December 1964, was very well defined. The maximum temperature on the 14th was 40° F, the minimum 32° F; the maximum on the 15th was 32° F, the minimum, 11° F. The next two days had maximums of 16° F and 25° F and minimums of 13° F and 17° F respectively. The temperature rose slowly after that, with maximums in the high thirties or low forties and minimums at slightly above or slightly below freezing. The Arboretum's thermometer is in a screened box and the temperatures recorded probably are comparable to those in our lath house. The intense cold was accompanied by a very desiccating north wind.
The damage to the Arboretum's Rhododendron collections was not as severe as it might have been, although certain species and clones suffered badly and a few were probably killed, at least to the ground. At this writing (3/25/65) it is still too early to determine which are actually dead and which will eventually make a recovery. We learned in 1955 not to be too hasty in removing a seemingly dead plant.
Below are three lists of (1) plants very badly damaged, dead or killed to the ground; (2) plants with moderate leaf or bud injury; (3) plants with minor leaf injury or none at all despite a reputation for tenderness.

1. Plants very badly damaged or killed to the ground or probably dead.
arboreum 'Album', defoliated and old wood damaged, protected site
arizelum , killed to ground in open site and in lath house
basilicum , severe kill in lath house
bauhiniiflorum , killed to ground in protected site
beesianum aff. (R. 42), killed to ground in open
bullatum , killed to ground in protected site
'Cowslip', leaves & buds badly burned in open
grande , defoliated, wood damaged in open and in lath house
heptamerum (K. W . 5415), leaves & buds blasted in open
hormophorum var. album , killed to ground
lacteum , defoliated in lath house
lanigerum , defoliated and probably dead in partial shelter
niveum , defoliated in open
'Royal Flush', killed to ground in open
sino-grande , defoliated in lath house
spiciferum , defoliated in protected site
suberosum (F . 26486), killed to ground in protected site
'Thalia', defoliated and severe bud damage in protected site
triflorum , killed to ground in open
yunnanense , perhaps killed to ground in open damage not quite as severe in protected site
2. Plants with moderate leaf or bud injury
barbatum , moderate leaf damage; flowering March 1965
'Cincrass', moderate burning on leaves in open
cinnabarinum , flower buds blasted
'Cowslip', leaves burned and flower buds blasted: open to west
desquamatum , leaves burned; flowering 3/20/65
floccigerum (F. 19769), moderate leaf burning; flowering March 1965
fulvum , moderate leaf damage in lath house
griersonianum , in semi-protected site
lutescens (seedlings & F.C.C.), moderate leaf burning; flowering March 1965
macabeanum , leaf damage in lath house
peregrinum , mild leaf burn in shelter
thomsonii , leaves scorched on plants in open site
ungernii , leaves burned in open site, undamaged when protected
venator , leaf damage in shelter
3. Plants with slight or no damage
barbatum (L. & S. 17525), in sheltered site
'Cilpinense', flowered February 1965
coryphaeum , in lath house
elegantulum aff., flowering March 1965
fictolacteum , in open
hemsleyanum , in protected site
pennivenium , flowering well in protected site
praestans (R. 59234), in lath house
rex , in lath house