Logo for the Journal American Rhododendron Society

Journal American Rhododendron Society

Current Editor:
Dr. Glen Jamieson ars.editor@gmail.com


Volume 27, Number 2
April 1973

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals

Freeze Disaster Strikes Portland Area
Ted Van Veen, Portland, Ore.

        Sweeping down without warning, the storm of the century plummeted temperatures in and around Portland to a record low for early December. Somewhat passively we thoroughly enjoyed a warm November with rainfall far below normal, and gave little thought to the realization that a few light, early-season frosts had lost their effectiveness for dormancy. In some areas these conditions are great cause for concern but we Oregonians are not prepared to accept the 4th of December as the beginning of winter.
        On an average, Portland has only one day in December with a maximum temperature of 32° F or lower. This particular December of 1972, readings that did not rise above freezing for ten consecutive days were the cause of extensive damage to our rhododendrons and other plant material. These temperatures, and the official recorded low of 8° F for Portland are not too impressive in other parts of our country. But under unusual circumstances, in an area unaccustomed to such a severe test, they were killers. A survey by the Oregon Association of Nurserymen indicates probable losses of $10 million to ornamental stock in the nurseries around Portland and the Willamette Valley.
        While conditions along the Columbia River at Camas and Vancouver, Wash., were quite similar to Portland, the weather was much more severe south and west of the areas. Incongruous as it may seem, the easterly winds so often deplored by Portlanders, along with the light snow cover, saved many rhododendrons in the city. This might call for some explanation.
        We all recognize snow cover as an insulator, but this insulation is somewhat dependent upon its depth. As an insulator, the snow prevents conduction of heat from the earth below to the low level air immediately above the snow surface. The recorded two inches of snow in the Portland area was not sufficient to prevent soil heat from escaping and warming the low growing plant material. The winds stirred up the air and prevented the cold from settling.
        Now snow also is a radiator in that at night it quickly loses heat from its own mass by long wave radiation to space. As the snow surface grows colder at night, heat is taken from the layer of air touching it and is radiated into space. This results in a rapid lowering of air temperatures. The radiation process is aided extensively by cloudless skies and lack of wind.
        South and west from Portland the winds diminished and the snow cover at depths of four to seven inches became much more of an insulator, as well as a more efficient radiator. The combination of light winds, greater snow cover and clear skies resulted in much more severe damage in these areas. Meteorologists tell us official temperatures measured at four feet above the ground could be 10 to 15 degrees warmer than at ground level under radiated conditions just described. This extreme cold accounts for the browned or blackened bark on small trees immediately above the snow line. And, this too is the reason for much of the bark-splitting on our rhododendrons.
        Typical record low temperatures reported outside of Portland were Aurora 6°, Newberg 0°, Forest Grove -4°, Monitor -10°, Salem -12°, and Silverton -17° F. Judging by the extent of damaged and killed rhododendrons, it is apparent that temperatures must have been much colder than the official recordings. Most H-3 and H-4 hybrids, and even many H-2 rhododendrons were killed in the nursery fields at Hillsboro, Monitor and Silverton.
        Cecil Smith's beautiful garden near Newberg in a woodsy environment clearly shows the advantage of tree cover. This is exemplified by some plants showing no damage, i.e., R. hanceanum var. nanum, R. proteoides, R. strigillosum, R. wardii, R. xanthocodon, 'Bric-a-brac', 'China', 'Gold Strike,' 'Nestucca', and 'Odee Wright'. On the other hand, these are frozen to the ground: R. diphrocalyx, R. mallotum, R. sperabile var. weihsiense, R. spiciferum, 'Barclayi', 'Cornubia' and 'Noyo Chief'. A few other observations - 'Crest', a few buds frozen; 'Loderi King George', leaf burn and bud damage; 'Yellow Hammer', leaf burn and some bloom: and R. falconeri, one with leaf burn and one frozen back.
        The elevation at the Portland airport weather station is 30 feet. The hills around Portland rise to approximately 1000 feet. A cross section survey of rhododendron damage in the Portland area under these widely variable conditions proved too inconsistent to be of much value. Some of the varieties reported as killed or badly damaged were R. arboreum, R. cinnabarinum, R. davidsonianum, R. leucaspis, R. macabeanum, R. mollyanum, R. neriiflorum, R. tephropeplum, R. yunnanense, 'Berryrose', 'Cotton Candy,' 'Grosclaude,' 'Lady Roseberry,' 'May Day', and 'Royal Flush'. Portland's Crystal Springs Rhododendron Garden seems to be in surprisingly good condition. There is a considerable amount of foliage burn and blasted buds, but not too many are frozen back or killed.
        At our nursery in Portland there are three different blocks of 'Noyo Chief' in the same lath house. One group is in almost perfect condition, one is partially defoliated, and the third block appears to be completely killed. The group in good condition is one year older than the others, the middle block is two years old and transplanted in September, and the decimated 'Noyo Chiefs' are also two years old but transplanted in June.
        There are many variables to cold hardiness. A study of these will indicate why there is such a wide divergence in reporting successes and failures with rhododendrons under cold conditions, and why we should not be too hasty in judging the hardiness of our rhododendrons.

1. Age of a rhododendron is an important consideration. Comparing hardiness of a two-year plant with a ten year-old is not valid. Rhododendrons generally reach full maturity for hardiness when they are five years old.
2. General health of plants must be compared.
     a) Best nutrient level for winter. 
     b) Control of diseases such as root rots, die-back and leaf fungus. 
     c) Absence of pests such as root weevil, moles, borers, nematodes. 
     d) Thriftiness of growth.
3. Plant exposure is one of the greatest variables in the comparisons of hardiness.
     a) Freedom from desiccating winds. 
     b) Protection from quick-thawing eastern exposures brought on by the early morning sun.
     c) Amount of overhead cover as shade house, trees, house eaves and cloud cover.
     d) Good air drainage and freedom from frost pockets.
     e) Artificial winter barriers. 
     f) Effectiveness of mulching. 
4. Transplanting and soil conditions have a definite effect on hardiness. 
     a) A transplanted rhododendron cannot be judged fully hardy until established in one location for a year.
     b) Soil of good texture, proper plant depth and good drainage will help a plant through the winter.
     c) Plants grown in containers are more subject to winter damage. 
     d) Amount of moisture around the root ball at the onset of a long freeze.
5. The final variable in hardiness comparisons is the degree of dormancy. While latitude and elevation may have some effect on dormancy, the key factor is temperature. Light frosts will temporarily slow down rhododendron growth, but with warmer days a plant is "de-hardened" and will continue to grow depending upon the amount of food stored in its system, and the water available.

        All our apprehensions of those ten chilling days last December are now somewhat forgotten, as at this writing (last week in February), the R. dauricum has been blooming for some time, R. mucronulatum seems to be everywhere, 'Lee's Scarlet' is in full flower, and the R. sutchuenense is in beautiful color at the Portland Crystal Springs Rhododendron Garden.


Volume 27, Number 2
April 1973

DLA Ejournal Home | QBARS Home | Table of Contents for this issue | Search JARS and other ejournals