Rhododendrons and our 1950 Winter
By Clarence Prentice
After the winter of 1949, which was very severe and unusual for this country, we thought we knew the tolerance of rhododendrons to certain degrees of temperature, and felt certain varieties would stand down to certain degrees.
I think about 10° F. above was the lowest in 1949. This defoliated R. 'Cornubia' and 'Gills Triumph' and a few others. The buds on R. maddenii and 'Fragrantissimum' were destroyed, but it did not interfere with the bloom on most varieties, and the damage was not very great, with very few actual losses. But the past January and February has bettered all records for at least fifty years, and the picture is a complete change.
These notes are from my observations mostly of nurseries in Washington and Oregon. The temperatures ranged from 3° above to 12° below. I did not visit out Hillsboro way. Bainbridge Island registered 8° above; Bothell, Washington and Salem, Oregon were 12° below.
Through all the various degrees of temperature at different places I have noticed a great difference of damage. I can hardly explain the exact degree of tolerance or to put my finger on a certain number of degrees that would kill certain plants. R. 'Penjerrick' at 3° above was injured just as much as 12° below. The plants were completely defoliated, but the wood seems to be alive. R. 'Cornubia' generally seem to have been lost. Nearly all flower buds on rhododendrons in nurseries in Oregon and Washington are completely gone, although at Gresham, where the wind blows seventy miles an hour down the Columbia gorge, I have found plants with good buds and the temperature was 8° below.
I have come to a very definite conclusion that one cannot estimate and depend on a definite degree that a plant will withstand cold. There are a great number of factors besides degrees of coldness to consider. Air drainage has a lot to do with resistance also, plant condition, and I also suspect the percentage of humidity of the air have a lot to do with it. We, here in certain areas of Washington, had a cold rain that completely covered leaves and buds with ice, and this may have had some effect.
I am going to further expound my theory on humidity. We all know that the easterners tell us that 30° below is not uncomfortable, but that this cold is a dry cold, and summarily not noticed as much. Also, during hot, humid days in summer, the heat is more penetrating, and we perspire much more. So, I feel sure the humidity of the air is more responsible for damage in different areas, than other factors. We speak of air drainage, and I suspect the fogs and more moist cold air settle in low areas and the wind does not circulate this air and plants in these areas are subjected to a more moist penetrating cold than those in higher areas, even though freezing is a drying out process. I suspect these areas may get a little warmer in the daytime to cause higher humidity in the air while at night the temperature drops and drives this moist cold through the plants to a greater degree, so in a humid area 3° above zero may do as much damage as 12° below in a drier cold.
The other thing we all observed after the thaw was the apparent absence of damage. Then day by day we keep seeing bad effects and at this writing I am convinced we will not know the complete damage until the middle of the summer.