Some Observations of Frost Damage In the Puget Sound Region
First I will give the extreme low temperatures in different localities and how the species and hybrid rhododendron were effected by the weather. A little south of Everett the low as recorded was 2 degrees above zero on two, separate occasions; with several weeks of eight to 25 degrees above, and never above freezing during the entire time.
South of Seattle at Glendale, the low was 14 degrees. At my garden 7 miles south of Tacoma, and about a mile from Puget Sound, the temperature went to a ten degree low. In a different location in Tacoma close to the Sound it was 14 degrees while a short distance in an open location the temperature was 6.
I heard several reports that at Spanaway, south of Tacoma the temperature went to a minus 5. I was not able to check on this low.
Now as damage to rhododendron there was practically none. At Everett, no hybrids were injured, (except for a little. leaf burn,) and not even flower buds were hurt. The species came through well and only the most tender were damaged. R. johnstoneanum came through with only some burned foliage.
At the Arboretum at Seattle nothing appeared to be injured except in the R. megacalyx subseries of R. maddenii. This was to be expected at ten above. At fourteen above the same plants in another locality survived well.
I had just moved all my plants from Puyallup, and so late that none of the plants had a chance to really get established. Some of the plants were injured and some lost. The only plants killed outright were R. scottianum, R. megacalyx, and R. odoriferum. There was a number of plants moved in the preceding year and none were affected by the cold. Among them are R. griffithianum, R. diaprepes, grande, sinogrande, crassum polyandrum, and johnstoneanum, all are supposed to be tender. Some large plants of R. decorum and R. thomsonii that dried out before they could be planted lost their buds and some foliage, but are now coming back.
None of the hybrids were injured in any way and all bloomed beautifully this spring.
In Puyallup, about 8 miles from the Sound the temperature went to a low of eight degrees, but not even the R. Foxes (Helen and Robert) were injured, both made a show during blooming time.
In Tacoma, there seemed to be very little damage. Some leaves burned or a few buds lost in the more exposed locations.
In conclusion it seems that any rhododendron that had been established at least a year, and was in good condition suffered little from the coldest winter in 65 years. I think then, in the right locations, we can grow practically anything including even the most tender plants if they are given the most sheltered spots.