JARS v38n2 - Polyethylene Houses Offer Protection for Potted Rhododendrons

Polyethylene Houses Offer Protection for Potted Rhododendrons
R. L. Ticknor
Oregon State University, Aurora, Oregon

The normally mild-winter-Portland-area received a blast of Arctic air December 20-28, 1983, effects of which will be seen in the months to come. By January 6, 1984, flower buds of cultivars classed as H-4 were brown when cut and some plants classed as H-3 have brown florets in the buds. A potentially more serious problem is root injury to plants grown in containers which were not protected. Roots are much more susceptible to cold injury than the leaves and stems. An example is Japanese holly - Ilex crenata - the top of which will take -20°F but the roots are killed at +20°F.
Dr. John Havis at the University of Massachusetts and Dr. Peter Steponkus at Cornell University have conducted freezing studies with many plants including azaleas and rhododendrons. Bruce Briggs of Briggs Nursery, Olympia, Washington, reported his experiences with root injury at the 1980 Far West Seminar. The consensus is that root killing temperatures for most azaleas and rhododendrons are between 10 and 18°F. Air temperatures remained in this range for many hours permitting the soil temperatures in unprotected containers to drop into this critical range.
At the North Willamette Experiment Station, there is an official weather recording station. Measurements taken at 8 a.m. during the December cold period are shown in Table 1. Lowest temperature recorded was 6°F on 12/24 at the official 5 foot height above the ground. At 6 inches above ground the temperature was 5°F. An anemometer records the miles the wind has blown (wind run) at 2 feet above ground. There were very high readings on December 24 and 25 when 211 and 185 miles were recorded. Sustained periods of 20-30 m.p.h. with a peak gust of 52 m.p.h. on the 24th and 56 m.p.h. on the reading taken at the end of the weekend were recorded by another anemometer mounted on top of the office. From 12/21 to 12/23 the wind run was also strong with close to 100 miles per day.

These high winds which were low in relative humidity were responsible for some of the tan to brown leaves that are showing on broad-leaved evergreens. The strong winds would also be responsible for root injury to bunched containers by carrying away the heat normally trapped by the close spaced foliage.
Recorders were placed in the root zone of containers on December 21 after the temperature dropped into ranges associated with root injury. The recorders were programmed to monitor the number of hours the temperature was below 45, 30, 25, 20, and 15°F. They were placed in an open container yard and in clear and white polyethylene covered 14' x 100' houses. Table 2 shows that no hours below 20°F were recorded in the unheated poly-houses and that the number of hours between 25-20°F was much reduced. Apparently the white covered houses stayed slightly warmer as fewer hours below 25° and 30°F were recorded. Plants in the open were subjected to 23 hours below 15°F which could cause root injury to many container grown azaleas and rhododendrons.
It was good the National Convention was in Portland in 1983 instead of 1984 since there may be less color for visitors to see this year.