JARS v37n3 - Low Temperature Injury to Rhododendrons

Low Temperature Injury to Rhododendrons
In the Winter of 1981 - 1982
Myron Shear, Blacksburg, VA

Blacksburg, Virginia, at an elevation of 2,200 feet, is subject to unusually cold temperatures for the State. During the winter of 1981-82, a low temperature of -16°F occurred January 17 which caused extensive flower bud damage to rhododendrons growing on a ¾ acre planting. These were well-established plants, mostly 6 to 12 years old.

Since our normal winter temperature is about -5°F, an attempt is made when purchasing rhododendrons to obtain information on hardiness. It has been found that these ratings are not always reliable. The winter of 1981-82 afforded an opportunity to evaluate a large number of plants at an abnormally low temperature. These data should be of value to both nurserymen and collectors.

The rhododendrons included in this report were planted in a mixture of top soil (50%) plus one part of sphagnum, one part of a fine pine bark and one part of weblite (a locally available expanded shale). Some sulphur plus a small amount of fertilizer especially prepared for azaleas and rhododendrons was also added to the mixture. The plants were planted so that the surface was several inches above the surrounding soil to assure good drainage. An inch or more of pine bark and pine needles was maintained around the plants during the early years of their growth and they were fertilized in early spring and June. Watering was done during dry periods in the growing season.

The plants are growing in moderate shade to full exposure. Once the plants were established, these exposures did not appear to influence hardiness. Wind, while prevalent in the area, would not appear to be a factor as the plantings were protected on the west and north by spruce and pine trees. No accumulation of snow occurred during January 1982.

The weather records for Blacksburg were obtained from the National Climatic Center, Climatological Data for Virginia. The recording station is located five miles from these plantings and therefore the records are not necessarily comparable to the immediate growing site.

Precipitation for 1981 was as follows: January - .60 in., February - 2.78 in., March - 2.84 in., April - 2.86 in., May - 4.81 in., June - 2.31 in., July - 5.66 in., August - 2.91 in., September - 3.58 in., October - 4.35 in., November - .49 in., December - 2.76 in. Additional water was applied to the plants several times during the summer and also in November and December.

Official temperatures for January 1982 show lows of -10°F January 10, -12°F January 11, -10°F January 17 and -8°F January 18. The coldest temperature of the winter, -16°F, was recorded on January 17 on a Taylor indoor-outdoor thermometer at the site of the rhododendron plantings and is used in assessing cold damage.

Hardiness ratings from the listed publications are given in the accompanying table. In addition to flower bud injury, leaf and/or stem damage occurred on 13 varieties, mostly those where 100% flower bud kill was recorded. All sources have some ratings that appear to be too cold hardy. All catalogs had what appeared to be excessively high hardiness ratings on from 35% to 38% of the plants listed. None of the catalogs had any plants that appeared to be rated not hardy enough, while all of the books had some plants not rated hardy enough. These comparisons show a wide range of hardiness ratings for many hybrids. This indicates a need for updating ratings whenever possible to improve their reliability. The use of H1, H2, H3 ratings used in the catalogs is not as reliable as temperature ratings in increments of 5°F.

Editor's Note: The Rating Committee Chairman, Newton W. Edwards, would like to receive any similar reports from other members to help build up his data base for the revision of the ratings in the next ARS handbook.

++ Sources of Hardiness Ratings
1. Rhododendron Information, Edited by J. Harold Clarke The American Rhododendron Society, Sherwood, Oregon. 1967.

2. American Rhododendron Hybrids, Editor, Meldon Kraxberger The American Rhododendron Society, Tigard, Oregon. 1980.

3. Rhododendrons in America, Ted Van Veen, First Edition, Sweeney, Krist and Dimm Portland, Oregon. 1969. Ratings (H1, H2, H3 from Van Veen Catalog)

4. Greer's Guidebook to Available Rhododendrons, Species and Hybrids, Harold E. Greer, Offshoot Publications Eugene, Oregon. 1982.

5. Hybrids and Hybridizers Rhododendrons and Azaleas for Eastern North America, Edited by Philip A. Livingston and Franklin H. West, Harwood Books Newton Square, Pennsylvania. 1978.

6. Hillside Rhododendron Gardens, Elmer Morris, Manasquan, New Jersey. Catalog.

7. Lazy S's Farm, Pete and Terry Sheuchenko, Barboursville, Virginia. Catalog.

8. Holly Hills Inc., Dr. Henry R. Schroeder, Jr., Evansville, Indiana. Catalog.