Tolerance of Flower Buds of Several Rhododendron Taxa to a Severe Fall Freeze
University of Minnesota Landscape Arboretum
The University of Minnesota Rhododendron breeding and cold hardiness research is supported in part by grants from the American Rhododendron Society Research Foundation, the Horticultural Research Institute, and the Minnesota Nurserymen's Research Corporation.
Inability of Rhododendron flower buds to survive the winter can be due to one of three primary causes; inadequate acclimation prior to early severe low temperatures, midwinter temperatures below the plant's capacity to harden, or warm weather induced deacclimation followed by low temperatures. Although there is some information indicating the maximum cold hardiness capability of flower buds of some Rhododendron taxa, (Pellett and Holt 1981, Pellett et al 1986, Sakai 1983, Sakai et al 1986), there is little information available concerning relative rates of acclimation in the fall or ability to resist deacclimation during warm periods in the winter.
This past autumn severe low temperatures occurred in Minnesota when minimum daily temperatures ranged from +3° to -2° F. from November 10 to November 13, eclipsing the previous all time low for that period by 8° F. The low temperatures at that period provided an opportunity to compare relative flower bud hardiness early in the winter season for a number of Rhododendron taxa that are in our evaluation plantings. Many of the taxa evaluated were planted this past season and thus we had no previous experience with their hardiness in our climate.
The information gained gives us a better picture of the importance of time of initiation and rate of acclimation on winter injury of flower buds. It is important to remember that the actual temperature of plant tissues may not be the same as the reported air temperature. However, the temperatures encountered remained low for a considerable period and thus the plant tissue temperatures were probably close to that of the air.
Table 1. Percent of flower buds of Rhododendron taxa that survived -3° F. on November 11, 1986, University of Minnesota Landscape Arboretum. Percent of Flower Buds Surviving. Variety 0 - 15 16 - 50 51 - 85 86 - 100 'Arctic Glow' X 'Balta' X 'Blue Peter' X carolinianum album X catawbiense X 'Caractacus' X 'Charles Bagley' X 'Cheer' X 'Chionoides' X 'Colts Neck Rose' X dauricum X 'Francesca' X 'Gibrosea' X 'High Sierras' X 'Holiday' X 'Hotspur Orange' X IIam 'Red Letter' X IIam 'White Cap' X 'Jane Abbott' X 'Joseph Hill' X 'Klondyke' X 'Laetevirens' X 'Lilokai' X luteum X 'Madame Carvalho' X 'Maori' X 'Marion Merriman' X 'Maximum Roseum' X 'Mrs. C. S. Sargeant' X 'Olga' X 'Orient' X 'Oxydol' X 'Purple Gem' X 'Ramapo' X 'Suez' X vaseyi 'Pinkerbelle' X viscosum X 'White Lights' X 'White Throat' X 'Windbeam' X
The varieties listed in Table 1 were in three different sites, but all three sites were within 1,000 feet of each other. In one site, plants of evergreen types were planted on the north side of a wooden fence with no other protection. In a second site, plants were growing in a lath house and shade may have moderated the temperatures slightly. Deciduous cultivars were growing in a third site in the open.
Unfortunately we did not have any record of actual flower bud or air temperatures in any of the sites. After the end of the record breaking cold period, flower buds were collected from each variety. The buds were sealed in poly bags to keep them from drying out and then stored at room temperature for several days to allow oxidation of tissues to occur in injured flowers. The buds were then dissected and the number of live and dead flowers in each bud was recorded.
Table 1 indicates the range of flower survival for each variety. Although flowers of many taxa were killed by the early severe freeze, there was little injury to most of the hardier cultivars. Thus it appears that the time of initiation and rate of cold acclimation is not often the most limiting factor in the ability of flower buds of most Rhododendron to survive winter conditions.
Pellett, Norman and Mary Holt. 1981. Comparison of Flower Bud Cold Hardiness of Several Cultivars of Rhododendron spp. HortScience 16:675-676.
Pellett, Harold, Susan Moe and Wayne Mezitt. 1986. Flower Bud Hardiness of Rhododendron Taxa. Amer. Rhod. Soc. Journal 40:203-205.
Sakai, A. 1983. Freezing Resistance of the Genus Rhododendron. J. Jap. Soc. Hort. Sci. 52(3):294ff.
Sakai, A., L. Fuchigami and C.J. Weiser. 1986. Cold Hardiness in the Genus Rhododendron. J. Amer. Soc. Hort. Sci. 111(2):273-280.