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Cold Hardiness in the Genus Rhododendron


Cold hardiness of 101 Rhododendron genotypes was assessed. In hardy rhododendrons, the difference in hardiness between floral primordia and other tissues or organs was greater than in the less hardy species. All rhododendrons tested were divided into 4 hardiness divisions: very hardy, semihardy, hardy, and tender. In all hardiness divisions, the vegetative buds, leaves, and stem cortex were hardiest, whereas the floral primorida were least hardy, with a lowest survival temperature (LST) of −35°C or higher. The majority of the very hardy division of rhododendrons, such as R. brachycarpum D., R. catawbiense Michx., and R. maximum L., belong to the ponticum series. In general, rhododendrons from cold regions were hardier than those from warm regions. The hardiness of the floral primordia of rhododendrons growing near the timberline (3900 m alt.) of East Himalaya and the high altitudes of the northwestern Yunnan area were similar (−20° to −23°). These were less hardy than those found in the subalpine forests of Japan and eastern North America, probably due to moderate temperature and high humidity during the winter in eastern Himalaya and northwestern Yunnan.