Freezing Resistance of the Genus Rhododendron

Abstract

Freezing resistance of the genus Rhododendron of about 70 species or cultivars was assessed. In very hardy Rhododendrons, the difference in hardiness between florets and other tissues or organs became greater: The vegetative buds, leaves and stem cortex were hardiest, while the florets which survived freezing by extraorgan freezing were the least hardy. Even in the hardiest Rhododendrons, the flower buds did not survive freezing below -35°C, while the vegetative buds, leaves and stem cortex survived even -60°C. It is notable that very hardy Rhododendrons, except for the dwarf forms, such as R. brachycarpum, R. catawbience and R. maximum, all belong to the Ponticum Series of the genus Rhododendron. And most of very hardy cultivars belong to R. catawbience hybrids. The flower buds of Rhododendrons found near the timber line (3, 900m in altitude) of the East Himalayan mountains were marginally hardy at -20 to -23°C. Nearly the same results were obtained from the Yunnan′s Rhododendrons. These are less hardy than those found in the subalpine forests of Japan and eastern North America. Thus, in the Himalayas and Northeastern Yunnan, Rhododendrons seem not to have evolved species capable of resisting below -30°C, probably due to moderate temperatures and high humidity in winter in these areas.