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- Author or Editor: Mary A. Holt x
Five evergreen rhododendron cultivars were compared for flower bud cold hardiness in laboratory freezing studies on 6 dates. ‘Roseum Elegans’, ‘Catawbiense Boursalt’ and ‘Boule de Neige’ were more cold hardy on most sampling dates than ‘America’ and ‘Lee’s Dark Purple’. The relative cold hardiness of these 5 cultivars was consistent on several winter dates over several years. Injured florets were black and readily separated from uninjured white florets upon dissection of the inflorescence bud after freezing.
Leaves and stems from 7 field-grown cultivars of Rhododendron were frozen in laboratory tests from September to December or January for 2 successive winters to determine the temperature causing injury to selected organs. In all cultivars, the leaf midrib, petiole, and stem extraxylary (cambium, phloem, and cortex) organs were the least cold-hardy, while the stem xylem and leaf intervein were the most cold-hardy. Organs of ‘Lee’s Dark Purple’ and ‘Cataw-biense Boursalt’ rhododendrons were the most cold-hardy by December while ‘Caractacus’, ‘America’, and ‘Nova Zembla’ were the least cold-hardy. Field temperatures similar to those causing injury in the laboratory resulted in leaf and stem damage in early December.