Seasonal frost tolerance of some ornamental evergreen broad-leaved and coniferous tree and shrub species

Abstract

Abstract The seasonal frost hardiness of several ornamental species was evaluated by exposing intact plants to controlled advective frost conditions in spring, midsummer, autumn, and midwinter. Cultivars of the evergreen broad-leaved species Camellia japonica, Daphne odora, Desfontainea spinosa, Grevillea × gaudi-chaudi, Hardenbergia violacea, Hypericum × moserianum, Nandina domestica, and Photinia serrulata × glabra ‘Rubens’ all developed limited winter frost hardiness with the most hardy (Camellia and Photinia) being killed at temperatures below-17°C. Generally, evergreen broad-leaved species from northern zones were more tolerant than Australasian species. Of the ornamental conifers examined, Cupressus macrocarpa was only moderately hardy in winter, being injured by temperatures lower than-8°C and killed below-14°C, whereas Juniperus chinensis tolerated temperatures down to-19°C without injury. The summer frost tolerance of all of these species was similar and all were injured or killed by temperatures in the-1 to-5°C range. Yucca filamentosa, a species with a xerophytic morphology, was damaged at-13°C but survived-17°C in summer and was not injured by winter temperatures down to-17°C. Lethal temperature values obtained in the controlled environment evaluations generally agreed well with hardiness zone ratings.