Abstract
Frost hardiness of winter twigs from about 180 tree species of flowering and ornamental trees native to different climates were determined. Dormant one-year- old twigs collected from mature trees wintering under different climates during mid-winter were artificially hardened at sub-freezing temperatures for about 20 days to overcome the differences of the sites of collection and to develop maximum frost hardiness. Almost all of the trees which ranged from tropics to sub-tropics in both Asia and America sustained freezing injury to some of their tissues below -5°C. Evergreen broad-leaved trees native to warm climates survived freezing from -7 to -17°C. Most of the deciduous trees and conifers distributed in temperate climates were observed to be much hardier than evergreen broad-leaved trees, which survived freezing to -20 to -40°C. These hardy trees all can be grown in Sapporo where the annual average minimum temperature is about -20°C. A marked variation in hardiness between flower and vegetable buds was observed in many flowering trees. Also, a considerable variation in hardiness was noted among Japanese camellia from different provenances.
Winter minimums seem to be among the important factors governing the northern limits of artificial plantings in most of the flowering and ornamental trees.
