Frost hardiness of some New Zealand shrubs

Abstract: Frost resistances, LT50, were decided for stems and leaves of several New Zealand woody plants belonging to Olearia (Asteraceae), Coprosma (Rubiaceae) and Podocarpus (Podocarpaceae). That was part of the aims, which included trying to find species potentially winter hardy in southern coastal Sweden and study methods for frost hardiness assessment. Artificial freeze tests were conducted in Sweden in late November 2006 and mid January 2007. Plants had hardened in cool temperatures (ca 5-10C) and natural day lengths. Frost resistances were as expected found to be rather poor for New Zealand material (often between –8 and –13C) compared to European reference material like Taxus baccata and Buxus sempervirens 'Rotundifolia' (often resistant to –20C). The resistance measured in twigs of Olearia odorata, which topped all the New Zealand species with about –20C was less expected, but reasonable considering it has been growing in the rock garden of Gothenburg botanic garden for three decades. Twig tissues of subalpine Podocarpus tested showed resistances of between –16 and –20C in January, ranking them after the former. They are thought to be winter hardy in southern coastal Sweden, in the meaning they will survive there. Field observations are however needed to evaluate winter hardiness. Other findings including leaves were often of similar or poorer frost resistance compared to stems; in many species there were an increase of frost resistance from late November to mid January (Northern Hemisphere); and there was some correlation between the altitudinal distribution of the species and their frost resistances. In the method used, excised shoots were artificially frozen to a series of target temperatures, and evaluated for injury both visually and using the electrolyte leakage method. The dose response curves from electrolyte leakage data revealed that some species went from no injury to 100% injury over much larger ranges of frost temperatures than other, especially having implications for the accurate quantification of frost resistance of species having injury responses stretched out. Concluded from comparisons of frost resistances found in this study, compared with those found in similar studies, results in this study are believed to be valid and the methods used can be recommended with some modifications. A short time to reach results and good comparability between them are some of the major benefits using artificial freeze testing. Much remains to be discovered regarding frost resistance of New Zealand plants and the mechanisms behind it.