Seasonal frost tolerance of some ornamental, indigenous New Zealand plant species in the generaAstelia, Dicksonia, Leptospermum, Metrosideros, Phormium, Pittosporum, andSophora

Abstract

The frost hardiness temperature (i.e., the temperature that causes damage) and the lethal temperature (i.e., the temperature that causes death) were assessed in autumn (April-May), winter (July), spring (October), and summer (January) for 10 native plant species. The species in order of increasing winter hardiness/lethal temperature were Metrosideros kermadecensis and M. carminea ( -3/ -5°C), Sophora tetraptera and S. microphylla (-4/ -6°C), Leptospermum scoparium (-5/ -8°C), Sophora prostrata ( -6/ -11°C), Dicksonia fibrosa (-8/ -11°C), Phormium spp. ( -6/ -11°C), Astelia chathamica ( -8/ -11°C), and Pittosporum tenuifolium 'Irene Paterson' (-9/ -14°C). In summer, all species had frost hardiness temperatures between -2 and -5°C and lethal temperatures between -4 and -9°C, except the two Metrosideros species where these temperatures were -1 and -2°C, respectively. No differences among cultivars of Leptospermum scoparium ('Martinii', 'Burgundy Queen', and 'Nanum Huia') or Phormium spp. ('Maori Sunrise', 'Yellow Wave', and 'Sundowner') were detected. However, in Pittosporum tenuifolium the cultivar 'Deborah' was found to be least frost tolerant in winter and 'Irene Paterson' the most, with 'Pixie' intermediate; their respective winter hardiness/lethal temperatures were -5/-5°C, -9/-14°C, and -8/-8°C.Eachof these cultivars had the same summer hardiness ( -3 0c) and lethal (-5 0c) temperatures. Also, Sophora prostrata 'Little Baby' was more frost tolerant during mid winter than cultivars of either S. tetraptera or S. microphylla but all of these species had the same summer hardiness ( -2 0c) and lethal (-4°C) temperatures.

Are frost hardiness ratings useful predictors of frost damage in the field? A test using damage records from the severe frost in South Otago and Southland, New Zealand, July 1996

“…In cases where minimum temperatures for field survival were given, these were translated into USDA hardiness zones ratings by translating the limits for zones into °C (viz Zone 10, +4.4 to -1.1°C; Zone 9, -1.2 to -6.6°C; Zone 8, -6.7 to -12.2°C; Zone 7, -12.3 to -17.7°C, Zone 6, <-17.7°C). Similar equivalences have been used by Warrington & Stanley (1987) and Warrington & Southward (1995). For the purposes of analysis, the few species with USDA hardiness ratings lower than Zone 6 (<-23.3°C) were usually grouped with Zone 6.…”

Seasonal frost tolerance ofHebespecies and cultivars

“…Overall, therefore, within the range of species and cultivars studied, the frost tolerance limits are broadly similar to those for other New Zealand native species which have failed, under the comparatively mild oceanic winters experienced, to evolve the much greater hardiness limits of those taxa indigenous to the winter-cold, continental climates of North America and Eurasia (Sakai et al 1981;Warrington & Stanley 1987;Stanley & Warrington 1988;Greeret al 1989Greeret al , 1991.…”

“…Each of the species, cultivars, and hybrids studied showed the seasonal patterns of frost hardening and dehardening typical of most temperate plants (Weiser 1970;Larcher & Bauer 1981;Greer & Warrington 1982;Warrington & Stanley 1987;Greer et al 1991); the ability to tolerate frost was lowest during summer, it increased over a hardening phase to a maximum frost hardiness in winter, and progressively decreased over a dehardening phase in spring (Fig. 1 ).…”

“…At each evaluation, and particularly in winter (July), there were substantial differences in both the hardiness temperature and the lethal temperature among the different species and cultivars included in the study. The amount of frost tolerated in summer, when assessed using the hardiness temperature (Table 2), (Warrington & Stanley 1987;Greer et al 1989Greer et al , 1991 which have evolved to grow in climates where frosts, although more prevalent in winter, can occur throughout the year.…”

Biological flora of New Zealand 10.Phormium tenax, harakeke, New Zealand flax

“…although frost tolerance was investigated in some Phormium cultivars by Warrington & Stanley (1987), these were ornamental varieties of probable hybrid origin. Sparrow (1965) suggested that South Island plants have more rigid leaves and stronger fibres as a result of cold winters.…”