Seasonal frost tolerance of some ornamental, indigenous New Zealand plant species in the genera Astelia, Dicksonia, Leptospermum, Metrosideros, Phormium, Pittosporum, and Sophora

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. Each of these cultivars had the same summer hardiness (-3°C) and lethal (-5°C) 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°C) and lethal (-4°C) temperatures.

Keywords:

• manuka
• flax
• kowhai
• cold acclimation
• frost hardiness temperature
• lethal temperature
• freezing resistance
• seasonal frost tolerance
• Astelia chathamica
• Dicksonia fibrosa
• Leptospermum scoparium
• Metrosideros carminea
• Metrosideros kermadecensis
• Phormium tenax
• Phormium cookianum
• Pittosporum tenuifolium
• Sophora microphylla
• Sophora prostrata
• Sophora tetraptera