For many years horticulturalists have found the United States Department of Agriculture's Plant Hardiness Zones useful in assessing a plant's suitability for our climate.
In its simplest terms the USDA system has provided a classification of average minimum temperatures and horticulturalists from around the world have assigned plants to these zones. The classification relates to a plant's ability to handle cold temperatures. Thus, a plant that is able to tolerate a light frost is assigned to zone 9, where average minimum temperatures range from -1°C to -5°C.
Plants adopt a range of mechanisms to help them handle cold temperatures. For example many perennials can handle very cold winter conditions by becoming dormant and letting their top growth die off and "retreating" to the warmer soil conditions. They wait for spring warmth and shoot away again. Others just lose their leaves, the woody parts of the shrub or tree being better able to withstand cold conditions than the leaves.
We use the phrase "simplest terms" above cautiously - there are many factors that will influence a plant's ability to handle cold conditions - age, shelter, aspect, terrain, soil types, waterlogging among others. Thus, Hardiness Zones provide a guide only, which must be applied with local experience and knowledge.
In 2002 Liddle Wonder contracted NIWA to analyze weather station data to develop New Zealand Zones. They modified zone boundaries slightly to ensure that there is a difference between the North Island's central plateau and Wanganui for example. In the USA, zone 9 goes down to -7°C. If this was applied in NZ, Turangi would be Zone 9, the same as Wanganui and Tauranga!
Many international horticultural reference books contain USDA Zone assignments and these can be used with this map to provide a guide for planing in New Zealand.
However, as with all plant, growing and gardening circumstances, we cannot over-emphasize the importance of taking local conditions into account. Many areas have sites with very specific microclimates, north or south facing aspects, freely drained or cloggy soils. These and many other factors - cold together with wet soil for example - affect a plant's ability to handle harsher or more generous conditions. Above all we encourage gardeners to experiment. This way you'll soon learn the practical boundaries that your garden and site offer, and quickly establish a feel for whether you can garden up a zone, perhaps "half" a zone, and enjoy an even more diverse range of garden plants and activities.
Hardiness Zones are based upon the mean annual minimum air temperature for each zone. Air temperature data from widely spaced sites were used in research based spatial models to produce the original for this map at 1:4,250,000. Data mapped at this scale will not show variation that occurs at larger scales. NIWA used the best available information in preparing this map, exercising all reasonable skill and care. Nevertheless, NGIA and NIWA accept no liability, whether direct, indirect or consequential, arising out of the provision of the information presented on this map. In addition NGIA accepts no liability, whether direct, indirect or consequential, arising out of the provision of the information presented on this map or in its compilations of zones and plant assignments whether given verbally or published on labels, catalogues, tables, this web site or by any other means.