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USDA (United States Department of Agriculture) has divided North America into 11 plant hardiness zones on the basis of there average minimum temperature. These zones are also known as climate zones or growing zones and help you to find out which plants are suitable of which climate. This helps you to grow plants which will not die soon just because they can't manage your region's temperatures.
Among 11 zones defined by USDA for United States, the US Zones 2 to 10 are also subdivided into a or b, giving a total of 20 zones or sub-zones, as shown in Figure 1-21. These zones only apply to plants growing out of doors with no protection, but which are provided with adequate water.
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Figure 1-21
For other regions in the world you need know your annual minimum temperature over a number of years (say 10years) so you can match to USDA Chart and share our plant data across the world. The statistic used by the USDA is the average annual minimum temperature. This causes some confusion straight away, because in Australia the term average annual minimum temperature is used to mean the average minimum temperature over the whole year, whereas the USDA means the average, over ten or more years, of the very lowest temperature (the absolute minimum) observed for each year for each meteorological station. Thus the USDA term - average annual minimum temperature should preferably be called ‘the average annual lowest temperature’.
The figure given above tends to make places look very cold! For example, Florida, which we think of as a warm place, is in the US Zone 10 which has minimum temperatures from 30°F to 40°F (-1°C to +4°C). Zone 1 (e.g. central Alaska ) is below -50°F (-45°C) which is very cold!
For Australia: The same statistics can be used for Australia My Garden Pal desktop software shows 2 more zones (12 and 13) for Australia to cater to the warmer climate zones of Australia. This is because Australia, in winter, is much warmer than most of North America in winter, so the lowest US zones aren't needed. All of Australia (excluding Macquarie Island) is covered by just over four US zones (7b to 11)
Zones are very important part of sharing your plant data across the world. Therefore you must use the map given below as Figure 1-22 to choose your zone carefully. This ensures that someone in Miami Florida USA can share the same plant data as someone in Brisbane Australia or Taipei Taiwan.
Figure 1-22
The descriptions of 7 Australian zones are:
Zone7 covers the alpine areas of south eastern Australia.
Zone 8 the tablelands of south east Queensland, New South Wales and Victoria, and the uplands of central Tasmania.
Zone 9 includes much of the southern half of the continent, except for localities on or near the coast. Many of our weather stations are on the coast or on off-shore islands (some of them are lighthouses) and these are often a zone or two higher than adjacent mainland stations because of the warming effects of the ocean in winter.
Zone 10, because of this warming effect, covers a broad area from coastal Queensland across the continent to Shark Bay and Geraldton in the west, also includes the Mornington Peninsula, areas adjacent to Spencer Gulf and Adelaide, the south western coastal zone, Sydney and the north coast of NSW, along with a number of localities dotted all around the southern coast of the continent.
Zone 11 covers, some of the Queensland coast, Western Australia north of Shark Bay and across the top end.
Zone 12 includes the Queensland coast north of Cairns, Cape York Peninsula and the coast of the Northern Territory.
Zone 13 is mainly restricted to islands off the north coast.
For more information on USDA zones visit URL: http://www.usna.usda.gov/Hardzone/ushzmap.html
The Australian Map and information has been adapted from an article written by Iain Dawson Updated 19 June, 2007 Australian National botanic Gardens, Department of the environment and water resources viewed 7th October 2007 http://www.anbg.gov.au/hort.research/zones.html