The USDA zone, AHS zone, and Sunset zone.

It is probably the most frequently used in the world. The concept is fairly simple, many plants die in winter when the temperature gets too cold for them. The USDA zone is based on the average minimum temperature. The table 1 shows the temperature equivalent of the different zones.

This system has the advantage of being simple. Finding out your zone is generally fairly easy, even if you are in an area outside the United States. It is well adapted to the temperate and subtropical areas.

The main problem is that it doesn’t take in account the humidity and difference of temperature between day and night. For instance, an area in Arizona that would be in zone 9a might reach 20°F (-7°C) on a cold night, this cold temperature might last a couple of hours, and early afternoon the temperature might reach 50°F (10°C). By contrast an area in zone 9a in England might go down to 20°F (-7°C) but stay below freezing all day. Obviously, the plants resisting in Arizona, might get promptly destroyed in England.

Another problem is related to summer temperature. The summer temperature of this hypothetical Arizona situation will probably reach 100°F (38°C) at least a couple of weeks in summer, cooking plants that would do just fine in the English location.

In summary, USDA is a quick valuable reference, but should be correlated to other data.

To compensate for the shortcomings of the USDA cold hardiness zones, the American Horticultural Society introduced in 1997 another climate zone system called the AHS Heat zones.

The AHS compiled US Weather Bureau data for several years and calculated the average number of days per year in each location where the high temperature exceeded 86°F (30°C). The theory is that above that temperature, many plants will start suffering, and some plants are either better adapted than others to grow under high temperature conditions, or actually need these high temperatures. The zones go from 1 with no day above 86°F to zone 12 with 210 or more days.

It is better used as a supplement to the USDA zone. It is unfortunately less common, and it might be difficult to 1) locate in which zone you belong, and 2) finding reliable data on the plant you are looking for.

You can get more information on the AHS temperature zones at the AHS site and you can find a US map also at their site.

Generalized Western Plantclimate Zones (Sunset Zones)

Because of the shortcomings of the USDA Zones, many Western US gardeners and farmers use the so-called "Sunset Zones." The Sunset Zones (or more accurately, the Generalized Western Plantclimate Zones) are based on and adapted from the California Plant climates developed by the University of California Cooperative Extension. These are often called Sunset Zones after the Sunset New Western Garden Book (Lane Publishing Co). Both this book and its alter ego for the whole USA, the National Garden Book are very useful encyclopedias at extremely reasonable costs. They factor in length of growing season, timing and amount of rainfall, winter lows, summer highs, and humidity.

The zones from 1 to 24 are for the western USA (1 is the coldest), and the zones 25 to 45 are for the eastern part (45 is the coldest).  In Arizona, Phoenix is zone 13, and Tucson is zone 12.

In the plant information of Desert-Tropicals.com, if it says that a given plant will grow from zone 8-18, it probably means that I am missing the data for the east coast zone data.  It generally does not mean that the plant doesn't grow on the east coast.

You can find a description of the zones for the USA at the Sunset site.