Answer: The USDA zone numbers are based only on average annual minimum temperatures, i.e., how cold tolerant or "hardy" will any plant be. For example, Las Vegas in zone 8 has average annual minimum temperatures of 10 to 20F. However, many more factors influence a plant's ability to thrive, including soil type, elevation, rainfall, humidity, etc. For example, zone 6 occurs coast to coast, in such varied states as Washington, Colorado, Tennesee, and New Jersey. Obviously the same plants are not all going to do well in all of these areas. Are you familiar with the Sunset Western Garden Book? Many nurseries have it on hand as reference for their employees. Sunset has its own zone system, which allows more detail than the USDA zone system, because of the West's many weather/elevation variations. Las Vegas falls in zone 11 in Sunset's system. Perhaps that is what the nursery staff was referring to. Monrovia uses the USDA system to rate its plants, as well as the AHS Heat Index. I think your area is a little too extreme with sun, heat and aridity for this tree, but I would follow the advice of your local nursery. They should have experience with how a plant might perform in specific microclimates in the area, for example. Good luck!
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