Answer: You're not alone in your confusion and pinpointing your gardening zone. According to the USDA, you're in Zone 9 with minimum temperatures of 20 to 30F. But that's a pretty general statement which is qualified by the following: "One of the smallest regions in the plant hardiness zone map, Zone 9 includes central Florida, the Gulf coast of Louisiana and Texas, much of California, and the southern Oregon coast. This mild-winter region is characterized by a long growing season and almost no winter freeze. Summers are either hot and dry or hot and humid. Although it might seem that just about any plant could do well in such a mild area, Zone 9 (and warmer) is not good for plants that require a winter chill. Those that flourish include Asparagus setaceus (Asparagus fern), Eucalyptus globulus (Tasmanian blue gum), Syzygium paniculatum (Australian bush cherry), Fuchsia hybrids (Fuchsia), and Schinus molle (California pepper tree). Microclimates develop within cities, neighborhoods, and even your own yard due to elevation (including low spots), influence of bodies of water, and protection from tall trees or buildings."
If you've gardened for any length of time in Phoenix, you know that plants which grow well in Florida, Texas and Oregon do not grow well here because of the dry air and extreme summer temperatures.
One of the best sources I have found for determining at least a reasonable assessment of gardening zones is through Sunset Western Garden Book. The publishers have taken into account the major elements (elevation and proximity to water) to further divide the USDA zones into mini-zones. The book also lists plants that are likely to thrive in all gardening zones. Your library should have a copy of the book (ISBN# 0-376-03851-9) and your garden center will have copies, too. In the Sunset Western Garden Book, Phoenix is listed as zone 13. While there are many plants that do well in zone 13, there are many, many more that will not. Another source of information is the ASU cooperative extension website. You'll find tons of publications on gardening on the site. Here's one such publication: http://cals.arizona.edu/pubs/garden/az1100.pdf
You can find more by visting the homepage: http://cals.arizona.edu/pubs/
Once you get there, type the word garden in the search field and you should come up with a list of publications.
Hope this helps clarify things!
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