The new 2012 USDA Plant Hardiness Zone Map is the standard by which gardeners and growers can determine which plants are most likely to thrive at a location. The map is based on the average annual minimum winter temperature, divided into 10-degree F zones.
The Phoenix valley has been assigned two zones, roughly the area east of 17 and south to the I-10 falls within the 10a zone which has an average minimum winter cold of from 30-35 degrees F, west of 17 the zone is a bit cooler with lows of from 25 to 30 and is designated as zone 9b.
No posters of the new USDA Plant Hardiness Zone Map have been printed. But state, regional, and national images of the map can be downloaded and printed in a variety of sizes and resolutions.
The Sunset magazine has assigned the section of Arizona surrounding the Phoenix valley a 13 designation, using a range of temperature wider than the USDA scale. We will refer to the Sunset magazine planting times and tips because we receive their information monthly while only annually do we hear from the USDA!
All of the Arizona zones are subject to extreme temperature variations for short periods, and any home gardener must pay attention to the weatherman and protect plants that are supposed to survive any given temperature either high or low.