Understanding the specific climate conditions of a location is the first step toward successful gardening or landscaping, especially when selecting plants that must survive the local winter. The United States Department of Agriculture (USDA) Plant Hardiness Zone (PHZ) map is the standard reference tool for this purpose. It provides a standardized framework, helping gardeners determine which plants are most likely to tolerate the coldest temperatures in a given area. Knowing the zone for a specific region like Phoenix, Arizona, is foundational for making informed plant choices.
The Official USDA Hardiness Zone for Phoenix
The Phoenix metropolitan area is designated as a mix of USDA Zones 9b and 10a on the most current maps. This designation reflects the region’s typically mild desert winters. Local variations in elevation, proximity to urban heat islands, and microclimates determine which of the two subzones applies to a specific neighborhood within the city.
The two zones indicate different average annual extreme minimum temperatures that plants must endure. Zone 9b is characterized by average minimum winter temperatures that fall between 25°F and 30°F. The warmer Zone 10a has a higher minimum temperature range, between 30°F and 35°F.
How the Hardiness Zone System Works
The USDA Plant Hardiness Zone system is a standard for gardeners across the United States. It divides the country into 13 major zones based on a 30-year average of the lowest annual winter temperatures recorded at weather stations. Each whole number zone represents a 10°F difference in temperature.
This system is further refined by dividing each major zone into two sub-designations, labeled ‘a’ and ‘b.’ The addition of ‘a’ and ‘b’ allows for a more precise measurement of cold tolerance. Each letter represents a 5°F temperature increment. For example, Zone 10a (30°F to 35°F) is consistently 5 degrees colder than Zone 10b (35°F to 40°F).
Considering Heat: The AHS Zone Factor
While the USDA zone is useful for understanding a plant’s cold tolerance, it is insufficient for gardening in a hot desert environment like Phoenix. Extreme summer heat is often a greater limiting factor for plant health than winter cold in this region. Gardeners must also consider the American Horticultural Society (AHS) Heat Zone system.
The AHS system measures heat tolerance by counting the average number of “heat days” a region experiences per year. A heat day is defined as any day where the temperature exceeds 86°F (30°C), the point at which many plants begin to experience heat-related stress. The Phoenix metropolitan area typically falls into AHS Heat Zone 10 or 11. This signifies the region experiences between 151 and 210 days above that 86°F threshold annually.
Selecting plants that can withstand both the winter lows of USDA Zone 9b/10a and the intense summer heat of AHS Zone 10/11 is necessary for successful desert gardening. A plant may be perfectly hardy for the winter but will not survive the long, hot summers without significant intervention. Informed planting decisions in Phoenix require consulting both the USDA cold zone and the AHS heat zone information.