Note: The Arizona Plant Climate Zones described here are neither USDA Plant Hardiness Zones nor are they Sunset Western Climate Zones. However, Arizona Plant Climate Zones can be cross referenced with other climate zone systems to further assist gardeners in plant selection. Cross reference information has been included where each of the Arizona Plant Climate Zones is described.

Nowhere is home gardening more challenging nor potentially more rewarding than the Southwest. Because our climate ranges from subalpine to tropical desert, an almost bewildering array of ornamental plants can be grown in this region. However, very few plants grow satisfactorily over the entire range of varied growing conditions found in the Southwest. It is important, therefore, that we know how local climate influences plant growth and which ornamental plants grow well in our area.

Many climatic factors play a role in determining the kinds of plants that will grow in a given location. Minimum winter temperature and frost occurrence, maximum summer temperatures, rainfall amount and distribution, humidity, day length and light intensity are all important. Of these, minimum winter temperature is a major limitation and is often used as an indicator of where plants are adapted.

The plant climate zone map presented here is based on expected minimum temperatures throughout the Southwest. It shows five different zones, each of which represents an area of winter hardiness for certain ornamental plants. These five plant climate zones give adequate information for most horticultural purposes. However, important differences in plant performance may be found within a given zone. Most often these differences will be due to a change in elevation and a corresponding cooler or warmer climate.

The climates of adjoining zones grade into one another near their boundaries. This sometimes makes it possible to grow plants that are too cold tender for a given zone, or it may exclude certain plants at the coldest extremes of that zone.

Plants that perform well in one plant climate zone are often adapted to the next warmest zone. However this is not always the case. The milder winters of a warmer zone are usually accompanied by higher summer temperatures which are tolerated by some plants but not by others. In a milder zone erratic occurrence of frosts and warm spells during late winter and spring may affect plant performances, especially kinds that bloom at this time of year. Drier air, strong seasonal winds and more alkaline, salty soil may also create a hostile environment for plants that are quite cold hardy in a given zone.

In the discussion of each climate zone, a number of adapted plants are listed. These "indicator plants" may lead you to others that will succeed in the area.

Microclimates also play a part in determining the kinds of plants that will grow in your landscape. A microclimate is simply the local climate on a small site.

Microclimates are formed by hills and valleys, structures, paved areas, hedges or windbreaks. These features may change airflow patterns, alter day length or light intensities, trap heat during the day and slowly release it during the night, or in other ways modify local climate.

On a somewhat larger scale the growth of urban areas also affects local climate. For example, summer high temperatures have increased significantly in both Tucson and Phoenix over the past forty years. This is often referred to as the urban "heat island effect." The warmer, drier environment of urban heat islands has a strong impact of landscape plant performance.

Note: Originally published in March 1990 by Charles M. Sacamano, Extension Horticulture Specialist Emeritus and Randall H. Hagen, Research Assistant for the University of Arizona Cooperative Extension. Adapted and amended for the World Wide Web in February 2000 by Jeff Schalau, Agent, Agriculture & Natural Resources.