USDA Plant Hardiness Zone Map Updated - February 22, 2012
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
The USDA Plant Hardiness Zone Map (PZHM) was developed using minimum temperature data and the most recent version was released just weeks ago. Zones in the new map are based on 1976–2005 weather data. Each zone represents the mean extreme minimum temperature for an area, calculated from the lowest daily minimum temperature recorded for each of the years of record. These zones do not represent the coldest it has ever been or ever will be in an area, but it simply is the average of lowest winter temperatures for a given location for this time period. Plant Hardiness Zones are based on average annual low temperatures using 10 degree increments. For example, the average low temperature in zone 3 is -40 to -30 degrees Fahrenheit, while the average low temperature in zone 10 is +30 to +40 degrees Fahrenheit.
The previous edition of the USDA PHZM, revised and published in 1990, was drawn from weather data for 1974–1986. The longer period (30 years) of data was selected by the group of horticultural, botanical, and climatological experts who led the review of the latest revision as the best balance between smoothing out the fluctuations of year-to-year weather variation and the concept that during their lifetimes, perennial plants mostly experience what is termed "weather" rather than "climate."
Two new zones have been added to this edition: Zones 12 and 13. These have been introduced for regions with average annual extreme minimum temperatures above 50 degrees and 60 degrees F, respectively. They only appear on the maps for Hawaii and Puerto Rico. But the additional frost-free zones will enable better definition of conditions for tropical and semitropical plants, which often serve as house or patio plants in many parts of the country. The two new zones will provide a way to share information about differences in cold sensitivity of tropical ornamental plants and may help gardeners decide when to bring tropical plants indoors from a deck or patio as the temperature cools. The interactive PZHM is available online at http://planthardiness.ars.usda.gov.
The new PZHM was created digitally with a higher level of resolution and can show smaller areas of zone delineations than ever before. For example, cities tend to hold more heat because they have large amounts of concrete and blacktop, so a city or town may be assigned to a zone warmer than the surrounding countryside. Higher elevations tend to be colder than surrounding lower areas, so the top of a mountain may be an area of cooler zones. A location near a large body of unfrozen water may provide milder winter weather and be in a warmer zone.
Like previous versions, the new PZHM also has a tool that allows users to input their zip code and returns their plant hardiness zone. I tested this tool and found that Prescott was in Zone 7b (minimum temperature 5 to 10 degrees F), Cottonwood and Camp Verde were in 8b (minimum temperature 15 to 20 degrees F), and Sedona spanned Zones 8a and 8b (minimum temperature -10 to 20 degrees F). The areas in 8a are the low lying areas near Oak Creek Canyon and the higher elevation areas around House Mountain.
Interpretation of this map is enhanced by local knowledge and experience. Remember that low lying areas and canyon bottoms are going to be colder in the winter. South and western exposures are going to be warmer and microclimates created by natural or constructed features will also influence suitability of a site for a given plant species. I also highly recommend monitoring temperatures in your garden and landscape using a wireless thermometer. These handy devices monitor outdoor temperatures using remote sensors that transmit data back to the display module via radio signal, are available with many options, and many can handle multiple outdoor sensors.
In closing, one of the best tools for determining whether or not to plant a given ornamental plant in your garden is the Sunset Western Garden Book. This fantastic reference uses its own climate zones which integrate temperature extremes, latitude, elevation, average precipitation, and humidity. It is regularly updated and there is a brand new 2012 edition available.
Follow the Backyard Gardener on Twitter – use the link on the BYG website. If you have other gardening questions, call the Master Gardener help line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at email@example.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.
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Last Updated: February 16, 2012
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