Understanding Climate Zones - February 7, 2001
Jeff Schalau, County Director, Agent, Agriculture & Natural Resources
Arizona Cooperative Extension, Yavapai County

I know everyone has been scanning their favorite seed and nursery catalogs lately because the Cooperative Extension offices have been receiving calls about climate/hardiness zones. The USDA Plant Hardiness Zone maps provided in the supplier catalogs are small, often compressing the entire United States map into a two by three inch spot in some obscure location within its pages. To add to the frustration, we sometimes are challenged to compare the USDA system to the Sunset Western Garden Book system. Now I am going to tell you that the University of Arizona uses yet a third system. In this week's column, I will attempt to bring some order to this chaos.

The USDA Plant Hardiness Zone map was developed using minimum temperature data and was most recently revised in 1990. It represents the lowest average temperatures that can be expected in that broad area based on a 12-year average (1974-1986). There are 10 different zones rated for plant hardiness and an additional zone that is essentially frost-free. The 1990 map also divided zones 2-10 in half to denote 5-degree differences in average minimum temperature rather than the old 10-degree differences.

I consulted Arizona Climate 1931-1972 by Sellers and Hill for some local area minimum temperatures and record lows. All data are in degrees Fahrenheit. In Cottonwood, the average daily minimum was 28.4 and the record low was 5. In Sedona (at the Ranger Station), the average daily minimum was 29.7 and the record low was 0. At Montezuma Castle, the average daily minimum was 25.6 and the record low was -1. For comparison, Prescott had an average daily minimum of 21.5 and the record low was -21.

These average temperatures would grossly underestimate hardiness zones and their potential for cold weather. We need to plan for the worst-case scenario (i.e. consider the record low), especially if we want to get fruit production in our lifetime. If the record low is considered, then I am fairly comfortable in projecting our local area's Plant Hardiness zones. It also matches fairly well with the fuzzy, indistinct Plant Hardiness Zone map I pulled off the Internet.

So, what about it: all you really care about is your zone, right? In Cottonwood, Camp Verde, and other Verde Valley locations use USDA Zone 8a (15 to 10 degrees F). In Jerome, Oak Creek Canyon, and other cooler spots, consider using USDA Zone 7b (10 to 5 degrees F). In Prescott, use USDA Zone 7a (5 to 0 degrees F). Just Remember that some locations can have much colder microclimates.

Now What about the Sunset Western Garden Book Climate Zones? Sunset starts their climate zone section with a lengthy disclaimer about the limitations of the USDA system. For instance, the USDA system groups the Sonoran desert will the Olympic Peninsula in Washington. Sunset Climate Zones are based on latitude, elevation, ocean influence, continental air influence, mountain ranges, and local terrain. The Verde Valley is in Sunset Zone 10. Prescott and the cooler canyons areas of the Verde Valley are in Sunset Zone 2. See the Western Garden Book for the details. As an aside, if you are an avid gardener, then I highly recommend the Sunset Western Garden Book.

The third (and least known), plant climate zone system was developed by the University of Arizona Cooperative Extension. The map is identical to the Sunset map, but it uses different numbers. I have digitized this map on the Yavapai County Cooperative Extension web site (http://ag.arizona.edu/yavapai/anr/hort/climate/intro.html). You can simply click a location and a description is displayed. I have also tried to cross-reference the various plant hardiness/climate zone systems.

Microclimatic factors should also be considered when selecting plant suitable for a given location. The web site referenced above has an introduction that may shed light on microclimate considerations. I highly recommend reading this section. Lastly, I hope this column has not added to the confusion on hardiness vs. climate zones.

If you do not have access to the internet, you may call and request a copy of this publication from our office. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 or E-mail us at mgardener@kachina.net and be sure to include your address and phone number.

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: March 15, 2001
Content Questions/Comments: jschalau@ag.arizona.edu
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