Understanding the American Southwest's Three Main Climate Zones
If you live in one of the arid or semiarid regions of the U.S. Southwest, this gardening zone guide is for you
David Cristiani
January 1, 2000
The dry Southwest ranges across arid valleys and up into adjacent foothill areas, below around a 6,000-foot elevation. Many areas have a thin cover of desert bunch grasses, such as black grama or bush muhley, and scattered shrubs including creosote bush or brittlebush, with desert willows and mesquites lining dry washes, also called arroyos. Foothill areas sport abundant, often large prickly pear cacti, scattered evergreen oaks and other bold accent plants. Lower elevations have even more exotic elements, such as feathery green palo verde trees and immense cacti.
Each main zone, listed below, varies due to changes in elevation and distance from the equator. Plus there are smaller microclimates within areas, including arroyo and wash bottoms, north- or south-facing walls and slopes and heavily built up areas without trees to mitigate the daily effects of hard surfaces.
The Southwest is divided into the following three zones of overall and extreme temperatures.
Each main zone, listed below, varies due to changes in elevation and distance from the equator. Plus there are smaller microclimates within areas, including arroyo and wash bottoms, north- or south-facing walls and slopes and heavily built up areas without trees to mitigate the daily effects of hard surfaces.
The Southwest is divided into the following three zones of overall and extreme temperatures.
The Low Zone
- Growing season: 300 to 365 days on average.
- Summer: Hottest areas of the three zones, with most locales experiencing highs above 120 degrees Fahrenheit; at least 90 days average above 100 degrees; at least 150 days average above 90 degrees (American Horticultural Society, or AHS, heat zones 11 to 12).
- Winter: No consistent hard freezes below 28 degrees Fahrenheit; some winters have only a few light frosts or even no frosts at all; no daytime highs average below 32 degrees; one to 15 nights average below 32 degrees (USDA cold hardiness zones 9 to 10).
- Precipitation: Arid, with the impact of summer monsoons increasing farther to the east and light winter rains increasing farther west (yearly rain averages are 3/4 to 7 inches, with some spots up to about 9 inches).
- Sample locations: In Arizona — Phoenix, Yuma, Mesa, Tempe, Scottsdale, Lake Havasu City and Bullhead City; in California — Palm Springs, Palm Desert, Needles, El Centro and Borrego Springs; in Nevada — Laughlin and Mesquite.
The Middle Zone
- Growing season: 240 to 320 days on average.
- Summer: Hot; 125 to 165 days average above 90 degrees (AHS heat zones 9 to 10).
- Winter: Few consistent hard freezes; no more than one daytime high averages below 32 degrees; 10 to 35 nights average below 32 degrees (USDA cold hardiness zones 8 to 9b).
- Precipitation: Arid; semiarid in highland locations (yearly rain averages 2 to 12 inches, with some spots getting up to 15 inches).
- Sample locations: In Arizona — Tucson, Wickenburg, Carefree, Cave Creek, Anthem, Superior and Safford; in California — Barstow, Palmdale, Twenty Nine Palms, China Lake and Yucca Valley; in Nevada — warm areas of Las Vegas, Pahrump, Searchlight, Henderson and Boulder City; in Texas — Presidio, Terlingua and Lajitas.
The High Zone
- Growing season: 190 to 250 days; 170 days in the coldest valley and foothill spots.
- Summer: Moderately hot; 60 to 120 days average above 90 degrees (AHS heat zones 7 to 9).
- Winter: Short, definite winters with consistent hard freezes; up to five daytime highs average under 32 degrees; 50 to 120 nights average below 32 degrees (USDA cold hardiness zones 6b to 8).
- Precipitation: Arid; semiarid in highland locations (yearly rain averages 4 to 12 inches, with some spots getting up to 17 inches).
- Sample locations: In Arizona — Prescott, Payson, Page, Kingman, Sedona, Ash Fork, warm areas of Winslow and Holbrook; in California — Victorville, Hesperia, Bishop, Independence; in Nevada — Beatty, Alamo and Hawthorne; in New Mexico — Albuquerque, Las Cruces, Rio Rancho, San Felipe Pueblo, Bernalillo, Belen, Roswell, Tucumcari and Silver City; in Texas — El Paso, Midland-Odessa, Fort Stockton, Alpine, Marfa and Sierra Blanca; in Utah — Saint George, Moab, Springdale and Mexican Hat.
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Which zone is Roswell, New Mexico?
Roswell is listed under the high zone in my description...but this is general since I had to address people in very hot low deserts that rarely get frost (Phoenix, Yuma), and areas as cold in winter as Albuquerque or Roswell.
Santa Fe and Taos are in the SW but had to be left out, but they are both different and fit better into the Rocky Mountain area.
I would have had 10+ zones if I didn't have to keep it simple.
Some may surprise some people, but it's based on experience, extensive climate data, how the weather tends to work, and the types of plants native and adapted. Noelle who now writes for Houzz, breaks things down a little differently.
Where I live in El Paso is in the "high zone" with Roswell, only because I kept this simple...we are much milder than Roswell in the winter each time a cold front blows through. Even Albuquerque is a little milder than Roswell as to cold extremes (even though Roswell is often one of the warmest in the area when it's hot). The plants native or adapted here are 2/3 the same, but 1/3 much different than Roswell.
I hope that helps.