Knowing the American Southwest's Three Main Climate Zones
The arid Southwest ranges across rocky valleys and up into adjacent foothill areas, below around a 6,000-foot altitude. Many places have a thin cover of desert lot grasses, including black grama or bush muhley, and scattered shrubs including creosote bush or brittlebush, with desert willows and mesquites lining dry cubes, also referred to as arroyos. Foothill places sport abundant, often large prickly pear cacti, sprinkled evergreen oaks and other daring accent plants. Lower elevations have more exotic elements, such as feathery green palo verde trees and enormous cacti.
Each main zone, recorded below, varies because of changes in altitude and distance from the equator. Plus there are smaller microclimates in regions, such as arroyo and clean bottoms, north- or south-facing slopes and walls and heavily built up areas without trees to enhance the everyday effects of difficult surfaces.
The Southwest is divided into the following three zones of overall and extreme temperatures.
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The Low Zone
Growing season: 300 to 365 days on average. Summer: Hottest regions of the 3 zones, with most locales experiencing highs above 120 degrees Fahrenheit; at least 90 days moderate over 100 degrees; at least 150 days average over 90 degrees (American Horticultural Society, or AHS, warmth zones 11 to 12). Winter: No consistent hard freezes below 28 degrees Fahrenheit; a few winters have just a few light frosts or no frosts whatsoever; no more daytime highs average under 32 degrees; you to 15 nights moderate under 32 degrees (USDA cold hardiness zones 9 to 10). Precipitation: Arid, together with the effects of summer monsoons increasing farther to the east and light winter storms increasing farther west (annual rain averages are 3/4 to 7 inches, with some spots around about 9 inches). Sample places: 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. Shown: A Paradise Valley, Arizona, residence’s entry drive, with organ pipe (Stenocereus thurberi) in the foreground and ironwood (Olneya tesota) in the background.
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The Middle Zone
Growing season: 240 to 320 days on average. Summer: Hot; 125 to 165 days average over 90 degrees (AHS heat zones 9 to 10). Winter: Few consistent hard freezes; no longer than one daytime high averages under 32 degrees; 10 to 35 nights moderate under 32 degrees (USDA cold hardiness zones 8 to 9b). Precipitation: Arid; semiarid in highland places (annual rain averages 2 to 12 inches, with a few spots getting around 15 inches). Sample places: 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 Regions of Las Vegas, Pahrump, Searchlight, Henderson and Boulder City; in Texas — Presidio, Terlingua and Lajitas. Shown: A Tucson residence, with blue palo verde (Parkinsonia florida) and saguaro (Carnegia gigantea).
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The High ZoneGrowing season: 190 to 250 days; 170 days in the coldest valley and foothill spots. Summer: Moderately sexy; 60 to 120 days average over 90 degrees (AHS heat zones 7 to 9). Winter: Brief, definite winters with consistent hard freezes; around five daytime highs average under 32 degrees; 50 to 120 nights moderate under 32 degrees (USDA cold hardiness zones 6b to 8). Precipitation: Arid; semiarid in highland places (annual rain averages 4 to 12 inches, with a few spots getting around 17 inches). Sample places: In Arizona — Prescott, Payson, Page, Kingman, Sedona, Ash Fork, hot Regions 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. Shown: A Marfa, Texas, home using chinquapin oak (Quercus muhlenbergia), gulf muhley (Muhlenbergia capillaris) and century plant (Agave americana).