Welcome to growing a garden in one of the most fascinating and diverse places on earth – the Southwest. Diverse – because there are many different growing zones, not to mention the geographic region itself, with strongly upthrusting mountains, windswept plateaus, and canyons that reach down into the dawn of earth’s prehistory. We have elevations close to sea level in Yuma, Arizona and some of the tallest peaks in the lower 48 states, such as the snow-capped San Francisco peaks outside of Flagstaff AZ, and the Sangre de Cristo Range in New Mexico. Mountains and mesas may be of igneous, metamorphic or sedimentary rock, providing unique parent material for the soils of our region. (More on soils coming soon under this website subheading).
With this wide geographic variation in the Southwest comes vast climate variation, as much as you could see in a journey from the tip of Florida to the Alaskan tundra! So welcome to gardening in a very unique corner of the world!
Note – I’m a scientist and like to know the “why” of things. If you just want the facts – skip to the bottom.
Moving from West to East, the Southwest includes: southeastern California, Nevada, southern Utah, Arizona, New Mexico, southern Colorado, western Texas, and western Oklahoma. The analogous regions in Mexico that could also use the informations on this site include Baja California Sur, Baja California, Sonora, northern Sinaloa, Chihuahua, Coahuila, and northern Nuevo Leon.
Deserts of the Southwest
The deserts of the Southwest include areas of the Chihuahuan, Mojave, and Sonoran Deserts found in Arizona, California, New Mexico, Nevada, Texas, and corresponding states in Mexico. While most people think of deserts as dry and hot, these deserts also feature notable wet and cold, even freezing periods.
Summertime highs over 100 degrees are common, but winter lows in the teens are also common. Areas that get sufficient hours of cold (called “chill hours”) can grow temperate fruit trees such as apricots and apples. The vegetable growing season is generally year-round, with marked times for planting cool-season or warm-season plants. Many garden favorites such as roses and iris do well here.
In these Southwest deserts, there are two marked rainy seasons. Gentle soaking winter rains off the Pacific Ocean grace the Mojave and Sonoran Deserts. Torrential summer thundershowers sweep in off the Gulf of Mexico, and fall upon the Chihuahuan and Sonoran Deserts. Some areas commonly get both rains so that the average rainfall in the intermediate desert region is 2.5 to 12 inches, depending on your location.
Local vegetation varies from dense to sparse. If you plan on watering your garden, at least occasionally, you can grow virtually anything from these three deserts in your garden, along with a plethora of arid-adapted plants from around the world, like Greek oregano, Roman chamomile, or palm trees from the Sahara.
Gardening Regions in the Southwest
I developed these simplified gardening regions for the books I wrote for Cool Springs Press. These regions are based on three factors, elevation, USDA hardiness zone, and seasonal rainfall patterns – in other words, your local climate. Even within these three regions, there is huge variation in growing conditions. Indeed, within your own yard, there are variations in growing conditions, termed microclimates. You may be able to grow something not rated for your regional climate by using such highly local microclimates.
May never freeze, but if it does, between 15 November and 15 February.
Summer highs in the 110’s
Average summer humidity 10 to 40 percent
Average winter humidity 0 to 80 percent
Middle Desert Elevations
First frost average 1 November
Last frost average 15 March
Summer highs in the 100’s
Average summer humidity 10 to 90 percent
Average winter humidity 0 to 80 percent
High Desert Elevations
Averages! – your local area may differ.
First frost average 1 October
Last frost average 1 May
Summer highs in the upper 90’s
Average summer humidity 20 to 70 percent
Average winter humidity 0 to 30 percent
Southwest USDA Zones with Gardening In the Land of El Sol Zones
USDA Cold Hardiness Zones in the Southwest range from 10B to 4A – totaling 14 categories. Rather than tediously referring to these zones, I divide your gardening tasks primarily into “lower,” “middle,” and “upper” elevations.
USDA Zones 10B to 9A – includes Ajo, Alamogordo, Bullhead City, Las Cruces, Parker, Phoenix, Yuma. Some of these cities routinely experience mild winter freezes while others rarely do.
USDA Zones 8B-7B – includes Alamogordo, Albuquerque, Alpine, El Paso, Gallup, Kingman, Las Cruces, Las Vegas, Lordsburg, Safford, Sierra Vista, Socorro, Tonopah, Tucson, Wilcox. These cities routinely experience moderate to severe winter freezes.
USDA Zones 7A to 4A – includes Carson City, Colorado Springs, Durango, Elko, Ely, Fallon, Flagstaff, Gallup, Holbrook, Prescott, Reno, St. George, Santa Fe, Sedona, Show Low, Taos, Window Rock, and Winnemucca. These areas always freeze in winter, and often experience below zero weather.
There is always an exception!
The exception is – USDA Zone 7B – which includes the cities of Albuquerque, Socorro, and Tonopah. In general, gardeners in 7B should consider themselves an “upper elevation.” The exception occurs when you experience an unusual season, like a warm spring, then you can do “lower elevation” tasks. Every so often I will mention this specifically to remind you. One factor you can’t fudge in 7B are the chill hours certain fruit trees need. You will need to select those with fewer required chill hours, because of those unusual years.