Extreme Heat: Gardening in the Low Desert

One Arizona gardener shares her ongoing journey to combat the high heat and make her desert garden thrive.

Marconi peppers

As a result of the unusual climate in the low desert, many traditional gardening practices are altered to adapt to the high heat.

Photo Courtesy www.RareSeeds.com

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My garden in Arizona lies in USDA Zone 1 Million. No, that’s a lie. The USDA Plant Hardiness Zone Map only goes to Zone 13. It just seems that hot.

In fact, the USDA map is not based upon summer heat at all. Instead, it helps gardeners identify their average minimum winter temperatures. Truthfully, I live in Zone 9A, where I can expect the average low temperature in the winter to be between 20-25 degrees.

Sometimes I wish the map would address the extremes of summer heat. If it did, I’m sure my garden would be in Zone 1 Million, because the high temperatures kill even the toughest of plants.

As a result of the unusual climate in the low desert, many traditional garden practices are altered. New and exciting opportunities arise, such as near year-round gardening. However, daunting challenges also surface to threaten success including extreme summer heat, drought, and alkaline soils. Solutions often turn time-honored practices upside down — planting in rows and furrows, for example. One thing is certain; growing food crops in Zone 1 Million is very different than elsewhere.

The wonderful thing about living in the Southwest is that there are two planting seasons. Summer, of course, is the traditional planting time. Due to the mild climate here, however, many vegetables can also be planted during the winter. For a gardener normally accustomed to only one season, this sounds like paradise, but things are not quite that heavenly.

My winter crop is the most productive and is, by far, the easiest to grow. This is where the USDA Hardiness map comes in handy. Knowing the average low temperatures in my area helps me select plants that grow well in this range. For example, broccoli, cabbage, and lettuce thrive in the mild climate, because they are somewhat frost hardy. Beans, melons, and squash, however, are more “frost tender” and must be planted in a warmer season.

Winter gardening is more enjoyable, as well, because the garden grows with little maintenance. Grass, weeds, and bugs are minimal. It’s a joy to work outside in the cool weather.

My winter crop is first planted near the end of September. At this time, summer is stubbornly hanging on, still delivering temperature highs exceeding 100 degrees Fahrenheit. Not many seeds germinate in that kind of heat, so I do this inside the house where it’s cool. I roll them up in a clean moistened cloth and then place this inside a glass jar labeled with variety name. Since the seeds need air, the jar’s lid is left off. However, I don’t want them to dry out, either. So I place another moistened cloth on top of the seeds to maintain humidity in the jar. After they soak overnight, the seeds are planted.

Many local gardeners plant seeds indoors in the sterile soil of preformed growing trays. Seedlings grow underneath artificial light and are transplanted outside as weather improves. Sometimes I do this as well.

Usually, however, I prefer direct seeding from presprouted seeds. One of the most common problems with this is exposure to a soil-borne fungus called “damping off,” which kills seedlings before or just after emergence. Fortunately, the excellent drainage from my sandy soil prevents this culprit from causing problems.

After I plant the seeds outside, it’s important to keep them moist and shaded from the hot sun. To do this, I have developed my own framework for row covers that easily convert to support shade screen, plastic, or bird netting — depending upon the season and type of plant. To make the simple scaffolding that holds the fabric, I cut and bend limbs from a bush that grows natively on my property. It contains resins that keep it from drying out, allowing me to reuse them for several years.

The crops planted in September grow and produce all throughout winter until about March or April, when they blossom and produce seed. At this time, I either remove them from the garden or allow them to go to seed to supply the raw material for next year’s planting.

All throughout the mild winter, however, there are enough vegetables for daily meals for myself and my husband plus excess produce for family members and friends. Visitors never leave without taking some of the harvest home.

Abundant Harvest

Broccoli is always a heavy producer. ‘Calabrese,’ one of my favorite heirloom cultivars, quickly makes a small central head. Once this is harvested, the plant continues to produce small tasty offshoots, which soon outweigh the central head harvest. In my opinion, the small side shoots are more tender and flavorful, anyway. I like to snack on them while working in the garden.

For lettuce, I prefer loose-leaf varieties, as they can be harvested leaf by leaf in an ongoing fashion. I’m careful, however, not to remove too many leaves from the same plant at one time. Turnip and beet leaves can also be harvested in this way.

I plant broccoli and leaf lettuce only once at the beginning of the season, as they produce all winter. However, cabbage, carrots, beets, turnips, and head lettuce are planted periodically throughout the season to ensure continual harvest.

Sometimes I freeze or home-can vegetables, but not as often as do my gardening counterparts who live in other sections of the country. In those areas with only one growing season, produce must be harvested and saved for use throughout the year. Here in the low desert, however, two seasons supply ongoing harvest that offsets, but does not eliminate, the need for preserving.

For example, if I wish, I can preserve some of the winter-grown broccoli for use later — but why? Since there are two growing seasons, we just change our eating habits. The winter stir-fry that was cabbage and broccoli with a lettuce salad on the side becomes a similar stir-fry in summer with squash, okra, and peppers and a slice of melon on the side. The recipe is essentially the same, only with different types of fruit and vegetables, depending upon the season.

Of course, by the end of winter I am tired of broccoli and yearn for squash. This is why I preserve a small portion of the harvest as a change of pace. I also home-can or freeze vegetables when the ongoing harvest exceeds our needs for daily meals, which is not often. I generally do not over-plant for excess.

Extreme Heat

I know all of this sounds just wonderful, but there is one small complicating factor I’ve not discussed yet … the Arizona summer. By far, my greatest challenges result from growing plants in extreme heat.

I think the Arizona summer is similar, in some ways, to winter elsewhere. That is, it’s a time when the garden often lies fallow due to adverse weather.

In fact, because most plants cannot withstand the hottest part of the summer, that season is essentially cut in half, creating a very short spring and fall season. As a gardener, this often leaves me struggling with short-season crops that must produce quickly before the heat sets in.

In the hottest part of the summer, I’ve recorded soil temperatures of 140 degrees (3 inches under the ground). Roots literally cook in the hot sandy soil. Leaves curl under from sun scald. Then they turn dry and drop off. Without relief, my garden withers and dies. The dry debris is carried away in a late afternoon monsoon wind.

What gardener could possibly be successful given these extreme conditions?

The ancient Native Americans, of course, knew how it was done. The ancestors of the nearby Gila River Pima and Tohono O’odham tended productive gardens. In addition to the traditional “Three Sisters” (tepary beans, corn, and squash), they grew melons, multiplying onions, chili peppers, and other crops.

The seeds grown by the ancient people were adapted to the unique conditions of the low desert. They were tolerant of heat, drought, and high soil alkalinity. The methods they used to plant, tend, harvest, and prepare the food crops also evolved into systems that were effective for them and the environment.

Fortunately, there is a non-profit organization dedicated to preserving the ancient seeds of the American Southwest and northwestern Mexico. Native Seeds/SEARCH (www.nativeseeds.org), located in nearby Tucson, is the source of many of the seeds I plant in the hot, challenging summer.

Using heirloom seeds historically grown in the local area is one good way to increase my success growing food crops. But this alone is not enough.

The summer garden fails if the soil temperature is not reduced. I employ several methods to accomplish this, including protecting the ground from the hot sun, varying irrigation methods, and altering traditional planting techniques.

Mulch is the traditional method for protecting the ground from the heat of the sun. I make heavy use of it in the orchard, around bushes, and sometimes around vegetables.

The problem with mulch is that it attracts bugs. This can be a problem with a seedling newly emerged from the ground. The tiny plant is often gobbled up before it has a chance to grow. That’s why I also employ other methods including shade screens and mixed plantings.

I am experimenting, for instance, with planting heat-tolerant crops along with more sensitive vegetables. This can protect the soil and veggies from excess solar radiation. For example, the Native Americans planted the Three Sisters crops together. From an Arizona perspective, could this be because they wanted to reduce the heat stress for sensitive squash by shading them with hardier tepary beans and native corn?

Watering Challenges

The method by which I irrigate my soil also affects the temperature underground, and I vary techniques with the season. In winter, I use drip irrigation hoses with built-in emitters. The programmable timer adjusts to accommodate changing weather and works effectively with minimal water.

However, in the summer, black hoses attract heat — even when buried. High soil temperatures warm water trapped in the hose. Since it cannot evaporate, it acts like a hot water bottle, increasing the temperature in the ground.

Additionally, a drip emitter delivers water from a point. In my sandy soil, it travels downward out of root range, not outward. This creates a small circle of moist earth, surrounded by hot, dry, sandy soil. The heat surrounds the little circle, pressing inward toward the roots.

Consequently, all of the hoses are removed from my garden in the spring. During this time, I favor irrigation methods that flood a large zone, as the sheet of water cools the entire area and washes unwanted salts downward.

Planting Techniques

Hot weather has also forced my planting techniques to evolve into non-traditional methods. Most gardeners, for example, use the hoe to make long hilled rows alongside lower valleys. They plant seeds along the upper crest line, using troughs to transport water.

This method is, in fact, employed successfully by local large-scale agriculture. They use traditional farming tractors, tap into flood irrigation from local canals, and plant heat-tolerant crops such as cotton and sorghum.

My experience with backyard gardening, however, is that planting on the hilled sections isn’t the best method for crops struggling in the hot, arid summer. The elevated earth collects heat and dries out. Harmful salts are attracted to the highest point and concentrate there, leaving a line of white deposits along the crest of the rows.

For these reasons, I usually plant seeds on level ground. If the planting area is surrounded by a raised perimeter of earth, the entire section can be flood irrigated.

Planting in large patches, as opposed to narrower row formats, also helps cool the soil. Unplanted walkways between rows collect heat, even when covered in straw.

Thank goodness, a few treasured vegetables love the hot sun! Okra thrives in it, producing prolifically all summer. Native chili peppers are also quite hardy.

Many of my spring-planted seeds, however, must produce before mid-summer heat kills them. It is essential, therefore, to start them as early as possible. Isn’t it odd that here in Arizona, with nine months of summer, I struggle with many of the same short-season issues as someone gardening in Alaska?

Consequently, with plants that are less likely to survive the extreme mid-summer heat (melons, squash, and tomatoes), I jump-start the spring season by planting presprouted seed as previously described. Since these prefer warmer germination temperatures, I place the jar in a warm spot, such as on top of the refrigerator.

When I plant them outside in late January or early February, I place clear plastic sheets over the row framework to protect them from frost. With a knife, I gently cut “Xs” to provide air circulation. If I’ve planted in a patch format, the framework for row covers changes to a series of movable PVC T-shaped structures that spin around to accommodate almost any size of plot. (When the weather improves, I roll-up the plastic for reuse later.)

Another unique way to get a head start on spring is to plant tomato seeds in a trench. First, I dig a long ditch about the depth and breadth of a standard spade. Then I plant presprouted seed in the bottom and lay clear plastic directly on the ground (with cut “Xs” over trench). As the seedling emerges, it is protected from frost in its underground greenhouse. Once it grows tall enough to reach the top of the plastic, the weather is generally warm enough to remove the sheet altogether. (If not, then I add PVC “T” supports to elevate the sheet.) Later, as the summer warmth progresses to horribly hot, the roots sit low in the trench’s cool soil.

I also plant melons and squash using this technique. However, I am careful with these to prevent the trench from filling in with dirt. They are fussier than tomatoes about backfill against their stems.

As I discuss my specific challenges with other local gardeners, I find that conditions and solutions vary greatly from garden to garden. For example, my sandy soil on virgin desert property amplifies the soil temperature. Often, people who live in town have richer soil and more ground cover, producing a cooler overall environment, allowing veggies to grow through even the hottest part of summer. Also, my daughter, only a few miles away, has a type of soil called “caliche” in which calcium carbonate forms a chemical bond similar to cement. She also experiments with alternative solutions that are very different from my own.

Like gardeners everywhere, I strive constantly with ways to improve my techniques, often employing unconventional methods, trying different heirloom vegetable varieties, relearning ancient traditions, and stubbornly progressing forward in spite of adverse conditions. Overcoming challenges, in fact, is part of the fun of gardening. The joy and satisfaction derived from growing plants and saving heirloom seeds overshadows the difficulties.

This is true whether a gardener lives in USDA Zone 1, Zone 1 Million, or anywhere between.


The Desert People

The Tohono O'odham Native Americans live in the Sonoran Desert of Arizona and northern Mexico. Their name literally means, "Desert People." However, the Spanish conquistadors are credited with calling them the Papago, which means "tepary-bean eater."

On the surface, the dry and desolate Sonoran desert doesn't seem to be a gardener's lush paradise. However, these Native Americans knew how to adapt to their landscape and carefully select seed varieties that would yield bountifully despite their dry conditions. We can thank the Tohono O'odham for delicious dried bean varieties that can be found in the hands of seed collectors today. By growing and cooking these varieties, you are able to touch the past and savor the same flavors that were enjoyed centuries ago by conquistadors and Native Americans alike.

If you have the opportunity to drive through the reservation, be sure to stop at their native art shops where you can purchase traditional handwoven baskets and bags of their cherished dried tepary beans.


Donna Hamill is a 20-year veteran of heirloom gardening. She and her husband are striving to be self-sustainable on 1.5 acres in the low desert of Arizona. Their property is a life-force integrated into an ancient system of mutually-beneficial relationships between garden, animals, humans, and the planet.