Sunken Garden Beds: Water-Wise Gardening in the Desert

Article and photos by Kara Adams

I’ve spent the last year building new sunken garden beds in Southern Arizona. I live in the Sonoran Desert, about 25 miles from the Mexico border. This region is very hot from May through October. We do get monsoon rains from July through September, but it is very dry otherwise.

The heat, intense sun, and dry conditions create challenging conditions for gardening. However, it is possible to create a productive garden in the desert. I want to share how I’ve been able to do just that.

To begin, I had a few requirements:

• Water Efficiency: I needed to find a way to use as little water as possible. I didn’t want to spend several hours and hundreds of dollars every month watering this new space.
• Time Efficiency: I needed to find a time-efficient way to maintain my new garden. I work, so a high-maintenance garden was not for me.
• Productivity – I wanted to maximize the space to grow as much as possible. This would allow me to eat fresh, organic produce in season.

That’s a tall order for a little desert garden. I began experimenting to find a way to meet all 3 of those requirements.

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History of Basin Beds in the Sonoran Desert

I am fascinated by the relationship between people, plants, and places in the Southwest region.  The Sonoran Desert is the most biodiverse desert on earth, and I love living here. Isn’t it amazing that 3500 species of plants, 500 species of birds, and 1,000 species of bees call this beautiful region home? I’m learning all I can about the history of this place and the people who thrived in such a beautiful yet challenging environment.

The U.S. Forest Service defines ethnobotany as “the study of how people of a particular culture and region use indigenous (native) plants.” 

Through Gary Nabhan’s book, I began learning about many different gardening methods in arid climates. One of them stood out as a possibility for my garden: basin beds, also known as sunken beds.

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What is a sunken garden bed or basin bed?

Basin or sunken garden beds were used by the indigenous peoples in this part of the Southwest. Basin beds function just as any traditional garden bed or raised bed, but are recessed or sunken. 

Sunken beds are dug down several inches, allowing irrigation water to slowly sink in rather than run off and keep soil and roots cooler during hot seasons. 

Since I didn’t have to construct a raised bed, the start-up costs were low. Using a basin-style sunken bed would help me with the water and heat challenges, but what about the time efficiency and productivity piece?

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Combining Basin Beds with Square Foot Gardening

I’m sure many of you have heard of the square foot gardening method, introduced in 1981 on a wide scale when Mel Bartholomew published his book, “Square Foot Gardening.” In this book, Bartholomew introduced a straightforward method to save time, effort, and space in gardening. 

Instead of long rows and wide paths between, he focused on using a 4ft by 4 ft square to grow intensively in a small space. Along with the 4×4 design, Bartholomew increased productivity through intercropping, succession planting, and amending the soil. If you haven’t read this book and want to garden in a small space, on a small budget, or with a busy life, I highly recommend it. 

I combined the two methods, sunken basin beds and square foot gardening, into one hybrid method. Maybe then I could get all the benefits of each.

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Sunken Garden Beds Step One: Test Your Soil

An in-ground garden bed will only be as good as its soil, so the first thing I did was take some soil samples. A local lab tested the soil to give me an idea of the soil makeup and which amendments I needed to add. I chose the “Complete Soil Test with Soil Amendment Recommendations” for $85, which included a nutrient analysis of my soil profile and amendment suggestions. 

I started with a very bleak space; not much of anything was growing here, except for wild amaranth and Bermuda grass, so I was surprised when the test results came in a few days later, showing I didn’t need to amend much. 

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Sunken Garden Beds Step Two: Remove Unwanted Vegetation and Weeds

My chosen garden area had Bermuda grass that needed removal before installing the new beds. The simplest way to remove vegetation is to use solar energy to do the hard work, something we have plenty of in the Southwest.

Two methods create a greenhouse effect and work well for this task: solarization and occultation. 

Solarization controls unwanted vegetation by covering the area with clear plastic. The 2 – 6 mil thickness plastic allows solar energy to penetrate, heating up the soil and creating a greenhouse effect that smothers the vegetation in as little as 3 weeks. 

Occultation is similar to solarization but uses a black plastic tarp instead of a clear one. This process takes longer as the black material absorbs solar energy and reduces the heat passed to the vegetation. A heavy opaque plastic, called a silage tarp, is used and the process takes 4-6 weeks.

An additional benefit of both solarization and occultation is the reduction of pathogen and nematode populations in the soil below.

I used occultation and laid a thick silage tarp to cook the weeds and seeds underneath for 6 weeks. When I removed the tarp, just like magic, there was nothing but bare earth and some decomposing plant material.

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Sunken Garden Beds Step Three: Make a Plan for the Space

I planned just four beds in the beginning. Every gardener knows that plans tend to grow exponentially, and I ended up with twelve beds. For now, let’s focus on those first four beds! 

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Sunken Garden Beds Step Four: Dig Out Beds and Add Compost

I measured out and marked an exact 4ft x 4ft square. After digging down 6 inches, I removed that soil. I mixed the native soil with compost in a 50/50 mix and then filled the beds with a little of that mixture. I ended up with a bed recessed by about 4 inches. 

Now, you may be thinking…“wouldn’t doing that remove all the good soil you had tested?” The answer is…not at all! When doing a soil test, you dig down about 6 inches to the root zone, where the plant will be accessing all that goodness. That is still there once I dig out the top, so what I’m doing is adding some amendment in the form of compost to the top while leaving the soil in the root zone intact. 

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Sunken Garden Beds Step Five: Create Pathways Between Beds

My backyard is covered in Bermuda grass, and if you’ve ever dealt with it, you know it’s tough to eliminate. I decided to cover the ground with landscape fabric around the garden beds and then layer wood chips to smother and suppress the Bermuda grass. This was after using a silage tarp for occultation before beginning the garden project. That’s how tough it is to get rid of the stuff!

Generally, I don’t like to use landscape fabric. I think it’s best for soil health to cover the ground with a thick layer of untreated wood chips. This keeps weeds in check and adds organic matter to the soil as the wood breaks down. When planning this garden area, I made an exception. 

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Sunken Square Garden Beds Step Six: Add a Square Foot Garden Grid and Border

Once the beds were dug and amended, I needed to make a square-foot garden grid. To save time in measuring, I made a template grid that I could use each time I plant a new bed. I used 4ft. green plastic garden stakes, measured and marked 1 ft sections, and fastened them with outdoor-rated zip ties that will withstand the high UV here better. Using a grid template allowed me to plant each square quickly.

I used the tons of rock around my property as a border for all of my garden beds. It’s rustic, but I’m happy with the results. Best of all, it was free! 

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Sunken Garden Beds Step Seven: Plant Desert-Adapted Varieties 

The next step was to select varieties of plants that are well-suited to this region of southern Arizona. I start nearly all my plants from seed, but if you don’t want to sow seeds, many great organizations in the area sell arid-adapted seedlings at seasonal plant sales. 

After moving from the Midwest nearly 20 years ago, one of the first gardening lessons I learned was that what grew there probably won’t grow here. Varieties matter, so choose wisely! There are so many varieties that perform well here. Don’t be afraid to explore!

I planted herbs and flowers amongst my vegetables (polyculture style), hoping this would give a habitat to many different pollinators and beneficial insects.

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Sunken Garden Beds Step Eight: Add Mulch and Shade 

Next, I added mulch to the top to prevent moisture loss and shade cloth in certain areas.

I use shade cloth when planting new seedlings. The sun here is very intense, and it takes those little seedlings a while to toughen up. Shade cloth is also important to use in certain seasons for established plants. 

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Adding Long Basin Beds

Late this summer, I dug a new bed – the garden is never done, remember? This time I wanted to experiment with a long basin bed, rather than a 4×4 square. The new garden bed was 3 feet wide and 28 feet long. 

I am a little (okay, a lot) obsessed with the beauty and variety of dried beans and wanted to grow them. So, I added a new long bed with a cattle panel down the middle. This would allow me to grow pole beans down the center and other crops along the bottom.

I made the most of that space, growing beans, okra, squash, roselle, and zinnias. This bed became a superhighway of butterflies, bees, and hummingbirds. It was a great success; I’m putting in a second long bed this fall. Because, you guessed it, the garden is never done!

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The Result: A Thriving Garden in the Desert 

Overall, I’m so pleased with the results of this gardening season. I’ve happily watched this dusty, bare backyard transform into a space alive with color and the hum of pollinators. I’ve learned a few lessons, of course, that I’ll change next season, but that’s true of any garden. I don’t think a garden is ever truly finished – there is always something new to learn and just one more plant to add. 

Most importantly, I’ve done almost no weeding and only had to water the beds twice a week during the hottest part of the year. As the temperatures cool, I will change the watering schedule to once a week. This winter, I’ll install rainwater harvesting tanks to water the garden, reducing the water I must pay for.

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Sunken garden beds are an effective, water-wise, efficient, and affordable option for any space

• Even if you have a small space, one of these sunken square-foot garden beds will allow you to grow a surprising amount of food.
• If you have little money to start a garden, this method can fit any budget.
• If you’re short on time, gardening this way can be done in just a few minutes a day. 

I hope this encourages you that any space, no matter the size, the climate, or the time constraints, can be a place for you to sow your dreams.

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Kara is an advocate for sustainable gardening practices and environmental stewardship. She creates written content and educational videos on Sonoran Desert plants, gardening practices, and soil health. Her topics include organic gardening, pollinator-friendly landscapes, and water-efficient gardening practices.
Follow Kara on her journey through the garden, where her love for it continues to bloom, one word and video at a time. You can follow along with her journey on YouTube and Instagram.

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