Is an “oasis” landscape perfect for Denver and arid climates?


The term and technique of xeriscaping was invented by Denver Water for the high desert, but it’s often been a tough aesthetic sell among consumers. Some considered it “zeroscaping” — like zero attraction — and weren’t impressed with how much landscape water the desert-style designs could save in their home gardens.

But the city and suburban dwellers of the Front Range are beginning to realize that the days of endless expanses of indelible green lawns are over. They suck way too much water from the Colorado River and are being banned in some major cities like Aurora.

So is an “oasis” home landscape Goldilocks’ solution, just right for water consumption and urban climate comfort?

A new study by the Desert Research Institute and scientists from two Western universities seems to think so. Researchers compared both water use and air temperatures at three different landscape scales: mesic, or traditional turf, surrounding thirsty trees; what they call xeric, an arid landscape of dripping plants surrounded by dry rock or mulch; and a midground, an oasis where shady trees or shrubs and small islands of peat break up a drier, resource-friendly landscape.

Play areas and structures reference a more natural landscape at High Prairie Park.
Play areas and structures reference a more natural landscape at High Prairie Park. (Kathryn Scott, special for The Colorado Sun)

Examining various scenes around Phoenix found that the oasis style could be a winning compromise for arid urban climates such as Arizona, Colorado, Nevada and Southern California. The oasis uses much less water than lawns while still keeping ambient temperatures within the comfort zone created by the shadier, greener lawn.

According to the study, published in the journal Hydrology, air temperatures in the xeriscape plots averaged 5.4 degrees higher than in the oasis or mesic tree-lawn style.

Researcher Rubab Saher of DRI’s Department of Hydrologic Sciences says this “best-of-both-worlds” effect is key to convincing the public, developers and city planners to go for the oasis style: They have to Relying on a water-friendly design won’t exacerbate urban “heat islands” that can turn less thoughtful sections of the Denver, Las Vegas or Phoenix Metro into a daytime hellscape.

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In addition, the oasis designs are simply more pleasant in the eyes of many residents.

“Removing peat grass from the landscape is an excellent approach to conserving water, but if we remove all of the peat grass, the temperature goes up,” Saher said. “For every hectare of peat grass removed, we must also plant native and/or rain-fed trees to make arid cities livable long-term.”

Water analysts praised the report, noting both the number of metropolitan areas where turf is being saved and the abundance of locations in Colorado where the oasis techniques are being demonstrated on site.

“Even over the past year, the momentum just keeps building,” said Lindsay Rogers, water analyst at the nonprofit Western Resource Advocates. “I’m glad these institutions are trying to gain a better understanding of the landscaping materials that could reduce or worsen the urban heat island effect, because this is a critical issue when we talk about best practices for aquatic landscaping, and it’s coming up a lot.”

Rogers would like to see the same scientific study of Colorado city environments to measure whether local practices produce lower temperatures than similar designs in Arizona or Nevada. Colorado landscapers typically use more wood-based mulch than stone to replace large patches of grass, and she wonders if these different materials would change the dimensions of the heat island.

Colorado developers and water researchers are demonstrating different levels of oasis style along the Front Range in an encouraging display of drought and climate adaptation, Rogers said.

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Aurora’s Painted Prairie housing development is building water conservation into its public parks and housing development. Northern Water, which supplies dozens of communities in the North Front Range, has a number of demonstration gardens at its Berthoud headquarters. Denver Botanic Gardens, Colorado State University and others are also experimenting with plant and material mixes that can conserve water while cooling the landscape, she said.

Completed and under-construction homes provide the backdrop for raised beds at Columbine Gardens. (Kathryn Scott, special for The Colorado Sun)

Meanwhile, cities discouraging traditional curb-to-curb turf are trying to promote alternatives to the southwestern desert-style landscaping that has given xeriscaping a bad name.

“We are seeing many of our cities incorporating parameters into their lawn replacement programs that recognize the benefits of living plant materials, from reducing temperatures to creating more habitat for pollinators and providing shade,” Rogers said. This may include a minimum percentage of living plant matter or a minimum number of trees.

“They’re really trying to balance the need to reduce water use in the landscape with all the other essential benefits that our landscapes have to give us,” she said.

The Desert Research Institute study, with co-authors from Arizona State University and the University of Nevada Las Vegas, also looked at the effects of landscape on nighttime temperatures and even whether the placement of buildings or homes causes its own climate effect.

The mid-size oasis style, while cooler during the day, didn’t show cooler temperatures than overnight xeriscaping, Saher said. The oasis design does not cool down as quickly at night as a traditional green lawn. You cannot fully explain why that would be true.

And it seems that building placement can create chilling effects similar to lawns by strategically targeting the shadow the structures cast.

Saher says her next project is to compare the increasingly tight-fisted Southern Nevada Water Board’s official watering recommendations with how residents actually water their lawns.

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“To see how far away or how close we are,” she said.

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