Many Utah cities want to ditch thirsty grass. But trees are a different story.
It comes down to cooling. As climate change ramps up Utah heat, a leafy canopy provides relief from the blistering sun in ways that grass can’t.
“Ask yourself: Would you rather park next to a small lawn or would you rather park under the shade of a tree?” said Doug Bennett, conservation manager with the Washington County Water Conservancy District. “It makes the point very quickly.”
On a bright, 100-degree day in St. George, Bennett walked beneath the shadow of a mesquite tree. It’s one of roughly 200 trees growing at the Red Hills Desert Garden, which the conservancy district runs on a cliff above downtown.
He aimed a handheld infrared thermometer at the concrete sidewalk next to his feet. The shaded surface read 93 degrees. The exposed asphalt parking lot nearby registered 45 degrees warmer.
That’s a good example of how Southwest native trees like mesquite, desert willow and palo verde can mitigate urban hot spots while barely sipping on the region’s precious water.
“This garden uses about 11 gallons per square foot per year,” Bennett said. “That is about one-fifth of what people would apply to a lawn.”
Now, the water district is paying residents to swap their grass for some drought-tolerant shade. This summer, it began offering a $100 incentive to motivate people to plant trees as part of a lawn makeover.
Residents can qualify for the “treebate” by notifying the district of their plans and confirming the desired species is on the list of climate-appropriate options. People can get one tree rebate for every 700 square feet of landscaping that’s replaced. In northern Utah, the Jordan Valley Water Conservancy District and Central Utah Water Conservancy District offer a similar rebate at $50 per tree.
Just one-tenth of a typical urban area is devoted to landscaping, Bennett said.
On one hand, that means cities can’t expect plants to fix all their warming problems. Dark roofs, asphalt and concrete block walls contribute to heat islands, too. On the other, it means cities need to get the biggest possible impact from that small sliver of green space. Grass isn’t holding up its end of the bargain.
“The biggest misconception is that lawns are cooling our urban areas,” Bennett said. “Don't get me wrong — if you lay down on well-watered grass, you're going to feel a cooling effect. That cooling effect is not the grass. It is the water being transpired out of that grass and past you to the atmosphere.”
That’s a big part of why grass requires so much irrigation, he said, which runs afoul of the district’s conservation efforts.
One recent study of desert cities worldwide suggests that grass can actually warm things up because the water it releases has minimal cooling impact and its dark leaves absorb plenty of the sun’s heat.
That finding may be skewed by parts of the world that don’t irrigate grass as much as the U.S. does, said Diane Pataki, chief scientist and vice president of science with the conservation group National Wildlife Federation. But it points to a tricky balance arid communities need to strike.
“If we stopped irrigating cities, they'd get a lot hotter,” Pataki said. “So, we have to use some water, and the question is how much.”
Pataki previously researched urban heat and landscaping as a professor at the University of Utah, seeking to find the best tradeoff between landscape cooling effects and water use.
Because trees generally need a different style of irrigation than lawns — less frequent, more targeted — she said they can provide a lot of cooling on a small water budget. Trees can even make grass less thirsty.
“We found that adding trees to landscapes can lower the water requirement of the landscape, even if you don't do anything else,” Pataki said. “It's counterintuitive because you're adding more plants, and you think, ‘Oh, I have to add more water now because I added more trees.’ But the shading effect reduces the water requirement of the lawn so much that you can actually lower the water requirement of the whole landscape.”
That illustrates how the benefits can start with a single tree, she said. Another high-impact planting spot is on the west side of a home, where a tree could keep the living space cooler and reduce air conditioning demand.
“It's definitely worth doing in your own backyard,” Pataki said, “even if you're the only person on your block doing it.”
As Utah cities rapidly grow and the state gets hotter, she said it’s vital for communities to consider trees’ cooling benefits as part of their future designs.
