But overall, says Ian McRae, a University of California, Berkeley innovation designer who studies climate resilience in the built environment, lawns are an inefficient way to cool a green space compared to building a diverse grouping of native plants that are more aesthetically pleasing , Water -efficient and conducive to biodiversity. “We love our lawns for different reasons,” says McRae, “but they’re overrated and overused compared to the variety of planting pallets that we have at our disposal to create spaces we love to be in, spaces that are owned by cooling and far better performance from a water usage standpoint.” (He was not involved in the new research.)
Much of the cooling attributed to lawns actually comes from the soil itself, says McRae: Because grass is such a short plant, the sun hits the soil below directly, evaporating any water that has seeped into the soil.
As the world warms and urban populations grow, more and more city dwellers will be exposed to extreme heat. “That sounds like bad news, but it also means there’s an opportunity,” says Christa Brelsford, an environmental scientist who has studied cash-for-grass programs at Oak Ridge National Laboratory but was not involved in the new study. “This paper shows that through different landscaping decisions, there are small decisions people make regarding landscaping in urban forums that can have a significant impact on people’s heat experience.”
Still, scientists have some concerns about how this might play out in the real world. First, certain plant species may find it more difficult to survive in cities as the urban heat island effect intensifies. You would hate spending time and money on tree planting campaigns when all your trees are going to die in a decade. So, in addition to continuing to explore how different types of vegetation can help cool cities, researchers need to find out which types can withstand the heat.
Second, when a particularly bad heat wave hits, even plants that can survive the heat stop releasing water vapor, a defense mechanism to prevent dehydration. “You lose that evaporative advantage when it’s really hot,” says Ariane Middel, an urban climatologist at Arizona State University and a co-author of the new paper. But that’s when people need to cool down the most.
The trick is to green cities in a way that provides the most cooling with the least amount of water. “There is no one-size-fits-all strategy,” says Desert Research Institute’s Rubab Saher, who led the new study. “I wish they existed. It would make our life so much easier. But it even depends from one district to another.”
A particularly effective solution could be ‘rurbanisation’ or moving food production to cities. While grass just gulps down water, urban farms could become ultra-efficient, growing food using recycled wastewater while feeding residents, cooling neighborhoods and attracting pollinators — helping increase diversity as an arid landscape might. Bonus: Growing plants under rooftop solar panels could cool buildings and generate free electricity.
Urban planners may even be able to reduce heat and increase shade in places that many plants cannot tolerate. Scientists are experimenting with reflective roofs and sidewalks that reflect the sun’s energy back into space, lowering surface temperatures. And when a neighborhood can’t afford the water to grow thirsty trees, building a “shady arcade” stretching overhead might help. This cover could even be made of the same material as reflective roofs, further increasing cooling performance.
“You can have really beautiful shade structures that don’t have to be a tree,” says Stephanie Pincetl, director of the California Center for Sustainable Communities at UCLA, who has studied turf replacement programs but was not involved with the new paper. “We’re so stuck. We have no conceptual architectural imagination.”
So what we need is fewer boring lawns and more creativity.