Testing Climate-Ready Landscape Plants Benefits Growers and Consumers
July 15, 2024 
The Climate Ready Landscape Plants project took place at University of California, Davis, and five other trial sites from 2021 to 2023. Photo: Jared Sisneroz/UC Davis
The Climate Ready Landscape Plants Project has been in the making for 20 years. With water availability a rising concern for many gardeners, drought-tolerant plants have become a trend. University of California (UC), Davis was right on time with this project, and it has expanded to much of the western U.S., including UC South Coast Research and Extension Center, the University of Washington, Oregon State University, Utah State University, and the University of Arizona.
The Climate Ready Landscape Plants project was initiated as the University of California Landscape Plants Irrigation Trials (UCLPIT) project. It evaluates the performance of plants exposed to three different deficit irrigation treatments based on reference evapotranspiration (ETo), according to Dr. Loren Oki, a retired professor from UC Davis.
This project started in 2004 as a graduate student research project, which was expanded periodically at UC Davis and then duplicated at the UC South Coast Research and Extension Center in 2017. Three years ago, the project expanded further to other universities, which facilitated the comparison of plant performance in response to deficit irrigation treatments across the western region of the U.S. Oki says the Climate Ready Landscape Plants project is a two-year trial, which took place from 2021 to 2023.
The data collected leads to recommendations for irrigating plants that are used by landscape designers and architects to design water-conserving landscapes and to comply with California’s landscape water conservation regulation, the Model Water Efficient Landscape Ordinance (MWELO), says Oki.
Now that Oki has retired, Jared Sisneroz serves as the Project Manager. Sisneroz started working with Oki on the irrigation trials while he was an undergraduate in 2008 before becoming a staff researcher, and currently is a Ph.D. student.
Trial Plants that Growers Want to See
In the irrigation trials that started in 2004, researchers selected plants to trial. The UC Davis Arboretum and Public Garden developed a list of 100 plants for the Central Valley. Researchers wanted to validate the plants on that list by evaluating them in trials. More recently, the trials have made a pivot toward industry-submitted plants.
“The rationale behind that was that we are not plant producers or nursery growers. To have the greatest impact, we want plants that growers or nurseries want to submit. Those are the plants that they want to grow and that’s what they want to learn about,” Sisneroz says. “By using that avenue, we can grow plants that people find at the nursery, garden center, or big box store.”
In the first year of the Climate Ready Landscape Plants project, UC Davis leveraged its network. For example, Sisneroz says Dr. Ryan Contreras, Professor and Associate Head of the Department of Horticulture at Oregon State University, is one of the few ornamental plant breeders at a university on the West Coast. UC Davis evaluated Conteras’ hibiscus in the trials. For the second year of the Climate Ready Landscape Plants trial, each university worked with an advisory committee to select plants.
“Each of our six sites has someone external in the industry on an advisory committee,” Sisneroz says. “In our second year of the trial, we asked the committee if there are plants that we should select — plants that have merit that we should evaluate.”
With the advisory committees at each site, unique varieties were introduced to the trials, including sterile lantanas from the University of Florida and varieties from the Chicago Botanic Garden.
Highlights from the Trials
The trial includes two growing seasons. Varieties are planted in the fall or the fall and spring, Sisneroz says. The first growing season is from spring to fall, and the plants are irrigated regularly in the first year so they can establish in the soil.
“Low-water-use plants are not low-water-use right out of the pot,” he says.
Deficit irrigation begins in the second growing season. UC Davis has developed a plant growth index to see if the irrigation treatment affects the plants’ size. Researchers collect data on the plants’ aesthetic measurements, such as floral abundance. They also study the overall health of the plant, including pest and disease resistance and vigor.
“We have open houses at the beginning and end of the deficit season,” Sisneroz says. “It’s helpful for us to get input from people in the horticulture, nursery, and landscape industries. We get a broader range of feedback. One thing we are cognizant of is that our recommendations match industry norms and expectations.”
There was a small group of plants that were trialed at all six locations of the Climate Ready Landscape Plants project. The rest of the plants included in the trial varied for each region.
“The year we tried to do the trial, we had a wet year at UC Davis. We had a cool, wet winter and spring. It was not the best year to evaluate plants from desert states,” Sisneroz says. “But it was helpful for us to learn which plants do well here. For example, Penstemon ‘Diablo’ is from a drier climate. It had no issues with our cool, wet winter. That is a silver lining. Even when plants die, you can learn something from it.”
The biggest challenge is selecting plants that will do well at all of the trial sites, including the cold in Utah, the rain in Seattle, and the heat in Arizona. There were a few surprises in the trial results.
For instance, UC Davis did a pair trial with Lantana ‘New Gold’, a common cultivar in the region, and a new cultivar, ‘Bloomify Red’. ‘New Gold’ survived, but ‘Bloomify Red’ perished because of the cold, wet winter.climatereadyplants.ucdavis.edu
“It was really interesting because lantana is commonly found at nurseries in this area,” Sisneroz says. “It reinforces the notion that it won’t assuredly do well here.”
He also highlighted a crape myrtle, Lagerstroemia ‘Centerstage Red’, which has dark burgundy foliage and watermelon-red flowers. Sisneroz said it had no issues and continued to bloom into the late fall.
“We were excited to see how ‘Centerstage Red’ emerged. It scored really high for us,” Sisneroz says. “You can’t miss it. That won’t fade into the background.”
The full report from the Climate Ready Landscape Plants project at all six sites will be available later this summer at . The report includes all collected data, graphs, charts, and photos. However, researchers realize that growers may not have the time to sift through all of that information. Therefore, universities chose to highlight a few plants that were highly rated during the trials with three curated lists.
Blue Ribbon plants had a good performance over the deficit season on low water. Happy Medium plants had good performance on the medium irrigation treatment, and Fan Favorites received high marks from consumers during open houses.
More Trials in the Future
Sisneroz says UC Davis hopes to continue its trials in the future. As for any university, trials can be a challenge because they are expensive and funding is limited. He says the university is grateful to have funding from USDA and the California Department of Food and Agriculture (CDFA).
“Each time we apply for a grant, we have a slightly different focus. That helps us be competitive and innovative,” he says. “Now, we’re working on evaluating vines to shade buildings as a stopgap while you’re waiting for a tree to grow.”
Sisneroz says UC Davis is lucky to have so much industry support, both from breeders that submit plants and other industry professionals, including Star Roses and Plants, Spring Meadow Nursery, Bailey Nurseries, Mountain States Wholesale Nursery, and Civano Nursery.
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