Resilient plants

Extreme weather conditions are impacting the way plants live in different environments, and the demand for climate-ready plants in the horticulture industry is becoming more prevalent. The weather challenges people face across the country are changing the way researchers and growers view plant production. Providing low water-use plants to landscapers, independent garden centers, wholesale and retail nurseries will be critical to meet the needs of consumers, standards of state regulations and water conservation trends.

“People are very interested in drought tolerant landscaping,” says Lloyd Nackley, associate professor of nursery production and greenhouse management at Oregon State University. “We are experiencing that the public is very aware of climate change, and it’s not something to shy from. It’s something to embrace.”

Soo-Hyung Kim, professor of environmental and forest sciences at the University of Washington, explains that one way climate change has already affected the horticulture industry is through the hardiness zones. The right plants to plant in the right zones are evolving, and the hardiness zones that growers use as a guide for selecting plants are changing rapidly.

“Keeping up with these changes is going to be a challenge,” Kim says.

The Climate Ready Plant Trials is a region-wide Western U.S. plant trials program. It is a collaboration between six universities to observe how plants respond to different irrigation treatments in different locations and climates. There are trial sites in Davis, California, Irvine, California, Portland, Oregon, Seattle, Washington, Logan, Utah and Tucson, Arizona.

This project is being developed in conjunction with the nursery industry, and there is an advisory board of nursery industry professionals who help select the plants for the trial. The plants are trialed in full sun or 50% shade cover, and three levels of irrigation treatments are provided to the plants at 20% (ETo), 50% (ETo) and 80% (ETo). The amount of irrigation that will be delivered is determined by evapotranspiration (ETo) data which is obtained from a local weather station.

This project has identified several plants that can not only survive but can thrive and continue to look beautiful with low water.

Kim explains that water is already a scarce resource, so it is critical for the nursery industry to get ahead of the curve and prepare to grow and select water-wise plants that will also provide other ecosystem services.

“We are seeing stronger interest and need for waterwise plants across the country,” says Katie Tamony, chief marketing officer at Monrovia. “This California and Southwest trend is now spreading to the Mountain West, even parts of the Midwest. Home gardeners are on the lookout for plants that have lower water needs, thrive in colder climates, and still add color and interest in the garden.”

The Climate Ready Plant Trials is a unique program that offers a multilayered wide plethora of research and data to growers across the country. It is an extension of the original project started at UC Davis in 2004, the UC Landscape Plants Irrigation Trials (UCLPIT).

“It is a useful marketing tool because we can advertise the Blue Ribbon winners as exceptional roses for low water use,” says Kristen Smith, rose evaluation manager at Star Roses and Plants. “The results for water use recommendations are published on the WUCOLS IV Plant List that can be found on this website: https://ccuh.ucdavis.edu/wucols.”

Smith explains that knowing the drought tolerance and superior performance of some of the varieties under low water conditions will hopefully give end consumers the confidence that their purchase will be successful in their home landscape, especially given the current extreme climate conditions.

“So often plants in the nursery industry are grown in one region but sold to another, at least that’s very common in Oregon, where 75% of our plants are shipped outside of Oregon,” Nackley says. “We don’t know exactly what’s going to happen with climate change, but what we’re seeing is a lot of extreme weather events. We’re seeing hotter summers, colder winters and late season frost. By having this large network of studies, we get to see if it is going to get hotter, how did [the plant] do in Arizona? If it is going to be colder, how’s [the plant] doing in Utah. Then also how’s [the plant] doing in these wetter sites if it is going to be wetter in the spring.”

The plants researched at the trial gardens in Washington showed little difference and no stress after being provided with the lowest level of irrigation treatment (20%) compared to the highest level of irrigation treatment (80%). This result was across all the species tested.

“So, in our area, reducing water by more than half, still provided very comparable plant aesthetics and performance,” Kim says.

Ursula Schuch, extension specialist and professor at the School of Plant Sciences at the University of Arizona, explains that extreme high temperatures and late low temperatures continue to accelerate and be a challenge for plants in her region.

“We had some really unusual late freezes in the spring that damaged some plant material, and the persistent high temperatures over 100 to 110 degrees Fahrenheit are a challenge to some of the plant material,” Schuch says.

Based on the trial, one of the top performers was Dodonaea viscosa ‘Emerald Ice,’ commonly known as hop bush. Schuch explains that the ‘Emerald Ice’ plants provided with the lowest irrigation treatment (20%) grew just as well as the plants that were provided 50% and 80%, and there were no significant differences in the growth.

‘Emerald Ice’ was discovered after surviving a freeze between five to 10 degrees Fahrenheit with minimal damage. Some of the older genus and species have limited cold hardiness and often become severely frost damaged and eventually killed if the temperature is in the low 20s.

“You can grow four times as many hop bush under the lowest irrigation regime (20%) compared to the highest (80%),” Schuch says. “So, if people would only irrigate as much as it’s needed, that would be very helpful in terms of conserving water in the long run.”

Some other plants that impressed Schuch during the trial and performed well under the lowest irrigation treatment (20%) were Leucophyllum frutescens ‘San Antonio Rose,’ Leucophyllum langmaniae ‘Rio Bravo,’ Ericameria laricifolia ‘Aguirre,’ Rosmarinus officinalis ‘Arp,’ Eremophila glabra ssp. Carnosa ‘Winter Blaze,’ Tecoma hybrid ‘Red Hot’ and Hamelia ‘Sierra Red.’

“Hamelia ‘Sierra Red’ was very interesting because initially they all froze back, which is quite normal especially with a freeze event into the low 20s,” Schuch says. “Then in March and April, we weren’t really sure what was going to happen because we saw the top of the roots system sticking out a little bit. Once it started to come back by April and May, it just performed amazingly [well]. By fall it was definitely one of the favorites among the people who rated the plants during our field days.”

Karen Reid, environmental horticulture advisor at the University of California Cooperative Extension, San Joaquin County, explains that the Climate Ready Plant Trials research team decided to make the trial more accessible to the nursery industry and reached out to several growers asking if there were any plants they wanted to introduce into the trade in California that didn’t have a water-use classification available. The research team has since worked with over 25 growers within the state, nationally and internationally, to attempt to get growers to introduce more climate appropriate plants.

“If the plants that use a lot of water are being produced in the industry, and if those continue to be used in certain settings, then that is going to have detrimental effects in terms of total water use, but also in other areas where sustainability is an issue,” Kim says. “So, I think this is an area where the nursery industry can take leadership on which direction things should go in terms of getting things prepared for climate change and its impact.”