Expansive study evaluates climate-ready landscape plants for the western U.S.

By Loren Oki and Jared Sisneroz

We have all experienced extreme weather conditions, from heavy rain and snow to prolonged droughts, which have become more frequent in recent years.

In response to the droughts that have threatened our water supplies, regulatory restrictions have been put in place to conserve water by limiting urban landscape irrigation. However, it is crucial to ensure the sustainability of our urban landscapes due to the environmental, social and economic services they provide.

Therefore, it is wise to use plants that contribute to the function of these landscapes, yet require minimal amounts of water to sustain them. There is increasing awareness and demand from retail customers for low-water plants.

This project aims to respond to that demand by examining how plants perform under different levels of irrigation based on weather, including high, moderate and low levels. Based on the results, recommendations will be developed and shared for how to irrigate the plants.

This information can be used to design landscapes that require low amounts of water and comply with regulations such as the Model Water Efficient Landscape Ordinance in California, which requires the estimation of annual landscape water use for new and certain renovated landscapes.

From an industry perspective, this project seeks to establish facts based on anecdotal recommendations and identify high-performing, low-water plants.

Study parameters

The California portion of this study began in 2004 as a graduate student research project examining a suite of mainly California native plants by our colleague, Karrie Reid, a retired University of California (UC) Cooperative Extension Advisor who was pursuing her master’s degree. The project was later expanded to include more fields and a shade house, and trials were opened to evaluate plants submitted by plant breeders, developers and nurseries.

In 2016, a California Department of Food and Agriculture (CDFA) grant was obtained to replicate the UC Davis trials at the UC South Coast Research and Extension Center in Irvine, California, allowing for the comparison of plant performance in northern and southern locations in the state. This grant also enabled the testing of cotoneaster genotypes developed at Oregon State University for disease resistance and water use.

To date, the program has evaluated more than 220 plants at UC Davis and more than 70 plants at South Coast Research and Extension Center, and it is known as the University of California Landscape Plant Irrigation Trials™ (UCLPIT).

In 2020, a CDFA/USDA grant supported the replication of the full sun fields at the University of Washington (Seattle), Oregon State University (research site at Aurora, Oregon), Utah State University (Logan, Utah) and the University of Arizona (Tucson, Arizona), allowing for the examination of plant performance across the Western Region of the U.S.

As the climate changes, future weather conditions are expected to become warmer and drier, with Sacramento’s climate resembling Phoenix’s current climate by the end of the century. With trial sites located across the West’s climate gradient, the project is uniquely positioned to respond to these changes.

The plant palette for the trials consists of a mixture of taxa grown at all six sites, regional selections grown at multiple sites, and local taxa grown only at a single site.

Researchers worked with cooperators to invite plant submissions from the industry and an advisory committee with representatives from the nursery industry and botanical gardens to select plants for evaluation. For example, at our site at UC Davis, we are growing research selections developed at Oregon State University, rose cultivars available nationwide, and species grown in Arizona that are rarely seen locally.

Methods

To evaluate how plants respond to water stress, we conduct multi-year trials starting in late fall or early spring. We use 24 individuals of each taxa, representing what a retail consumer would purchase, usually in a #1 sized container.

During the first irrigation season, from April to October at UC Davis, the plants are irrigated regularly to support healthy shoot and root growth, and to establish the plants in the native soil. Although we may need to irrigate during unusually dry winters, the plants rely solely on rainfall through the second fall and winter until the start of the second irrigation season.

During this time, we provide irrigation to the plants at one of three levels based on reference evapotranspiration (ETo), which is the estimated water used by a reference crop (in our case, cool season turf), determined by a local weather station. The three irrigation treatments are 80%, 50% and 20% of ETo for the high, moderate and low treatments, respectively, and the percentages function as a crop coefficient to reduce daily ETo.

In California, we obtain weather data from the California Irrigation Management Information Systems (CIMIS) program, which has stations across the state providing weather and crop water use data for growers, including a weather station on the UC Davis campus. By locating the trials at universities, the trial fields are often near existing weather stations, such as in Oregon, where the weather station and plant trial field are in the same research site.

We determine the amount of water required to rewet a soil cylinder that is 1 meter in diameter and ½ meter in depth, based on the soil characteristics in our fields, as this is where the plant roots are located. Using weather data, we estimate when 50% of plant available water in this cylinder has been depleted and apply this volume of water at each irrigation event. The volume of water applied is constant across all treatments, and the ETo percentage for each treatment adjusts the frequency of irrigations and consequently, the number of irrigations per treatment over a season.

During the 2021 testing season, our fields at UC Davis, which are located on a silty clay loam, had the 80% treatment receiving irrigation every 10 days on average, while the 50% treatment was irrigated about every 15 days, and the 20% treatment every 36 days between mid-April and the end of September. It is noteworthy that the 20% interval is an extreme treatment. During the establishment season, the irrigation frequency is set at 80% of ETo.

To evaluate plant performance during the irrigation treatments, we collect data on plant width, length and height monthly, and calculate the Plant Growth Index (PGI) using the formula: PGI = [(l+w)/2+h]/2, where l, w and h represent length, width and height of the plant. To account for differences in height at the start of the season, we determine a relative PGI (PGIr) for each plant every month during the treatment season. PGIr is calculated as PGIm/PGIi, where PGIm is the PGI for the month, and PGIi is the initial PGI at the start of the season.

The performance of the plants is assessed through various qualitative and quantitative measures. Qualitative performance ratings are made monthly on various categories, such as foliage appearance, flowering abundance, pest tolerance, disease resistance, vigor and overall appearance. The ratings are on a scale of 1 to 5, with 5 being the highest. A second rating for flowering abundance and overall appearance is collected mid-month for each plant in bloom to track their blooming period.

Results

The PGI, PGIr, foliage quality, floral abundance, disease and pest resistance, vigor and overall appearance results are analyzed statistically to determine the irrigation recommendations. The lowest rate at which growth, plant health and aesthetics are not compromised is recommended as the optimal irrigation rate. Plants that score 4 or higher on overall appearance at the low-water treatment receive the Blue Ribbon Award, while those performing at that level on the moderate treatment receive the Happy Medium designation.

Open House Field Days are organized up to three times each year during the treatment season, where growers, retail nursery professionals, landscape architects and designers, agency representatives and master gardeners are invited to visit the fields to view and rate plants. Attendees are shown a plant in each treatment and asked to evaluate them. These visits serve as an opportunity to identify plant preferences, and favorites are recognized with the People’s Choice Awards. The events also allow potential users to see how new plants perform on different irrigation treatments and determine if they would work well in a client’s landscape. Many landscape designers have noted that these visits provide a better understanding of how the plants would look in a landscape, compared to viewing them at a nursery in a container.

The information that is developed is shared on the UCLPIT project website. Industry cooperators who submit plants for evaluation are provided with a copy of our report detailing the performance of the plant and on which irrigation category optimal performance was observed.

We will typically recommend irrigating the plants at the lowest treatment rate where visual aesthetics were not compromised. Our irrigation treatments align with those listed in the Water Use Classification of Landscape Species (WUCOLS), the resource of reference used for calculating landscape water budgets in California.

We share our results with WUCOLS so they can be added to the list and utilized to develop water budgets for new and renovated landscapes across our region. As a result, our trials offer a means to introduce irrigation recommendation for cultivars and species introduced since WUCOLS was last updated in 2014.

About the authors

Loren Oki is a landscape horticulture assistant specialist at the UC Davis Cooperative Extension. He can be contacted at [email protected].

Jared Sisneroz is project manager of the UC Landscape Plant Irrigation Trials (UCLPIT) program. He can be contacted at [email protected].

Growing Knowledge from the June 2023 issue of Digger magazine | Download PDF