Lagerstroemia Delta Eclipse
Plant photos courtesy of Karrie Reid

For areas prone to drought, low water use plants could provide growers, retailers and landscapers with a steady revenue stream. An uncommon and valuable trial at the University of California, Davis provides the data needed to know which plants use the least amount of water in the landscape.

The UC Landscape Plant Irrigation Trial (UC LPIT) evaluates perennial landscape plants for two years and the plants’ performance is based on varied levels of reduced irrigation. Planted in the fall, the trial plants are established the first year with regular irrigation – roughly weekly during the summer with 8.3 gallons applied during each irrigation event.

By the second spring, the crew begins deficit irrigation at three levels based on percentages of what a well-watered, full-cover turfgrass surface 8-15cm in height uses (also called reference evapotranspiration or ETo), explains Karrie Reid, environmental horticulture advisor at the University of California Cooperative Extension.

Lagerstroemia ‘Purple Magic’

“We don’t let them dry out and the plants are watered deep that first year,” she says. “Our target is a ½ meter deep. And in the second year we only irrigate in the summer.”

Dietes ‘African Gold’

During that second season, plants are irrigated with one of three different irrigation frequencies that correspond to the Water Use Classification of Landscape Species (WUCOLS IV, https://ucanr.edu/sites/WUCOLS) categories of low, moderate and high. These categories are based on percentages of reference evapotranspiration with local weather station data used for estimates.

In the second season, the crew waits to irrigate until 50% of the plant’s available water in the soil has been used up. The highest irrigation treatment in the heavy clay soil averages around every 10-14 days, Reid explains. And the lowest irrigation treatment typically equals about twice a summer.

“For a sandy soil, that irrigation treatment would be more frequent,” she adds.

At the end of two years, plants are rated for foliage quality, flower quantity, vigor, health and overall appearance. Height and width are measured monthly to calculate a growth index for each species at each irrigation level. Monthly plant growth measurements (l x h x w) are used to calculate a Plant Growth Index (PGI= [(l+w)/2 + h]/2).

Overall appearance, flowering time and duration, and pest or disease problems are rated monthly to provide a comprehensive assessment of performance, allowing the trial managers to make irrigation recommendations for these plants.

Plants are deemed low water use (20% ETo), moderate water use (50% ETo) and high water use (80% ETo).

Rosa Pink Knock Out

The trial and the recommendations are a boon to green industry professionals in California because of the strict water use laws.

“The California state ordinance regarding landscape water conservation says that if you build a new landscape or renovate a landscape over a certain size, you have to calculate an estimate of the annual water use for that landscape,” says Loren Oki, UC Davis Plant Sciences faculty and Cooperative Extension Specialist. “To do that, you need species coefficients for irrigation management, but those coefficients exist for just over 4,000 plants, and there are an estimated 10,000 to 20,000 cultivars being used in California landscapes.”

Vitex Delta Blues

There are currently two trial sites – the original at UC Davis and a second at the University of California Agriculture and Natural Resources South Coast Research and Extension Center in Irvine. A Specialty Crops Block Grant from the California Department of Food and Agriculture provided the funds to set up the duplicate trial in Southern California. The trial team was recently awarded a grant from the Horticultural Research Institute, the research arm of AmericanHort.

The Davis trials are in USDA Hardiness Zone 9b in full sun. All plants are trialed in silty, clay-loam soil, Reid says. Each plant is spaced either 2 or 3 meters apart based on planting recommendations from the breeder. Plants are protected by 3 inches of mulch and all are on drip irrigation. There are eight plants of each variety in the trials. In Irvine, the trials are made up of sandy soil and plants are irrigated with reclaimed water.

(L-R) Karrie Reid and Loren Oki want to share trial results with growers, breeders, landscape professionals and consumers.
Photo courtesy of UC Davis

“We’re conducing the only irrigation-based plant trials,” Reid says. “In California, any time a new development or a permitted new landscape is approved, that project has to create a water budget based on the types of plants they use. WUCOLS is helpful, but it’s a static list as of now,” she says. “When the industry comes out with new plants every year, how can the developers and landscape professionals spec that plant if they don’t know its water needs?”

The trial staff wants to get the word to the green industry and explain the value of these ratings.

“There is tremendous value in identifying low water use plants that provide low water use lush to landscapes,” she says.

A more detailed website dedicated to the trials is in the works. For now, the industry and consumers can visit the UCLPIT Facebook page at www.facebook.com/UCLPIT.

This spring, the trial hosted an open house for landscape and horticulture professionals, as well as educators. Participants rated a representative sample of the plants in the field.
Photo courtesy of UC Davis

Recently, five plants received the UC LPIT Blue Ribbon designation, the trial’s highest award for plants that consistently performed at a very high level on low water, or 20% of ETo. The Blue Ribbon goes to: Dietes ‘African Gold’, Lagerstroemia Delta Eclipse, Lagerstroemia ‘Purple Magic’, Rosa Pink Knock Out and Vitex Delta Blues. These plants received only two irrigations between April and October – on June 25 and August 20 – for a total of 5.6 inches of applied water, or 29 gallons per plant. On this sparse irrigation treatment, these plants rated at least 4 out of 5 possible points in the category Overall Appearance over the entire period.