How Much Water Do Landscape Plants Need?

To meet California’s mandatory landscape ordinances for water conservation, Loren Oki, Karrie Reid, and Jared Sisneroz are conducting UC Landscape Plant Irrigation Trials to determine water needs for landscape plants. The goal is to develop water conservation landscapes in three locations and study their long-term performance.

UC Davis Landscape Plant Irrigation Trials

Regulations for landscape water use

California’s Model Water Efficiency Landscape Ordinance (MWELO) is rippling through the state’s large green industry. MWELO mandates that landscapes conserve water and it’s effective for any new or renovated landscape covering more than 500 sq. ft.

One of the ways in which the regulatory framework seeks to conserve water is through the judicious use of water on plants. Rather than simply turning the sprinklers on for a one-size-fits-all water approach, MWELO asks for an annual water budget for the landscape and a plan on how that water is to be used. Using the right amount of water for the plant in the landscape is key, rather than a one-irrigation-pattern-fits-all.

To get there, the state is implementing mandatory use of Water Use Classification of Landscape Species (WUCOLS). WUCOLS provides evaluations of the irrigation water needs for over 3,500 types of plants, including cultivars in some instances. While 3,500 might seem like a lot of plants, it’s not. The base calculations are based primarily on horticultural experience, not science. Enter UC Davis.

University of California researchers and extension specialists Loren Oki, Karrie Reid and Jared Sisneroz are conducting UC Landscape Plant Irrigation Trials to determine just how much water landscape plants really need. Loren Oki is a faculty member and Cooperative Extension specialist, and Jared Sisneroz is a researcher – both in the Department of Plant Sciences at UC Davis; Karrie Reid is a Cooperative Extension environmental horticulture advisor in San Joaquin County.

Loren Oki was in Columbus, Ohio, at AmericanHort’s Cultivate’17 garnering support for the water use trials among horticultural industry plant developers. While there, he also presented on his work.

Loren Oki evaluating plant performance in a field trial at UC Davis.

In addition to informing WUCOLS, Oki’s work is collaborative. He has formed partnerships up the Pacific Coast in Oregon and Washington. The plan is to develop water conservation landscapes in three locations and to study their long-term performance. Not only that, but using Specialty Crops Block Grant money that flows to the California Department of Food and Agriculture, Oki is extending the landscape water use trials to Orange County (Irvine). In the future, there will be a better understanding between Northern California and Southern California landscape plant performance under prescribed watering regimes.

The UC Davis trial

It’s a two-year process. Most rain falls from December to early March. July and August are typically rain-free. During Year 1, plants are established (planting is in November), where watering is “adequate” for establishment. During Year 2, plants receive irrigation based on evapotranspiration measurements. The high treatment is 80% (recommended for cool-season turf) followed by 60% (warm-season turf), 40% and the low treatment, 20%.

Irrigation is a fixed-water volume to water deeply. It’s applied beginning in late April after the trial is irrigated to field capacity, which then sets the ET “clock.” For a relative idea of how often treatments may be irrigated, the 20% treatment was watered twice from April to October (every 56 to 58 days); the 80% treatment every 12 to 18 days.

Ceanothus maritimus Valley Violet (photo) and Carex spissa are two species that performed well in in all irrigation levels.

Trials are randomized and replicated across treatments. Plants are measured monthly when width/height are recorded and qualitative measurements of appearance, flowering and disease/pest damage is rated.

“Landscape designers and landscape architects love the trial,” Oki said. “They can see the plants in the ground as they look in the landscape.”

UC Davis and Oki’s team are onto something of great interest. They’re in-demand speakers up and down California and the West, as well as Texas. Outside of the U.S., Spain has begun to use WUCOLS. And the UC Davis water landscape group is traveling to other water-stressed areas like Australia, Greece, and France, talking about the program.

You can access Loren Oki’s trial results at:

Like them on Facebook and keep up with their Open House Rating Days (conducted in late July) and other news. In addition to UC Davis support, the landscape water trials work is also supported by the California Association of Nurseries and Garden Centers, Elvinia J. Slosson Horticultural Endowment, Saratoga Horticultural Research Endowment and USDA Specialty Crops Block Grant funds through the California Department of Food and Agriculture.

(Adapted from original article by Debbie Hamrick, NewTerrain, Ball Publishing,, August 2017)

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