Lessons from the University of California Plant Trials

by Jodie Cook, ASLA, SITES AP

University of California Climate-Ready Field Trial growing grounds / image: Karrie Reid – used with permission

As a horticulturally obsessed landscape designer for most of my adult life, I’ve observed how our landscape irrigation practices, tools, and technologies have evolved over time. In the last few decades, we have radically changed our plant irrigation practices in public and private designed spaces, particularly in the West. While using a plant palette of so called ‘drought tolerant’ species, many large commercial landscapes, and managed communities such as mine, irrigate four times per week or more in a climate that has never experienced rain with such frequency. How many regions do experience this frequency of natural rainfall? I have often wondered, why do we irrigate non-lawn areas so much?

So, I was thrilled to be a participant in the Climate-Ready Landscape Plants trial evaluations at the University of California South Coast Research Center fields in Irvine, California. I was there, clipboard in hand, as a plant performance evaluator and was not involved in the research in any other way. I did, however, discuss the research at length with those who devised the trial. It was fascinating and eye-opening.

Evaluators observing ornamental species growth, health, and aesthetic qualities. / image: Karrie Reid – used with permission

This trial is a long-term irrigation deficit study initiated in 2004 by then graduate student, and most recently UC Cooperative Environmental Horticultural Advisor, Karrie Reid, under the supervision of UCCE Environmental Horticulture Specialist Dr. Lorence Oki, and in collaboration with universities throughout the western states. Originally confined to growing fields on the campus of UC Davis in Northern California, the study has now evolved to include southern California and additional fields in Arizona, Oregon, Washington, and Utah.

Plant trial evaluation rubric, University of California Climate-Ready Landscape Trials

This long-term plant evaluation project is important for a few reasons. First, it’s a deficit irrigation trial occurring in states that often suffer from drought conditions, most recently of historic proportions. Researchers are discovering exactly how far below reference evapotranspiration rates ornamental plant species can go while preserving their aesthetic qualities and requisite ‘wow’ factor. This means scoring as close to 5 on the trial’s 1 to 5 multivariate scale as possible. This is right up our alley as sustainability-focused landscape architects and designers. How little can we irrigate our landscapes to maintain high aesthetic performance? As the researchers discovered…not as much as we do right now.

Although the West is not in the dire drought condition we experienced earlier this fall, we know a few things for sure: we will find ourselves in drought again, and rainfall patterns are changing such that regions where drought conditions are not commonplace could experience drought in the future. This research is timely and important.

Growers involved in the trial selected common landscape species—shrubs, perennials, and grasses—that revealed unexpected drought resistance. The researchers uncovered surprising results, not only confirming that some of these species were indeed very low water users, but that with the correct soil preparation, irrigation frequency and duration, plus maintaining a three-inch layer of wood chip mulch on top of the soil, many other plants could be also.

Determining how little water a plant requires to retain high aesthetic value isn’t a totally unique experiment. Home gardeners do this informally all the time and agriculture does it with ample scientific rigor. But in ornamental horticulture, using a range of common landscape species, plant aesthetic performance as a function of measurable plant water reduction had not undergone a trial like this before.

Rosa ‘Limoncello’ growing under deficit irrigation / image: Karrie Reid – used with permission

The unique aspect of this ongoing trial is in how irrigation is applied. It is not a reduced watering duration in each irrigation cycle—say, applying ten percent less water, but still irrigating two or three times per week. In this case, it is a reduction in watering frequency over time, allowing the soil to dry to a specific level based on site soil texture before topping it up fully and allowing this oscillation to dictate ongoing watering frequency.

For example, if a plant is fully watered during one irrigation event, how many days will it take before the allowable root zone moisture depletion takes place? This aspect of the trial is significant because when automatic irrigation moved from agriculture to ornamental horticulture decades ago, the methods (spray, drip, etc.) and timing (often multiple times weekly) of agricultural irrigation followed. All plants—whether naturally low, medium, or high water—are still often irrigated on a weekly schedule. It wasn’t until relatively recently that weather-based controllers were manufactured to easily allow longer intervals of up to a month or more between watering events. Even with the focus on lower water species, the resulting irrigation is often delivered as a reduced percentage during each weekly set of irrigation run times—a reduced duration occurred, but the weekly frequency remained. For many low water plants (especially natives), this is not an ideal arrangement. It can lead to crown rot, invasion by pathogens, salt accumulation in soil, and to a significant waste of water over time.

Evaluating flowering shrubs and groundcovers / image: Karrie Reid – used with permission

The UC Climate-Ready Landscape Plants trials used a different method. After a one-year establishment period when plants were watered regularly at 25% root zone moisture depletion to foster healthy growth and a deep root system, the researchers began to alter the frequency of irrigation based on localized evapotranspiration rates. (Drip emitters uniformly placed at each plant provided the necessary level of precision for the trial, but the use of drip versus other irrigation methods was not specifically studied in this research project.) Irrigation frequency decreased slowly so plants would adapt by growing deeper roots to access soil moisture. Then, plant root zones were allowed to deplete 50% of plant available moisture. At each irrigation, that 50% was reapplied to refill the root zone volume calculated as 1m (3.3 ft) wide, by .5m (1.6 ft) deep for the trial’s primarily shrub species, plus a few grasses. The plants were not irrigated again until 50% of the root zone was depleted. Evapotranspiration rates were calculated daily but accumulated until the 50% depletion rate was achieved before irrigation was applied. The trial in which I participated did not evaluate native plants. The studied plants were a mix of common landscape species such as roses, larger flowering shrubs, and flowering groundcovers.

Three water requirements are used in the trial program: High (80% ETo), Moderate (50% ETo), and Low (20% ETo). The resulting irrigation frequency in the dry Mediterranean climate of my region revealed high water plants needing irrigation approximately every 10 days during the hottest months of the year, between April and October, and the low water plants every 51 days. In total, the high-water plants required 26.9 inches of water, and the low water plants required just 6 inches. Keep in mind that during the study period our region enjoyed just 1.56 inches of natural rainfall and the trial fields are in full sun under varying soil conditions.

Drip emitters placed at mulch surface at each plant / image: Karrie Reid – used with permission

It is relatively easy to calculate irrigation in this way—I teach the calculations to my students each semester—so, as professionals we can be reassured that when we irrigate designed landscapes using these methods it is also based on solid science, thanks to the researchers. Water plants deeply, infrequently, and use sustainable technology and methods when we do. We are heading into spring and another hot summer. Let’s remember these lessons from the University of California Climate-Ready Landscape Plants trials.

You’ll find more information about the trials here.

Jodie Cook, ASLA, SITES AP, is President of Jodie Cook Design, Inc., in San Clemente, California. It’s an exciting time to be working with landscapes and the people living and working within them! A paradigm shift is occurring in our midst and I’m delighted to be partnering with nature and like minded professionals on sustainability in landscape planning, design, and horticulture. In addition to landscape design and program management, I teach The Sustainable Landscape and Native Garden Design at Saddleback College. My education-based turf removal program, NatureScape, was featured in the ASLA’s Smart Policies for Climate Change exhibition. Jodie also serves as a leader for ASLA’s Planting Design Professional Practice Network (PPN).

One thought on “Lessons from the University of California Plant Trials

  1. David Hopman March 16, 2023 / 12:26 pm

    Thank you for this convincing information. It is a great way to make the case for balancing the root zone with the moisture in the soil column. I recently found out that one of my native polycultures on The University of Texas at Arlington campus was being watered four times a week–a little bit each time which is the opposite of what is recommended in your post. This Post is a very helpful short article to share with maintenance crews and supervisors which I have already done!


    David Hopman, ASLA, PLA

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