Mid-summer plant evaluations for University of California Landscape Plant Irrigation Trials

A year ago I blogged about the irrigation trials at UC Davis. As I said at the time, the objective is to determine how ornamental plants fare when they receive different amounts of water and are irrigated increasingly less frequently. The ultimate goal is to find the sweet spot where a plant still looks good with as little irrigation as possible.

On the UC Landscape Plant Irrigation Trial website, the project is described like this:
UC Davis horticulturists are evaluating landscape plants with the potential to be good performers in low-water use gardens. [...] the plants are exciting new cultivars provided by growers and breeders who want to evaluate their new plant varieties for low-water use in hot California gardens. The results of these trials are providing growers and retailers the information they need to successfully distribute and market these plants to the public.

This is how the project works:
After being grown for a full year on a regular watering regime [roughly weekly during the summer with 8.3 gallons applied during each event] to establish deep, healthy roots, plants are irrigated through the second growing season with one of three different irrigation frequencies that correspond to the Water Use Classification of Landscape Species (WUCOLS IV) categories of Low, Moderate, and High.  (These categories are based on percentages of reference evapotranspiration with local weather station data used to estimate these percentages.)
According to a recent article in Nursery Management magazine, “the highest irrigation treatment in the heavy clay soil averages around every 10-14 days, [and] the lowest irrigation treatment typically equals about twice a summer.”

The team performs monthly appearance evaluations of the trial plants. In addition, larger-scale evaluations are held in the spring, summer and fall where local horticulturists, landscaping and nursery professionals, master gardeners, and garden communicators are invited to walk the growing grounds and rate the plants growing under the different irrigation schemes.

Since my debut last summer, I've participated in every rating event. Yesterday was my fourth time. We performed a mid-summer assessment of this year's plants; the final evaluation will be in September. The next crop is well underway (it was planted last fall), and evaluations will begin in spring of 2020.

The trial plants are grown in three blocks which are irrigated at different intervals. Plants are chosen for evaluation on a random basis (presumably the same number from each irrigation block). Participants are asked to rate each plant on a scale from 1 to 5 in three different categories: foliage quality; flower quantity (not quality); and overall appearance. A plant with a 0 for overall appearance looks so bad that nobody would ever buy it. A plant with a 5 for overall appearance, on the other hand, is a real standout.

As I had noticed before, there's often very little difference in appearance between the different irrigation schemes. In other words, the Flip Side chaste tree or the Kings Fire grevillea you see in the photos above and below look equally good, irrespective of whether they're watered more frequently or less. That's what you want in a water-wise plant.

Flip Side chaste tree (Vitex × 'Bailtexone')

My favorite in this year's batch is the Kings Fire grevillea. Some specimens at UC Davis would look even better with judicious shaping, but in terms of vigor and overall appearance, it's a knockout. And it seems to be a virtually constant bloomer to boot.

Grevillea 'Kings Fire'

Meerlo lavender (Lavandula × allardii 'Meerlo') and Kings Fire grevillea (Grevillea 'Kings Fire')

I'm not a fan of traditional foundation shrubs, but I find the Olive Martini elaeagnus below very elegant.

Olive Martini elaeagnus (Elaeagnus × ebbingei 'Viveleg')

The most polarizing plant, based on comments I've overheard, seems to be this cute little dianella. Yes, I get it: It looks tousled and unkempt, but that's its nature. That's exactly why I like it.

Coolvista dianella (Dianella revoluta 'Allyn-Citation')

Crepe myrtles are perennial favorites here in the Sacramento Valley because they thump their virtual noses at our hot summers. The Bellini Raspberry below is one of a new generation of lagerstroemias that are bred for compact size, 3-4 ft. in this case. (I'm trialing an ever smaller cultivar for Walters Gardens in our own garden.) That means that every homeowner can have a crepe myrtle—even those with patio-sized lots.

Bellini Raspberry crepe myrtle (Lagerstroemia indica ‘Conlagras'), low-growing to 4 ft.

This year's batch of plants contains three roses. I'm not a fan of roses but I was impressed by how well they're doing in full sun, even on the stingiest irrigation regime, and how clean they are—I didn't see any disfiguring spots or pests.

Brick House rose (Rosa × hybrida 'Meitraligh')

In addition to UC Davis, the UC Landscape Plant Irrigation Trials team also has test fields at UC Irvine in Southern California. Since daughter #2 will be a student at UC Irvine as of this fall, I'm hoping that I'll be able to participate in one their ratings events in the future. It would be very interesting to see which plants they're trialing in the Goldilocks climate of Orange County.

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  1. This is such important work! And that's been my experience with 'King's Fire' as well, so nice to have it validated.

  2. I'm very impressed by this study and kudos for your participation!

  3. Very interesting and useful study. My 'Kings Fire' still looks wretched--compared to 'Superb', 'Peaches and Cream', and the rest. Wonder if I just got the two in a bad batch.

    The Walters Gardens Lagerstroemia will no doubt grow larger in CA than in the upper Midwest--longer growing season, no die-back due to winter freezes.

    That rose, though, why bother? Roses are meant to be luxurious, opulent indulgence, not little rags of gaudy color. Would rather have a Grevillea!

  4. Hello Gerhard, I was there on Tuesday, my first time. I am not a fan of Lavandula plants and yet that "Meerlo" really impressed me as well as the Elaeagnus, the Grevillea, and the Vitex. Cheers, Linnea the Lion-Arted!

  5. Hi Gerhard, sorry I missed you, but the spring days conflicted with a big horticultural trade show in Ohio... I'm laughing at the "tousled" comment on our Dianella Cool Vista. That variety has hardly bloomed at all for me in Berkeley, but would have been covered in blue flowers just a few weeks earlier. Timing is everything :)


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