Climate-Ready Plant Trials Underway at UW Botanic Gardens
UW Botanic Gardens (UWBG) has launched a longitudinal research project to help identify some of the best water-wise plants for our region. The findings will help the green trade and gardeners in the Puget Sound make smart plant choices for the hotter, drier summers climate scientists predict for the years ahead.
“My collaborators and I have been talking about doing this project for almost ten years,” says Professor Soo-Hyung Kim, UWBG faculty and Byron and Alice Lockwood Endowed Professor in Forest Resources at the UW School of Environmental and Forest Sciences.
Kim’s research dream, officially known as the Climate-Ready Landscape Plants Project, has come to fruition in the grassy field south of Merrill Hall at the Center for Urban Horticulture (CUH). The 0.4 acre research site, known as Plot 1, is filled with precise rows of roses, mock-orange, mahonia and other popular species being maintained by students and visited by pollinators.
The UW is one of six universities in western states conducting this plant irrigation trial. University of California, Davis (UC Davis) originated the project, which is designed to assess plant performance under three different irrigation regimes. Those that do best under low-water conditions will be recommended to the green industry. This hard data will guide nurseries in choosing appropriate plants to propagate and market for Western North America’s shifting climate.
One of the project’s major goals is to move away from anecdotal to more rigorous, robust data about plants’ water use characteristics. “This trial is significant as it really does put data to the observations–not just qualitatively, but quantitatively–about how plants are performing,” explains Ray Larson, Curator of Living Collections at UWBG. “I think this is the type of research that people are going to be increasingly interested in.”
Trials Methodology
Protocols and requirements are exacting, as all six sites must precisely adhere to the same methodology established by UC Davis.
The UW trial is testing 15 plants at three different irrigation volumes. “So 15 times three is 45, which is replicated eight times for each treatment in each species. So we have 360 plants total,” explains Kim.
Year one is the establishment phase during which all plants receive the same amount of water in the summer. Irrigation (of the non-rainfall variety) is suspended during the winter.
Year two, known as the deficit year, begins in spring 2022, and is when the three different water treatments (low, moderate, and high) commence. In other words, that’s when the rubber hits the road and the plants will begin to respond and grow at different rates.
The amount of irrigation that will be delivered is determined by Evapotranspiration (ETo) data which is obtained from the WSU AgWeather weather station located a mere 50 meters from the plot. Some researchers and farmers have to rely on weather data from stations hundreds of miles away, so the UW ETo data is enviably accurate. Arthur Hsin-Wu Hsu, a Ph.D student in the UW School of Environmental and Forest Sciences (SEFS), checks the data daily and makes adjustments to the irrigation as necessary.
During this second year, graduate students will rate the plants’ health and aesthetics in the following categories: foliage, flowering, pest tolerance/disease resistance, vigor and overall appearance (AKA, the “WOW factor”). Those which thrive best with the lowest amount of irrigation will be the “winners”, so to speak.
How the Plants Were Chosen
All 15 species are suited for full sun exposure. Six are common to all participating universities; the remaining nine, which reflect the tastes and climate of our region, were selected by the UW team based on advice from Larson, who is the Plant Advisory Committee representative for UW’s trial. To build the list, he consulted with Seattle-area peers in the horticulture and nursery worlds, including the well-regarded Great Plant Picks team. “It was a chance to nerd out a bit,” he laughs.
To make the selections, he points out, it’s important to have good knowledge about what growers are propagating, nurseries are selling, and what consumer trends are. “Plants fall out of fashion, just like clothes,” he points out.
Larson is particularly interested to see how Vitex Blue Diddley® fares. Blue Diddley® is a dwarf version of the straight species which some gardeners shy away from due to its potential to become quite wide and tall–in other words tree-like. “Vitex has proven to be really drought-tolerant for us and with no obvious negatives. So if you could get a compact, intensely colored, freely blooming one like Blue Diddley®, it could be a pretty popular landscape plant, especially as an alternative to the similar–but invasive–butterfly bush.”
Intensive Teamwork and Collaboration
Teamwork, problem-solving and perseverance were essential to launching the project. “Obviously, the project has its own meaning and implications in terms of research, but in terms of the process of working together, it has been really, really great,” enthuses Kim. “This has been the culmination of how staff and students and faculty can work together to achieve a goal. This is a very, very impressive outcome.”
Funding for the trial originated with USDA and is funneled through the California Department of Food and Agriculture to UC Davis, which in turn distributes it to the six participating universities. Once UW’s funding was approved, Kim and David Zuckerman, UWBG Manager of Horticulture, embarked on a multi-step process to bring the trial to life.
After identifying the CUH field as the ideal potential site last year, Kim and Zuckerman had to obtain approval to use it from UW Environmental Health and Safety. Because the land is close to a wetland buffer, it was a nerve-racking and long process–with a successful outcome.
The next step was performing soil analysis (result: sandy loam on top of irksome hard pan) and soil preparation. Annie Bilotta, UWBG horticulturist, began plowing in April, 2021. “I had started by using our walk-behind rototiller but that just bounced off the soil,” she recalls. In the end, she used a number of John Deere tractor implements (spring-tooth harrow, rototiller, sub-soiler and disc harrow) to break up deep soil compaction and clods in order to transform the field into a plantable area: “I had to go over the field over and over again–I felt like a farmer plowing a field, trying to get it to a usable state.”
Meanwhile, Hsu and M Stuke, SEFS graduate student and research assistant, were sourcing and transporting plant materials and coordinating plot layout.
Next came the irrigation design and installation. James Boeckstiegel, Irrigation Lead at UW Facilities, consulted with Kim and Zuckerman to ensure that the site had a reliable water source and the appropriate hydrology to support the project’s irrigation requirements. “I showed them how to do the installation and then they took over. They did all the measuring of the pipes, the cutting and gluing,” he says of their can-do spirit. “They were very enthusiastic. They were more than willing to jump in and learn and do all the aspects of whatever the project called on them to do.”
Then it was time to install 360 plants, which was accomplished by a team of staff, faculty and students. Zuckerman gave student volunteers a mini workshop on correct planting techniques before digging in. “It’s important that they get planted properly or else we would have high mortality. The students got to see how to do it. They were like, ‘I didn’t know you could be that ruthless with the root ball!’ Yes, you’ve got to get those roots spread out!”
Plants were installed based upon a plot randomization grid created by Hsu. “That’s the Holy Grail,” says Zuckerman admiringly of the brightly colored grid with 360 blocks, each representing a plant. “If we didn’t have that we wouldn’t have gotten anywhere.”
The installation took place a week before the infamous “heat dome” hovered above Seattle, creating record temperatures and unfortunate leaf scorch in the region. Of course, the team paid attention to the forecast so delivered extra irrigation in advance to mitigate the heat’s impact. There were only nine casualties. “We had to replace six Philadelphus, plus one Hydrangea ‘Pee Wee’, one Rosa Oso Easy® Double Pink, and one Rosmarinus ‘Arp’. So only nine out of 360–not bad,” Kim remarks.
Passers-by are definitely curious about the plot. “There has been huge interest,” says Zuckerman. “There have been a lot of people walking by the plot when we’re working there and they want to know what’s going on.” Stuke agrees. “It’s been fun to describe the design and objectives to the public and receive positive feedback. It’s exciting for regular users to be able to see the research occur in real time. They’ll be able to watch the plants grow and possibly even see the treatment effects when they start to show up next year.”
What’s Next
During next year’s treatment season there will be three open houses. “We’ll invite horticultural professionals and University Master Gardeners to view and evaluate the performance of selected plant material,” Kim says.
In Fall, 2022 data collected on Plot 1 will be analyzed and communicated to the green industry, horticultural community, and other researchers so that all can incorporate learnings about water-wise plants into their production plans.
Bilotta has begun soil preparation on Plot 2 next to the UW Farm, which will be configured slightly differently than Plot 1 in its plant selection, soil characteristics and physical dimensions. With Larson’s advice, the team is finalizing the plant list which currently includes: Lantana spp., Caryopteris x clandonensis, Galtonia spp., Cercis spp., Azalea spp., Physocarpus spp. and more.
Jessica Farmer, Adult Education Supervisor at UWBG, is keen to create educational programming related to the project’s insights. “With climate change so top-of-mind, especially after this summer’s heat dome, people are really becoming interested in learning about how to garden with plants more tolerant of heat and drought conditions. These plant trials couldn’t have started at a better time.”