Made in the Shade: Lessons from a Conventional Solar Site in California’s Central Valley

UC Davis Ph.D. Candidate Yudi Li collects data on plants inside a study quadrat on the UC Davis solar site.

Water in California’s Central Valley is increasingly scarce, and periods of extreme drought are motivating landowners to turn from agriculture toward solar energy generation. 

With PV solar projects expanding in the region, UC Davis researchers wondered how this infrastructure impacts the surrounding ecosystem.

How do the plants growing on solar sites respond to panels tracking the sun overhead? Could this infrastructure create new habitat?

Scientists dedicated a year to those questions by studying the vegetation and microclimates on a single-axis, ground-mounted solar project in Davis, California, owned by the university.

The results were recently published by the UC Davis Wild Energy Center, establishing an ecological profile of “business-as-usual” at a conventional California solar site managed with periodic mowing. While solar projects may appear static, the findings illustrate dynamic features of this infrastructure that could offer benefits for humans and wildlife as the climate changes.

Getting into the Weeds

Like many solar sites in the industry, the UC Davis project was developed on former agricultural land.

“Mostly, it’s weeds,” said UC Davis Ph.D. candidate Yudi Li, the first author on the paper. “We realize that’s a pre-existing characteristic of many large, ground-mounted solar projects.”

After a mower cleared all that weedy vegetation, Li returned to the research site every month for one year to collect data as the plants grew back.

Of all the species on site, 86% were non-native, like milk thistle.

The experiment divided the site into study ‘patches’ exposed to the sun during different times of day. Researchers studied those plots, collecting data on over a dozen different variables above- and below-ground, including air temperature, soil temperature, irradiance, wind speed, species diversity, canopy coverage, and biomass. Navigating around the panels to collect this data, Li braved encounters with several species of non-native thistles, which quickly grew back after the initial mow.

“It’s pretty common to be poked,” Li said. “I have done fieldwork all around them. I’m already physically immune to the pain.”

Yudi Li visited the research site at least one a month for a year to collect vegetation and microclimate data.

Micro-Patches Made in the Shade

All that time in the field eventually paid off.

At the end of the study period, what scientists observed was a mosaic of distinct vegetation patches growing across the site. Depending on the sun’s radiation and competition with other species, different plants seemed to prefer some areas over others. 

The research team confirmed one major hypothesis: ground covered by solar panels for part of the day was cooler than patches in the direct sun. In the face of climate change, the significance of that finding is wide-ranging.

“As temperatures warm and weather becomes more variable, it's possible these shaded micro-patches could provide a refuge for certain species,’ stated Dr. Rebecca Hernandez, UC Davis professor and director of the Wild Energy Center.

The tilting cycle of single-axis solar panels creates a dynamic habitat for native plant and animal species living below, which may seek a balance of sun and shade. That benefit isn’t limited to wild species. Those shady spots could also support certain food crops on an agrivoltaic farm, or shield domestic animals grazing a rangevoltaic solar ranch, like sheep.

“If sheep do prefer to lie down under the panels, this provides the quantitative evidence that soil temperatures there are relatively cooler, and this may drive their preference to use these micro-patches,” Hernandez said.

Best Practices, Grounded in the Field

Prior to restoration, only four native species were observed growing on the UC Davis solar park, including Bird’s Eye Gilia (gilia tricolor).

Li is excited for the research to reach solar project managers, who can directly apply the results to improve operations and cost-efficiency of their sites. For instance, the findings indicate that April and May are peak months for plant growth—a good time to trim back. Then, in fall, plants tend to die and dry out, which may present an increased fire risk, especially if non-natives are abundant.

“When plant litter starts to accumulate, it may be the time to start thinking about manual removal,” Li said.

All those non-native weeds likely hindered the success of native vegetation on the site; Li only found and identified four native plant species. Nevertheless, studying weeds did have an upside. It prepared the Wild Energy Center to share lessons learned the hard way with the rest of the solar industry.

The Wild Energy Center found that large, ground-mounted solar projects may consider alternative control measures, like the sowing of native plants, to compete with weed species. Extra resources may need to be allocated to the spaces between panels receiving direct sun, where plants typically grow taller thanks to greater photosynthetic radiation.

Li sees this research as an introduction to environmental conditions on average solar projects, which could be used to target above-average biodiversity, a benefit to humans and nature alike. With these findings as a guide, solar developers can take advantage of dynamic environmental conditions created by this infrastructure to better support native species, even if the weeds are stacked against them.

Co-authors on the paper include Rebecca R. Hernandez, Christopher Simmons, and Noah Z. Krasner of UC Davis, as well as Alona Armstrong of Lancaster University.