Worth the Weeds: Our New Study Unearths 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. As the climate changes, periods of extreme drought are motivating land owners to turn from agriculture toward solar energy generation. 

With PV panels replacing row crops in the region, UC Davis researchers grew curious about potential microclimates created by this new infrastructure moving in.

How are plants on conventional solar sites impacted by panels that rotate to track the sun overhead?

The Wild Energy Center just published the first paper to address that question. Scientists dedicated a year to examine the vegetation and microclimates on a conventional, single-axis ground-mounted solar project in Davis, California, owned by the university.

Getting into the Weeds

UC Davis Ph.D. candidate, Yudi Li, the first author on the paper, was inspired to conduct this experiment to capture a profile of “business as usual” at an average California solar site. Like many in the industry, the Davis site was developed on former agricultural land.

“Mostly, it’s weeds,” Li says. “We realize that’s a pre-existing characteristic of many large, ground-mounted solar projects.”

After a mower cleared all that weedy vegetation on our research site, Li returned each month to collect data on how it changed over the course of one year.

Of all the species on site, 86% were non-native, like Milk Thistle.

The experiment divided the site into five study patches exposed to the sun during different times of day. Researchers collected 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. Collecting data on the site did present challenges. Walking between the panels, Li braved encounters with several species of thistles.

“It’s pretty common to be poked,” Li says. “I have done fieldwork between them, among them, above 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 to 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. The significance of that finding is wide-ranging. For one thing, the tilting cycle of single-axis solar panels creates a dynamic habitat for wildlife underneath.

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

Those shady spots could also be alluring to domestic animals grazing a rangevoltaic solar ranch, like sheep.

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

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 land managers, who can directly apply these results to scenarios on the ground. For instance, the findings indicate that April and May are peak months for plant growth—a good time to trim back. Then, in the 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 says.

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. Based on this study, extra resources may need to be allocated to panels interspaces exposed to the sun, which typically grow taller thanks to greater photosynthetic radiation.

With this research as a guide, Li feels better prepared to manage conditions to support the growth of native plants on solar sites, even if the weeds are stacked against them.