As land become a more precious resource, figuring out how to use it as economically as possible is a top priority. It’s often where agrivoltaics – agricultural production underneath or adjacent to solar panels – comes into play. But how effective is farming beneath these panels?

Luckily, research is ongoing and extensive, taking into account many different crops. Here, we’ll look at work being done with leafy greens, squash and bell peppers.

Evaluating Leafy Green Production

Consumers want fresh produce and many of them try to purchase it locally. Looking at the growth and yield of leafy greens by examining how shade from photovoltaic arrays influences their production are Armando Villa-Ignacio, a Ph.D. student, and Jennifer Bousselot at Colorado State University and Maria Chavez at New Mexico State University.

The agrivoltaics system built at the Colorado State Spur Campus in Denver featured 45 cm deep substrate and three growing options: full sun, under opaque silicon photovoltaic (PV) modules and under bifacial silicon PV modules (with solar cells on the top and the bottom). The leafy greens used – arugula, kale, lettuce, spinach and Swiss chard – were transplanted in the system in randomized rows. The environmental conditions were monitored continuously using HOBO® sensors.

Over the course of the growing season, the average air temperatures were similar in both of the PV treatments and in the full sun. The substrate temperatures were lower under the panels at midday. Substrate moisture was best under the opaque PV panels. Solar radiation on the leafy greens was higher in full sun – until about 4:30 p.m., when the angle of the setting sun reached below all the PV panels.

At harvest, the plants’ fresh weights, dry weights and plant sizes were noted. Those grown in full sun had higher fresh weights (meaning higher yields). The plant size was higher in the lettuce and spinach grown under the PV panels compared to the full sun. Interestingly, spinach performed much better than the other veggies under the opaque panels.

It was also determined water use was reduced in leafy greens grown under the PV panels compared to the full sun – which could be useful information in regions prone to water scarcity.

Measuring ‘Multipik’ Production

A research group including Raymond Wilmes, Jacie Legois, Ajay Nair, Suzanne Slack, Matthew O’Neal, Anne Kimber, John Tyndall and Hongli Feng from Iowa State University went a different direction and studied how solar panels affect the production of the summer squash cultivar ‘Multipik.’

‘Multipik’ is a high-yielding, early and prolific hybrid straightneck squash known for its bright yellow color and resistance to certain mosaic viruses – a valuable crop.

The team thought the squash would yield more marketable produce under single-axis tracking solar panels – those that follow the path of the sun – compared to traditional open-field cultivation.

In their experiment, the squash was seeded May 7, 2024 and transplanted on May 31 in single row, 24-inch spacing. Weed fabric and plastic mulch were utilized to manage weeds. Flower count and nitrogen status data were collected on a weekly basis. HOBO sensors measured soil temperature, air temperature and light intensity. A common pest, the spotted cucumber beetle, was managed by spraying Mustang® MAXX and PyGanic®. Watering was done primarily via drip irrigation.

Between July 1 and Sept. 20, the researchers completed 31 harvests. They took place every two to three days. No custom equipment was used to plant, grow or harvest the squash under the PV panels. Commercial vegetable production practices and tools were employed.

The open-field plot yielded about 992 lbs. of marketable squash; nearly 1,323 lbs. were harvested from beneath the solar treatment – a 28% improvement in yield.

Notable measurements included air temperature, which remained cooler in the agrivoltaics plot throughout the growing season (June – September), as did soil temperatures. The PV treatment, for obvious reasons, also had more shade during the growing season, with much lower light intensity on the plants.

A Boost for Bell Peppers?

Dr. Ajay Nair teamed up again with Iowa State graduate student Raymond Wilmes to consider how solar panels may affect the production of the bell pepper cultivar ‘SVPB8415.’ Their study took place at the Alliant Solar Farm at ISU in Ames, Iowa.

The farm comprises 10 acres, and its topsoil was not graded at solar panel installation (it usually is). It had previously been a corn/soy field. More than 3,000 bifacial PV panels have been installed, both fixed tilt and tracking.

Like the summer squash, the bell peppers were grown between solar panels and in open-field plots. For the solar panel treatment, two raised beds with white plastic mulch were created on each side of single-axis solar tracking panels. There were also two rows of open-field peppers.

The peppers were transplanted on June 6, 2024. HOBO sensors kept track of soil temperature, air temperature and light intensity. In a pleasant turn of events, no pesticides were needed to manage pests. Drip irrigation was used for watering.

A total of six harvests occurred throughout the season on a weekly basis, from Sept. 6 through Oct. 14. The differences in growing condition were marked.

The pepper plants between the solar panels yielded 115 lbs./row while the open-field plants yielded 59.5 lbs. – a 48% increase in the solar treatment.

When it came to pepper grading, marketable weights were nearly double from under the panels; and peppers that were too small to be marketed were slightly more numerous in the open field.

Other measurements recorded showed a 37% increase in plant height under the panels; chlorophyll was 20% higher there; and around week 6 of growth, the plants’ stem diameters grew to be 24% thicker.

From June to September, the air temperature under the PV panels was on average by 4º F while the soil temperature was 3º F cooler. Nair and Wilmes believe these lower temperatures are what led to better yield, due to less stress on the plants.

Based on these first-year results, bell peppers show promise for cultivation within an agrivoltaics system. More data will be collected in the coming years on plant growth characteristics, soil moisture and the post-harvest quality of peppers.

by Courtney Llewellyn