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Drought and extreme heat in California’s Central Valley in recent years has meant shortages of tomatoes, particularly “processing tomatoes” used for sauce and ketchup. And such conditions are only expected to get worse with climate change.
Researchers note that the relatively nascent field of agrivoltaics — growing crops below and between solar panels — could offer help to the country’s billion-dollar-plus tomato industry.
Shade provided by solar panels can help conserve water, create humidity, and lower temperatures that can become too much even for heat-loving tomatoes.
An August paper by the National Renewable Energy Laboratory surveying agrivoltaic research sites across the country noted that on average, tomato yields doubled compared to non-agrivoltaic sites, whereas other crops like wheat, cucumbers, potatoes and lettuce showed negative impacts.
When it is too hot, tomatoes will abort the development of fruit from flowers since the plant senses that the fruit won’t flourish. Solar panels cool the air down enough to avoid this process, research has shown. And most importantly in a place like California, where the vast majority of the nation’s tomatoes are grown, solar panels can mean significantly less irrigation is needed.
Florida, the nation’s second-largest tomato producer, typically has plenty of rain. But even there, climate change means temperatures are climbing and drought has hit parts of the state.
At the University of Arizona’s Biosphere 2, cherry tomatoes doubled their yield when grown under solar panels, as noted in a 2019 study published in Nature.
“They got plenty of light, plenty of water, and the temperature stress was brought down just below that threshold so they could fruit through the summer and get an extra month of production, and more production per plant,” said Greg Barron-Gafford, lead author of the paper and associate professor in the School of Geography and Development at the University of Arizona.
Barron-Gafford found that not only do solar panels cool the air during the day, but they also warm the air at night — a benefit for tomatoes and other crops in desert climates like Central California and Arizona where temperatures can plummet after dark.
Even as the solar panels created cooling shade for the plants during the day, Barron-Gafford also observed a mutual benefit for the photovoltaic panels. Solar arrays are less efficient at generating energy when they are especially hot, and plants growing below panels create a cooling effect on the panels thanks to their transpiration.
Tomatoes often grow to about 5 feet high, so they can fit under solar panels that are elevated to 6 feet — taller than most commercial arrays but standard for agrivoltaics. Tomatoes are often harvested by hand, and Barron-Gafford’s research in Arizona noted that agrivoltaics could benefit farmworkers, with preliminary research showing skin temperature could be as much as 18 degrees F lower working under the shade of panels.
While processing tomatoes are grown in vast fields, many whole tomatoes sold in stores are grown in greenhouses. Researchers said that growing tomatoes under solar panels could mimic the conditions of a greenhouse in some climates, and energy from solar panels could also power on-site greenhouses.
“When the average person goes to the supermarket and sees tomatoes still on the vine, those are usually hot-house-grown tomatoes,” Barron-Gafford said. “So how do I use renewable energy to offset the immense cost it takes to keep a controlled environment like that cool?”
Researchers at Oregon State University examined tomato cultivation with agrivoltaics in a cooler climate for a 2021 study. They found mixed results that bolstered the idea that tomatoes are ideal agrivoltaic candidates in hot climates where most commercial production is done, but not a fit for small farmers in cooler climates.
“They’re one of the crops that would be an excellent choice in a warmer, more arid environment, whereas if you move to the northern USA, the Midwest where sunshine and temperatures are already edgy for tomatoes, it would push them out of contention,” said Chad Higgins, Oregon State associate professor of agricultural sciences. “For any crop, you have to consider the current climate and what agrivoltaics will do to that climate.”
In the Oregon State research, tomatoes were planted under solar panels, between rows of solar panels, and in a separate area as a control. The solar array was close to 500 kilowatts total, and each group had two plots with 40 tomato plants in each.
The soil and air temperatures were significantly lower in the plots below the panels, compared to the between-row plots and control. The soil temperature under the panels was a full 5 degrees Celsius lower than the between-rows and the control. The sunnier control plots’ tomato plants yielded the most fruit. But the control plot used more water than the other plots, even in a relatively cool climate — an effect that might be amplified in a hotter place.
The researchers concluded that agrivoltaics offered the opportunity to “trade a reduction in yields for reductions in water use.” The study notes that using water more efficiently may not matter in a place with lots of rain like western Oregon, but “could be critical in areas which are currently water stressed and expect to become more water stressed in coming years.”
Hadi Al-Agele was the lead author of the Oregon State study as part of his doctoral research. He lamented that their tomatoes suffered when “it became so cold and rainy.” But he is now working in his home country of Iraq, where there is blazing sun but little solar energy. He would like to see agrivoltaics take off there, providing clean energy and helping crops survive the heat.
The Arizona and Oregon researchers said that tomatoes are a good fit for agrivoltaic studies, providing insights that can also apply to other crops. Barron-Gafford noted that there are generally three types of specialty crops — leafy greens, tubers like potatoes, and fruiting plants like tomatoes and peppers, hence agrivoltaic studies try to include examples of all three.
“Tomatoes are a really well-studied plant,” meaning agrivoltaic results can be compared to other literature, Higgins added. “We could have studied ginseng, which is an interesting choice because it grows slowly and is only hand-harvested, and needs 80% shade — the agronomics match really well. But there’s just not as much literature about the plant physiology of ginseng.”
Whether agrivoltaics really are adopted by the tomato industry depends in large part on the economics of meeting the needs of both solar developers and farmers. Elevating solar panels higher to accommodate tomatoes underneath means extra costs, and machinery used to pick and plant tomatoes on many large farms might not work in concert with solar panels.
The Oregon State researchers studied tomatoes under tilted solar panels that were not ideal for planting and harvesting tomatoes — they had permission to work in a solar array that was not designed specifically for the experiment.
Higgins described Al-Agele stooping below the panels to prepare the soil with a hand-held rototiller held at an awkward angle, since the panels were only 18 centimeters off the ground at the lowest point. “It was hard,” Al-Agele said.
“I don’t know of any commercial grower who would grow tomatoes in the fashion we did. It would be too much work — too much hand labor,” Higgins said. “We were doing it for science.”