Cary Mitchell, left, and Celina Gómez harvest tomatoes grown around red and blue LED lights. Cary Mitchell, left, and Celina Gómez harvest tomatoes grown around red and blue LED lights, which use far less energy than traditional high-pressure sodium lamps in greenhouses. (Photo by Tom Campbell)


Greenhouse Tomatoes in Red and Blue

By Keith Robinson

Mitchell in 1957
Mitchell, 1957. (Photo courtesy of Cary Mitchell)

Young Cary Mitchell saw that bedding plants stored under a bench in his parents’ plastic-covered greenhouse were growing almost as well as those on top and fully exposed to light.

His father, Carl, was showing the 13-year-old that, unlike in a glass greenhouse, sunlight passing through the translucent material was so scattered that there were very few shadows, and the plants under the bench received almost the same amount of light. That was in 1956, when the Mitchells were the first nursery owners in the Midwest to erect a greenhouse made with plastic.

Cary Mitchell's, professor of horticulture and researcher at Purdue University
Cary Mitchell's curiosity is reflected in his work as a professor of horticulture and researcher at Purdue University, where he grows tomatoes in a greenhouse using light-emitting diodes, known as LEDs. (Photo by Tom Campbell)

“So right from the get-go we were thinking about how plants grow and how we could make that process more efficient,” says Mitchell, recalling his days on the family farm in Woodstock, Ill.

Fifty-six years later, Mitchell’s curiosity is reflected in his work as a professor of horticulture and researcher at Purdue University, where he and doctoral student Celina Gómez are growing tomatoes in a greenhouse using light-emitting diodes, known as LEDs.

During the daytime, the greenhouse behind the Horticulture Building looks like any other, the plants inside soaking up natural sunlight. But at night, it almost looks like it is strung with Christmas lights, the LEDs’ tiny red and blue lights embedded on towers only inches from the stems and leaves of several rows of 9-feet-tall tomato plants. The lights help to drive photosynthesis so the plants can produce their succulent fruit.

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The Beginnings

Cary Mitchell started selling vegetables as a teenager at a family farm stand in the 1950s.
Cary Mitchell started selling vegetables as a teenager at a family farm stand in the 1950s. (Photo courtesy of Cary Mitchell)

Mitchell’s interest in growing plants with LEDs began in the 1970s with NASA’s Advanced Life Support Program, in which he researched how to grow plants in a chamber with no sunlight to sustain space travelers of the future on such places as the moon and Mars. Although he and others on the research team grew cowpeas—also known as black-eyed peas—in a controlled environment with traditional lights hanging above the plants, they found that as the plants grew larger and filled the chamber, the light wasn’t getting below the upper leaves. That left the middle and lower parts in the plant’s canopy not producing the high-protein legume.

To grow many plants close together in a small space and make them productive, the researchers needed to come up with a way to feed light into the inner leaf canopy. They got short, fluorescent kitchen lamps from a hardware store, modified them to the need, hung them in the growth chamber from fish line and tried growing crops around them. While the lamps provided light in the canopy, they were too large and too hot so close to the plants, scorching some. Some plants would have to be removed to make room for more lights. That seemed counter-productive.

So Mitchell began to ask himself this: How could we get light into a foliar canopy without creating a lot of heat or needing lots of bulky lights?

Mitchell knew that scientists at the University of Wisconsin and NASA’s Kennedy Space Center had been experimenting with LEDs for plant growth for about 10 years. So with more funding from NASA, he and the aerospace research and development company Orbital Technologies Corp. of Madison, Wis., developed “light-sicles,” vertical strips of LEDs—cool to the touch—that could hang close to plants while providing the necessary light.

That got Mitchell and the team at Orbitec, as it is called, thinking about possible other applications for LEDs in horticulture. They decided to look at extending the LED technology to the greenhouse industry, using the lights to supplement natural, solar light. Mitchell’s work for NASA led to his current research, funded by a $4.88 million, four-year grant from the Specialty Crop Research Initiative Program of the U.S. Department of Agriculture’s National Institute of Food and Agriculture in 2010. He has collaborators at the University of Arizona, Michigan State University, Orbitec and Purdue. (Roberto Lopez of the Department of Horticulture and Landscape Architecture is using LEDs to propagate ornamental bedding plants in a greenhouse, and John Burr of the Krannert School of Management is looking into whether LEDs in greenhouses can be economical for growers.)

The Challenges

Using LEDs to grow plants isn’t new. Japan is using them as the sole source of light for growing leafy vegetables such as lettuce and spinach in tiered shelves in warehouses, a system called “vertical farming.” But whereas such plants are relatively easy to grow, Mitchell and Gómez are taking on the difficult task of growing greenhouse tomatoes, which require so much light and warmth that they are produced mostly in the Southwest and Mexico. They want to determine whether tomatoes can grow with the help of LEDs as a partial energy source in the semi-controlled environment of a greenhouse in colder, northern states, especially during winter, when direct sunlight can be scarce because of frequent overcast skies. Mitchell figures if tomatoes can grow in those conditions, just about any vegetable could.

“We chose tomato as a worst-case crop species, frankly,” Mitchell says.

Mitchell’s and Gómez’s research goes beyond just trying to grow tomatoes in a difficult environment. They want them to taste good—like luscious, home-grown tomatoes—so that northern consumers do not have to rely so much on tomatoes that are picked green before they are ready and then ripen as best they can while being trucked over long distances. New graduate student Michael Dzakovich is working on the issue of fruit quality from off-season tomato supplemental lighting in greenhouses.

The researchers also have environmental concerns that tomatoes shipped across the country leave a deep carbon footprint.

And with the increasing popularity of local foods, they want to help meet consumers’ demand for affordable, “clean and fresh” fruits and vegetables, including those that are out of season.

Their work with LED lighting is centered on saving energy, thereby helping to reduce production costs.

Greenhouses typically use yellowish high-pressure sodium lights—we most often see them as street lamps—to supplement sunlight. Mitchell and Gómez are using both HPS lights and LEDs and comparing their electricity use. Gómez, who monitors the electricity consumption, says the LEDs use about 25 percent of the energy that is needed to power HPS lights.

“We’re saving 75 percent of the energy, and we have not seen any yield differences,” she says.

While Gómez says the heating expense for a greenhouse with LEDs would increase if it did not use heat-emitting HPS lights, it could be offset by the lower cost of lighting. She is sending data from the LEDs’ energy consumption to Orbitec so that it might develop even more energy-efficient lights.

The Future

Using LEDs to grow plants isn’t new. Japan is using them as the sole source of light for growing leafy vegetables such as lettuce and spinach in tiered shelves in warehouses, a system called “vertical farming.” But whereas such plants are relatively easy to grow, Mitchell and Gómez are taking on the difficult task of growing greenhouse tomatoes, which require so much light and warmth that they are produced mostly in the Southwest and Mexico. They want to determine whether tomatoes can grow with the help of LEDs as a partial energy source in the semi-controlled environment of a greenhouse in colder, northern states, especially during winter, when direct sunlight can be scarce because of frequent overcast skies. Mitchell figures if tomatoes can grow in those conditions, just about any vegetable could.

“We chose tomato as a worst-case crop species, frankly,” Mitchell says.

Mitchell’s and Gómez’s research goes beyond just trying to grow tomatoes in a difficult environment. They want them to taste good—like luscious, home-grown tomatoes—so that northern consumers do not have to rely so much on tomatoes that are picked green before they are ready and then ripen as best they can while being trucked over long distances. New graduate student Michael Dzakovich is working on the issue of fruit quality from off-season tomato supplemental lighting in greenhouses.

Mitchell’s years in researching how to grow horticulture crops more efficiently has been a learning experience for him beyond the technical insights he has gained. He understands that with most of the world’s arable land already in use, agriculture will need to find new ways to grow more food to meet the needs of a global population that is expected to increase from 7 billion people today to 9 billion by 2050.

That is where he finds the relevance in his work.

“We need to come up with new staple food sources that lend themselves well to controlled environment agriculture and can be produced cheaply enough,” he says.

What that food of the future is we don’t know yet.

“But, for now,” Mitchell says, “tomatoes are really good for getting controlled environment agriculture going.”

LED Lights  


What is an LED?

An LED, or light-emitting diode, is a semiconductor device that converts electricity into light. As the electricity crosses the components of the diode, energy from the electrons in the current is released as photons, or light rays.

Developed in the 1960s, LEDs today are used in a variety of products: traffic signals, mp3 players, DVD players, computers, televisions and outdoor signs, among others.

LEDs consume less energy and so are more efficient than traditional lights such as incandescent bulbs and fluorescent lights. They also last longer, too. But they are more expensive to buy.

The technology in electrical-conversion efficiency is improving rapidly and is expected to become even more efficient in coming years.

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