Cool LEDs for hothouse tomatoes

Caren B. Les, caren.les@photonics.com

In the cold dark of winter, fresh ripe tomatoes bring a burst of summer to your lips – but fruits imported from more temperate climes are less tasty, and locally grown vine-ripened tomatoes are expensive. But that expense could drop, thanks to LEDs.

“LEDs are a hot topic in agriculture – especially for plant growth in controlled environments such as greenhouses, where plants are concentrated for high productivity,” Cary A. Mitchell, a professor of horticulture at Purdue University in West Lafayette, Ind., told BioPhotonics.

Mitchell and doctoral student Celina Gómez have found that LEDs could reduce growers’ energy costs in terms of light and heat during the shorter, colder days of fall and winter; the effort could enable more produce to be grown indoors year-round in northern areas.

Professor Cary Mitchell, left, and doctoral student Celina Gómez harvest tomatoes grown around red and blue LEDs. Courtesy of Purdue Agricultural Communication/Tom Campbell.

From January through June 2012, the researchers compared two types of supplemental electrical lighting for growing greenhouse tomatoes: LED intracanopy lighting (LED-ICL) towers, which are placed at intervals within rows of plants, and traditional overhead high-pressure sodium (HPS) lamps.

They found that both types of lighting produce the same size and number of tomatoes, but that the LEDs use about 25 percent as much energy as the HPS lamps. The findings are detailed in HortTechnology.

Unlike HPS lamps, LEDs are cool to the touch, Mitchell said. They can be attached to towers alongside today’s “high rise” plants, which are grown in Jack-and-the-beanstalk fashion up along a wire. They can be placed close to where light is needed along the sides of the plants. Sections of the LEDs can be turned on or off relative to the height of the plants, and exhaust fans in the towers remove waste heat from the LEDs.

“The leaves are photosynthesizing on the lower parts of the plants where they otherwise would be shaded, and that helps provide the plant’s growth energy,” Gómez said. “We’re getting the high intensity of the LEDs close to the plants because they’re not hot like a high-pressure sodium lamp. If you put one of those close to the plants, you’d scorch them.”

Also, LEDs are tunable, so you can prescribe the spectrum you want for optimum plant growth, Mitchell added. The LED-ICL towers in the experiment were programmed to mix 95 percent red (peak wavelength at 627 nm) and 5 percent blue (peak wavelength at 450 nm) light along the vertical height of the climbing plant. The 1000-W HPS lamps emitted blue-deficient yellow light, and only from above.

Right now, the researchers are using proof-of-concept LED units; manufacturers would have to figure out how to produce them cost-effectively in volume, Mitchell said. “Our main research goal is to continue to find ways to save money and energy, while having the same high crop yield.”