Tomato Yield Not Affected by LED-Lit Greenhouses

LED-equipped greenhouses can grow the same number of off-season tomatoes as those outfitted with traditional lighting, but using a lot less energy, a new Purdue University study finds.

The average tomato is shipped about 1500 miles from a warmer climate, where it’s grown, to a cooler climate that cannot produce the fruit cost-effectively in the winter, said horticulture professor Cary Mitchell. Not only do those trips cost a lot and increase the industry’s carbon footprint, but the fruit is also less flavorful because it is picked green to ripen during shipping.

Cary Mitchell, left, and Celina Gómez harvest greenhouse tomatoes grown around red and blue LED lights, which use far less energy than traditional high-pressure sodium lamps. Courtesy of Tom Campbell, Purdue Agricultural Communication.

“It makes it really hard for the greenhouse industry to grow tomatoes well in the off-season. We’re trying to change that and make it affordable,” Mitchell said.

Mitchell and doctoral student Celina Gómez experimented with LEDs, which are cooler and much more energy-efficient than traditional high-pressure sodium lamps used in greenhouses. They got the same yield — size and number of fruit — with high-pressure sodium lamps and LED towers, but the LEDs used about 75 percent less energy.

The scientists think that the method could have other advantages because the cooler LEDs can be placed much closer and along the sides of plants, lighting not only the top, but also underneath.

“The leaves are photosynthesizing on the lower parts of the plants, and that may be helping with the plant’s 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 it.”

The heat from high-pressure sodium lamps accounts for about 15 to 25 percent of that needed to warm greenhouses, but, Mitchell said, “that’s a very expensive way to heat a greenhouse, through lighting.”

The goal of the research is to reduce prices to the point where local growers could compete with shippers.

“The United States still imports one-third of its tomatoes from Mexico and Canada, as well as other countries,” Mitchell said. “This technology could allow US growers to create local jobs that shrink carbon footprints and produce better-tasting tomatoes.”

Future studies include comparing the flavor of the tomatoes produced through the different lighting methods.

The study was published in HortTechnology and was funded by the Specialty Crop Initiative of the US Department of Agriculture’s National Institute of Food and Agriculture.

For more information, visit: www.purdue.edu