by Michael Keller

The harvest season seems to whiz by every year in northern latitudes. Just as the time comes to sink a fork into early spring’s peppery locally grown lettuce and asparagus, the market’s crates are already brimming with winter squash. And the juicy tomatoes that yesterday took a quick ride from a nearby farm start logging thousands of miles from farm to table.

Unfortunately, the only two options for most consumers looking to buy fresh produce during the cold months are either to get them shipped from warmer regions or from greenhouses closer by. Efficiencies in the agricultural and shipping systems being what they are, fruits and vegetables grown in warmer climes—by necessity picked before they ripen to prevent spoilage in transit—cost less than premium-priced food from the greenhouse.

Either way, each of those February cucumbers is the product of a significant energy investment—whether it’s producing the fertilizer, burning fuel in shipping, or lighting and heating commercial greenhouses.

“The average tomato is trucked 1,500 miles from where it’s picked in the winter and it sits on that truck for a week or more,” says Purdue University horticulture professor Cary Mitchell. “By the time it gets to a northern market, it has been in the dark for a while and its quality is degraded. Yet you pay a premium for it—up to four dollars a pound in January.”

Fans of the local food movement who despair through the winter months at high-priced greenhouse-grown or unripe produce transported over thousands of miles might soon find some relief.

Major energy savings

Mitchell is leading a study of advanced lighting in greenhouses that is successfully using cool and efficient LED systems to grow tomatoes in northern greenhouses through the winter. Adopting this new equipment and better growing protocols, commercial growers could cut costs enough to provide produce locally when it’s cold outside.

“There is a lot of potential good to be gotten out of growing produce locally, including food quality, environmental and lower carbon footprint issues,” Mitchell says. “People realize this and the movement to buy local grown produce has grown. The problem has been lighting costs and heating over the seasons when produce can’t be grown outside.”

Mitchell’s team set out to quantify the cost difference between traditional high-pressure sodium (HPS) lamps and red- and blue-light LED systems. HPS lamps convert only 30 percent of the energy going into them to plant-usable light—the other 70 percent comes out as heat. LEDs are up to 50 percent efficient and are improving regularly. Plus, by choosing photosynthetically optimized blue and red lights, they don’t waste energy on wavelengths less useful for plant growth. “LEDs are much cooler to the touch and much more efficient,” he says. “They deliver the required amount of light using much less energy.”

Mitchell says the difference between the two types of lighting is a big deal to commercial greenhouse growers—a significant portion of their total costs is spent recreating the sun’s energy to help plants grow.

In their research greenhouse, they grew tomatoes from January to June 2012 to test the winter to summer production period. They split the plants into three groups: a control group received no supplemental lighting, one test group was grown under HPS overhead lighting, and a third group was subjected to LED lighting that was placed between the plants on vertical towers.

The HPS lights needed to be kept farther away from the plants because they throw off enough waste heat to burn nearby plants. The LEDs, on the other hand, could be placed within the tomato plant canopy, allowing more of the plant to soak up the energy. “The leaves are photosynthesizing on the lower parts of the plants, and that may be helping with the plant’s energy,” said doctoral student Celina Gómez, who is contributing to the research project. “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.”

Lower consumption, same yield

Both forms of supplemental lighting produced more fruit and higher total weight of fruit than the plants that only received sunlight coming into the greenhouse. Further, the researchers saw no differences in productivity between plants grown under sodium lights and those grown with LEDs.

The difference in energy costs was significant, though. Breaking down their findings to the individual level, they found that each plant lit by HPS consumed 1,224 kilowatt-hours of energy through harvest while those taking their light from LEDs required only 294 kilowatt-hours each. “The LED in-canopy lighting demonstrated considerable savings,” Mitchell says.

Since the plants grown under the two different types of light produced similar yields of fruit, the difference in energy costs to make produce was also sizable. “The lighting cost per average fruit grown under the [HPS] lamps was 403 percent more than that of using [vertical LED] towers,” they wrote in a study published in the journal HortTechnology.

There are still obstacles to overcome before greenhouse growers adopt the new lighting, which is still being developed by manufacturers. Once they appear on the market, their prices will need to come down to make switching over from HPS cost effective.

But Mitchell says the horticultural advances are just beginning with the new LED systems that can be tuned to produce different wavelengths of light. “This is opening up a whole new range of research now that we have these LEDs,” he says. “It’s the dawn of a new era of plant photobiology.”

Top Image: Cary Mitchell and Celina Gomez harvest tomatoes grown around red and blue LED lights, which use far less energy than traditional high-pressure sodium lamps in greenhouses. Courtesy Tom Campbell/ Purdue University.