Developing the right light recipes for greenhouse vegetable crops

Exclusives from Urban Ag News

Originally published in Issue 15

Canadian researchers at the Harrow Research and Development Centre are working with growers to determine which horticultural lights are the most effective and efficient for producing tall greenhouse vegetable crops.

When it comes to the collecting data on using LEDs on tall greenhouse vegetable crops, there isn’t a clear picture on how well they perform. Shalin Khosla, greenhouse vegetable specialist at Ontario Ministry of Agriculture, Food and Rural Affairs, said much of the early research that has been done with LEDs has been on ornamental flowering crops and leafy green vegetables.

“Not a lot of research has been done on tall-growing vegetable crops like tomatoes, cucumbers and peppers,” Khosla said. “Growers need a light recipe for growing tall vegetable crops. Another factor as to why more of these tall vegetable crop growers haven’t made the switch to LEDs is the cost of the fixtures compared to high pressure sodium lamps.”

Comparing HPS to LED lights

Khosla said the latest LED light fixtures have a better electricity to light conversion factor than the best HPS light fixture.

“Some LED light fixtures are better than others,” he said. “Some are more efficient than others. The efficiency of the latest LEDs is better than the best HPS, in terms of converting electricity to light.

“The best HPS lights produce 10 percent convective heat, 50 percent radiation heat and 40 percent light. With the best LED fixtures there is 35 percent convective heat, 15 percent radiation heat and 50 percent light. Comparing the two lights, the LED fixtures can be placed closer to the plants without harming them. Whereas with the HPS light, which is generating more heat, it needs to be placed further away from the plants. That is why LED lights can be used for interlighting. The HPS light would be too hot for interlighting and would burn the plants.”

Khosla said light manufacturers are working on designing HPS lights that don’t give off as much heat. They are also looking for ways to remove some of that heat.

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“Manufacturers of HPS lights are working on the conversion factor so that more electricity is going to produce light rather than heat,” he said. “They are improving the efficiency so more light is given off than heat. If I want to put a light fixture in a small space like a growth room or vertical farm, a LED light would be used because there is not as much heat given off. In a tall greenhouse, HPS lights can be used for top lighting because there isn’t as much of a concern of damage with the amount of heat given off. In fact this heat will offset some of the heat requirements to grow the crop in the winter.”

Khosla said the newer larger LED fixtures are more efficient and are suitable to use similarly as overhead HPS lamps.

“Light manufacturers have not yet achieved a one-to-one replacement LED for HPS, but they are getting closer,” he said. “HPS bulbs emit light in all directions so it has to be bounced back down towards the crop. The reflector has a huge influence on how good the light reception level is. LED light is more direct and it may be focused to specific parts of the plant.”

Greenhouse vegetable specialist Shalin Khosla (left) and research scientist Dr. Xiuming Hao at the Harrow Research and Development Centre are studying the effects of HPS and LED light on tomatoes, cucumbers and peppers.
Photos courtesy of Shalin Khosla, Ontario Ministry of Agriculture, Food and Rural Affairs

Developing light recipes

Khosla said HPS lights emit a broad spectrum of light with peaks in the blue and red range. With LED lights, which produce specific wavebands, researchers are able to study the impact these specific wavebands can have on plant growth, flowering and fruiting.

Khosla along with Dr. Xiuming Hao at the Harrow Research and Development Centre are studying the effects of HPS and LED light on tomatoes, cucumbers and peppers. They are comparing the growth of crops under HPS and LEDs separately as well as combining HPS top lighting with LED interlighting.

“There are three groups, Agriculture and Agri-Food Canada, OMAFRA, and Ontario Greenhouse Vegetable Growers Association, working together on this lighting project,” Khosla said. “We are conducting research with HPS top lighting and LED interlighting on tall vegetable crops. We’ve studied mini-cucumbers for three years. We began lighting a newly planted crop in September and continued through April. We were able to produce 60 kilograms per square meter. That is a very decent yield for a single mini-cucumber crop grown during the winter months under Ontario’s climatic conditions.

“We are also conducting studies to compare the difference between growing these three crops under the HPS and LED lights separately as well as using the two types of lights together. There is a difference in light intensity from the top of the plant to the bottom of the plant. We need a working light profile. We can alter the spectrum of the light within the crop itself by using different color LED lights.

Khosla said preliminary results have shown there is an improved yield when HPS lights and LEDs are combined.

“We have been able to increase the fruit dry matter content,” he said. “We also had slightly better quality fruit when we combined the two kinds of light than when we used HPS lights by themselves.

“All three crops showed improved responses, but at different levels. Cucumbers showed the best results. For peppers and tomatoes the results were not as dramatic. We still have to do more studies on both of these crops.”

Determining payback

Khosla said it’s very hard to determine the difference in payback between HPS and LED fixtures because their efficiency and prices are constantly changing.

“We are still working on what the payback would be,” he said. “We are working with some growers who have installed LEDs, but are still collecting the data.

“Most of the growers we are working with have had the lights installed for about a year. For this pilot project, we are only looking at a section of the greenhouse, not the whole operation. Growers are waiting to see the full results before they invest in the lights. The growers are using the lights on tomatoes, cucumbers and peppers. They are looking at the cost of electricity, the cost of the fixtures and the yields to determine how viable this is. We are expecting to have this part of the study completed by next year.”

Khosla said each of the Canadian provinces has its own energy rebate program.

“In Ontario, Hydro One offers a rebate program for greenhouse lighting for retrofits and new builds,” he said. “As long as growers can prove they are using less electricity with the lights or whatever equipment improvement they make they can qualify for a rebate.”

Canadian researchers are conducting studies to compare the difference between growing tomatoes, cucumbers and peppers under HPS and LED lights separately as well as using the two types of lights together.

Khosla advises growers considering purchasing any type of horticultural lighting to look at how efficiently are the lights converting electricity to light.

“If growers are interested in LEDs they need to be sure the light waveband combinations are correct for the crops they’re growing,” he said. “Growers should also work with a lighting company that can supply information about the light fixtures as well as providing reliable fixtures. Growers should contact other growers who are using the fixtures to find out their experience with them. Just like any other equipment purchase, growers need to do their homework before buying any type of lights, HPS or LED.”


For more: Shalin Khosla, Ontario Ministry of Agriculture, Food and Rural Affairs, Harrow Research Centre, 2585 County Road 20, Harrow, Ontario N0R 1G0; (519) 738-1257;

This research study is sponsored by Ontario Greenhouse Vegetable GrowersAgriculture Agri-Food CanadaOntario Ministry of Agriculture Food and Rural Affairs and Growing Forward 2.

David Kuack is a freelance technical writer in Fort Worth, Texas;

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