Is there really that much difference between LED lamps?

Many LED light manufacturers claim their lamps can be used for horticultural production. But do they provide the right wavelengths of light to meet plants’ needs?

Growers often ask Michigan State University horticulture professor Roberto Lopez if they can use the LED lamps available from their local home improvement store to induce flowering of their long day ornamental crops.

“Some growers in the Midwest ship flowering annual and perennial bedding plants into southern markets,” Lopez said. “Trying to grow these long day plants during February and March in the Midwest means they are being produced under short day conditions. Since incandescent bulbs have been phased out and compact fluorescent bulbs aren’t very effective, LEDs can be an affordable alternative.”

Growers want to know if the LEDs available from local retailers can be used as effective photoperiodic light sources. Studies by fellow Michigan State researcher Erik Runkle have shown that these LEDs work with some long day crops, but not all of them. This can pose a challenge for growers who produce a wide variety of crops.

“Another question I get from growers is related to the inexpensive horticultural LEDs offered on Amazon or other online retailers, some costing $10 or less,” Lopez said. “These lamps are considerably cheaper, sometimes $20 to $30 less than the LEDs being offered by other companies that manufacture horticultural LEDs. The growers want to know is there really much difference in the light delivered by the LEDs from different manufacturers. Growers also need to consider that not all lamps are designed to withstand greenhouse conditions. ”

Trialing photoperiodic lamps

Lopez will again be trialing several lamps marketed as “flowering, photoperiod or horticultural” from different lighting manufacturers, including the GE Arize Greenhouse Pro Photoperiodic LED lamp.

“The different lamps will each provide ~2 µmol∙m^–2∙s^–1 of light,” Lopez said. “The plants will be grown under the lamps just as growers would produce them. We will quantify the impact on flowering, height, flower bud number and overall plant quality. Each of these lamps has different red:far-red ratios so we should see some different responses.

“We are trialing petunias, calibrachoa, dianthus, ornamental oregano and snapdragons—the popular bedding plant crops that require long days in order to flower. We have included both facultative long day plants and obligate long day plants in the study as well.”

Lopez is using a spectroradiometer to determine the percentages of red, blue, green and far-red light emitted by each lamp.

“This study will enable us to report the red:far-red ratio for each lamp trialed,” he said. “None of the light manufacturers actually report the red:far-red ratio for their LED lamps. The only information they provide is that a particular lamp provides red, white or far-red light. If growers don’t have the proper equipment to measure the light, they have no idea of the percentage of red, white or far-red light their plants are receiving.”

Lopez said many growers are making the decision on which LEDs to buy based only on the cost of the lamps.

“We have an idea of what the red:far-red ratio needs to be in order to induce a large variety of long day plants into flower,” he said. “Stem elongation is one of the consequences of growing plants under far-red light. If growers are going to have to spend more money on plant growth regulators because of this stem elongation that is something they need to consider when choosing LED lamps. Also, if it takes a week longer for plants to flower with a particular lamp, even though the lamp is less expensive, this is another factor growers need to consider when making their decision.”

Impact of blue light on flowering

Another project with ornamentals that Lopez is studying is the impact blue light can have on flowering. Michigan State graduate research assistant Qingwu Meng and Lopez found that blue light can substitute for red and far-red light in terms of flowering responses.

“The blue light has to be provided at a moderate intensity,” Lopez said. “A red:far-red ratio at ~2 µmol∙m^–2∙s^–1 of light is sufficient to induce a long day plant into flower. With blue light, ≥15 µmol∙m^–2∙s^–1 is needed in order to induce a flowering response.”

Meng and Lopez are conducting this high intensity supplemental lighting study with perennials.

“In our first replication we found that some perennials flowered faster and were of higher quality when 30 µmol∙m^–2∙s^–1 of blue light were provided by LEDs vs. high pressure sodium lamps,” Lopez said. “Plants provided with 90 µmol∙m^–2∙s^–1 of supplemental lighting (30 µmol∙m^–2∙s^–1 of which were from blue light) had more branching, more flowers and overall were a much higher quality crop. Of the crops we studied, blue light had the biggest impact on Phlox paniculata. With LEDs, phlox flowered faster and was of higher quality.”

Benefits of LEDs on vegetables, herbs

Since Lopez is focused on controlled environment specialty crop production, 50 percent of his research is with ornamentals and 50 percent covers food crops, which includes vegetables, leafy greens and culinary herbs.

“As long as there is an interest from growers and the demand for information on food crops continues to increase, I will be working on these crops,” he said. “There are more growers who are transitioning from ornamentals to food crops. Also, there are many new operations that are opening that are focused solely on food crops.”

Michigan State graduate research assistant Kellie Walters is studying the impact sole-source LED lighting can have on the volatiles produced by herbs impacting their flavor.

“Kellie has found that light intensity has a major impact on flavor,” Lopez said. “When basil is provided with sole-source LED lighting (≥ 200 µmol∙m^–2∙s^–1), its flavor is much more intense. Kellie is conducting consumer taste panels to determine taste preference. We are also looking at how these research results can be profitable for growers. We have looked at basil and now we are going to expand this research to study more culinary herbs.”

Walters is also quantifying how daily light and temperature in the greenhouse influence the yield and quality of hydroponically-grown culinary herbs and leafy greens. She has looked at seven species at different light and temperature combinations to determine which combinations produce the most biomass and highest quality plants.

Lopez and Walters are also studying the impact of light intensity and carbon dioxide concentration on producing young starter plants.

“Producing herbs in an indoor facility is expensive so we looking at producing high quality transplants indoors and then transplanting them out into the greenhouse for finishing,” Lopez said. “We are looking to see if there are any residual effects from starting the transplants under high light intensities from electric lighting and supplemental carbon dioxide.

“Graduate research assistant Charlie Garcia is looking at the effects of supplemental light quality on vine crop transplants such as tomato, pepper and cucumber. The transplants are being grown to the point where greenhouse growers would receive them as young plants for finishing. We are growing them up to five weeks depending on the species under supplemental LED lighting. At this point the transplants would be shipped to growers to finish. Right now, whether it’s culinary herbs, vegetables or ornamentals, lighting these young plants is more economical than lighting the finished crops.”

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For more: Roberto Lopez, Michigan State University, Department of Horticulture, East Lansing, MI 48824; (517) 353-0342; rglopez@msu.edu; https://www.canr.msu.edu/people/dr_roberto_lopez.

This article is property of Hort Americas and was written by David Kuack, a freelance technical writer in Fort Worth, TX.