Originally published in Issue 3
When it comes to using supplemental lighting on their crops, growers have options whether they’re trying to achieve a photoperiodic or a growth response.
An increasing number of growers are using supplemental lighting for photoperiodic control and for accelerating plant growth. Growers have a variety of options when it comes to the type of lights available and how to use them most effectively. Growers who decide to use supplemental light to accelerate crop growth should expect to have to modify production schedules and possibly change some of their cultural practices.
High intensity discharge lights
James Grouzos, U.S. greenhouse consultant at P.L. Light Systems, said 90 percent of the greenhouse growers in the United States and Canada who are using supplemental light to increase plant growth are operating high pressure sodium lamps. High pressure sodium and metal halide are high intensity discharge (HID) lights.
“The bulbs contain a mixture of gases that are hit with electrical energy.
Basically the energy is vaporizing the gases,” Grouzos said. “It’s high intensity discharge since there is a lot of energy being produced by these gases. The mixture of gases in the bulb can be changed to create a blue spectrum or a yellow spectrum. When the electricity goes through the tubes the gases burn at a specific temperature called a Kelvin temperature. For high pressure sodium it’s usually around 3,400 Kelvin and it’s around 4,500 Kelvin for metal halide.”
Grouzos said metal halide lamps are not used extensively in the greenhouse industry. Metal halide bulbs produce more blue light, which can be used to control vegetative growth. He said metal halide lights are used in some lettuce production facilities, primarily by growers who are producing red leaf varieties and want more blue light in order to intensify the red leaf color or to produce shorter plants.
“Some growers may want the additional blue light from metal halide lamps to keep their plants short and stocky,” he said. “For most growers the blue spectrum of natural sunlight coming into the greenhouse is usually enough most of the time. If plants were grown inside, like in a growth chamber, where there is no exposure to outside light, they would normally be grown with metal halide lights or a mixture of metal halide and high pressure sodium.”
Grouzos said high pressure sodium bulbs produce more of an orange light similar to the light given off by street lamps.
“High pressure sodium lamps put out more light per watt and they hold their light levels longer than metal halide,” he said. “The high pressure sodium lamps provide more orange, yellow and red wavelengths. The reason sodium lamps are used in greenhouses is because they don’t degrade as fast and they produce more light per watt of energy. Growers produce more light and get more photosynthesis with the sodium lamps.”
Grouzos said metal halide bulbs have to be replaced after 8,000 hours. High pressure sodium bulbs usually have to be replaced after 10,000-12,000 hours.
Light emitting diodes (LEDs)
Johann Buck, technical services manager at Hort Americas LLC, said growers of both ornamental plants and vegetables are using light emitting diodes or LEDs.
“LEDs are being used for tissue culture propagation and seed germination of both ornamental and vegetable crops,” Buck said. “LEDs are also being used with ornamentals for photoperiodic lighting. Production of leafy greens and microgreens is another segment where LEDs are seeing increased use. Then there is interlighting in which LEDs are placed in the plant canopy.”
Interlighting has been used primarily with greenhouse vining crops like tomatoes and cucumbers and with fresh cut flowers including roses.
“The lights are placed down in areas of the plants where they are not receiving enough micromoles or photosynthetic active radiation (PAR),” Grouzos said. “By putting the LEDs down inside the canopy more light is being delivered to the plants.
“The interlighting is usually done with LED strips. There are LED lights on both sides of the strip so that when they are placed down inside the plant canopy there is light on both sides of the strip. Interlighting using high pressure sodium lamps can’t usually be done because of the amount of heat given off by the lamps.”
Buck said the combination of overhead high pressure sodium and interlighting LEDs may allow growers to reduce the light intensity overhead.
“This might result in less electricity used by the high pressure sodium lamps without a reduction in light levels because of the interlighting and similar crop yields,” he said. “This could also lead to a reduced heat load from the high pressure sodium lamps above the plants. Since high pressure sodium lamps produce a lot of radiant heat, this reduction might be beneficial because too much heat could impact the growing points of the plants.”
Grouzos said the heat given off by high pressure sodium lighting can help to offset heating costs during the colder months when the lights are needed the most.
Buck said with ornamental crops the biggest opportunity regarding LEDs is photoperiodic lighting.
“For photoperiodic lighting, most growers know that incandescent light bulbs are being phased out,” Buck said. “Growers understand whether or not incandescent bulbs are removed from the market that there are other sources available that can be used to grow a good crop. LEDs offer the potential to do programmable flowering. LEDs can replace incandescent and compact fluorescent bulbs. The possibility of actually programming flowering or initiating earlier flowering has been observed. Researchers at Purdue University and Michigan State University are looking at honing in on the specific wavelengths to figure out what would be a general application and even a more specific application with LED lighting.”
One area where light manufacturers are working is to develop LEDs to replace high pressure sodium and metal halide bulbs for high bay lighting.
“The footprint for high pressure sodium lamps covers more area than current LEDs,” Buck said. “Lighting technology is changing and the potential is there for horticultural LEDs to eventually replace traditional high bay lighting.”
Buck said another application for LEDs is multilayer production or vertical farming for crops including lettuce and microgreens.
“A lot of what is being done with multilayer production isn’t new,” he said. “As far as vertical farming, the Japanese have been operating plant factories for quite some time. Plant factory type production maximizes volume, which can be heads of lettuce or grams of microgreens. The technology is gaining attention in other parts of the world, including the United States. Even ornamental plug growers are starting to consider large scale multilayer production.”
Buck said LEDs lend themselves well to the production of leafy greens, herbs and microgreens. “These are relatively low growing crops which are less than a foot tall compared to high bay vine crops that are 8-12 feet tall. LED wavelength recipes have been developed to produce lettuce, herbs such as basil, and a variety of microgreens in a timely fashion. LEDs like the deep red/blue combination available in production modules are great for producing lettuces and microgreens. Some growers have used a combination of fluorescent lighting and LED production modules. Fluorescent tube lighting has been the lighting source of choice for multilayer production because it is a more compact lighting source. The fluorescent lights also don’t produce as much heat compared to high pressure sodium lamps.”
Jonathan Frantz, who has been a research scientist at DuPont Pioneer since January 2013, previously worked nearly 10 years at USDA, Agricultural Research Service. While at USDA Frantz worked on several projects, including the grower software program Virtual Grower. During his last eight years at USDA, Frantz did most of his research on plant growth lighting, including LEDs, high pressure sodium, metal halide and induction lighting.
“Induction lighting technology isn’t new. It just never gained traction because of the timing of when the invention came out,” Frantz said. “Thomas Edison and Nikola Tesla developed different strategies for creating lighting. Edison invented the incandescent bulb and Tesla designed induction lighting.”
Frantz said an advantage to induction lighting is that there is no filament so there is nothing to burn and nothing to burn out.
“With incandescent lamps that fail, it’s because the filament has been heated so much that it breaks so it no longer conducts current and no longer glows. Induction lamps don’t have those filaments and so don’t have this characteristic as part of their failure. As a result, induction lamps can be very long lived compared to regular incandescent bulbs. I have not yet seen an induction lamp fail.”
Frantz said the specifications for induction lamps indicate that that they are supposed to last twice as long as LEDs.
“The life expectancy for an LED is often sighted at 50,000 hours,” he said. “A spec sheet for an induction lamp says life expectancy is 100,000 hours. Even if it lasted for only half that time that would put it in the same ballpark as LEDs.”
Frantz said modifications can be made to induction lamps to adjust the light wavelengths, but they are not as flexible as LEDs.
“With LEDs, growers can basically pick the wavelengths they need,” he said. “Induction lamps, at least the ones that I have seen, the internal coatings or the ratios of those coatings have to be changed, which don’t have just one wavelength associated with them, but several. So that changes the peaks and valleys of several wavelengths rather than just one like with LEDs.”
Frantz said even with the long life of induction lights, the large footprint of the fixture would be a drawback to trying to use them in a greenhouse.
“The ones that I was using had about a 2-foot by 4-foot footprint of the lamp plus the reflector,” he said. “The ballast, which is located on top, was smaller than the one that would come on a comparable wattage high pressure sodium lamp. The footprint of the induction lamp is large.”
Frantz said one of the advantages of the induction lamps compared to high pressure sodium and metal halide is they give off very little heat.
“You can get very close to an induction lamp without feeling too warm,” he said. “If you have been in a greenhouse when high pressure sodium or metal halide lights come on there is a considerable amount of heat given off. This much heat can potentially alter the leaf temperatures, which can be good or bad. That can potentially alter how much additional heat you have to put in the greenhouse if you’re using high pressure sodium, metal halide or induction lamps. Applications where there is a need for “cool” lights as in a growth cabinet or chamber where there are a lot of seedlings, that’s where induction lighting is going to be most useful. Like LEDs, induction lights would require some cooling, but not nearly as much as high pressure sodium or metal halide.”
Another use for induction lamps is with the production of red leaf lettuce. That was the one crop that Frantz saw a major difference in growth.
“Red leaf lettuce responded to induction lighting by producing deeper red leaves,” he said. “I did not see much difference in plant growth in other crops like zinnia and tomatoes with the different induction lamps that I tested. The plants all grew well. They flowered all about the same time. It was all related to the amount of light rather than wavelengths with the plants I tested. I also tried a number of cultivars of lettuce.
“If a grower produces a lot of red leaf lettuce and his customers value the deep redness, they might perceive that as higher quality. In one particular grower’s case, this deep red color was what his customers valued. He felt it was justifiable to make the switch from high pressure sodium to induction lighting because he was growing red leaf lettuce and his customers valued that difference in color.”
For more: James Grouzos, P.L. Light Systems; (800) 263-0213; James@pllight.com; http://www.pllight.com. Johann Buck, Hort Americas LLC; (469) 532-2369; firstname.lastname@example.org; http://www.hortamericas.com. Jonathan Frantz, DuPont Pioneer, email@example.com.
David Kuack is a freelance technical writer in Fort Worth, Texas; firstname.lastname@example.org.