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Researchers have been hard at work this year, exploring the best lighting options and ways to improve produce production under cover. We rounded up some of the best tips and findings of 2020 here.

Lower energy lighting

Produce Grower, February 2020

An experiment out of Wageningen University & Research in the Netherlands has shown that there may be some more efficient options in supplemental lighting for growers to consider.

Comparing the traditional high-pressure sodium lights often used as supplemental lighting in greenhouses with a broad-spectrum white LED light, the university found that tomato crops have the same, or higher, yields under LED lights.

Dr. Ep Heuvelink, associate professor at Wageningen, says the university was interested in comparing HPS lighting with white LEDs as more and more growers use supplementary lighting in their operations. Researchers set out to observe the role of the different wavelengths in the spectrum on crop development and crop physiology.

“For tomatoes, we’re talking about hundreds of hectares with supplementary light, and the norm is the high-pressure sodium lamp,” he says. “But everybody knows that LED is more electrically efficient than the high-pressure sodium lamps.”

Wageningen was particularly interested in the effects of broad-spectrum white LED lights because most growers using LEDs opt for 95% red lighting with a small amount of blue. “And that is not because that’s the best for the plant, but it’s because of economic reasons — because they’re LEDs, they’re most efficient in turning electricity into light,” he says.

Shedding some light

At the end of the experiment, Wageningen researchers found an 11% higher yield in the Tomagino cultivar and a somewhat higher yield in the Malice cultivar, although Heuvelink says it was not a statistically significant increase.

The study also included a look at lighting effect on brix levels, finding no differences between the two types of lighting when it comes to fruit sugar content. And while the experiment did not particularly look into quality issues like acidity or shelf life, Heuvelink says researchers didn’t observe any external quality differences.

If you’re thinking of making the move into LED lighting, there are definite cost benefits, Heuvelink says. “The numbers are always a little bit different, but I think it’s safe to say that it’s at least $40 more efficient with LED compared to HPS light,” he says.

But be aware of the investment costs as they can vary greatly, he notes.

Supplemental lighting

Produce Grower, March 2020
Photo © warloka79 I adobe stock

Michigan State University Professor and Extension Specialist Erik Runkle and Assistant Professor Roberto Lopez collaborated with Greenhouse Lighting and Systems Engineering (GLASE) to discuss supplemental lighting management techniques. Here are some of their findings:

Stages 1 to 2: Stick to callus formation — For stage one, “you want to try to target a DLI of anywhere from 4 to 7 moles during this time period,” Lopez says. “Also, it’s very important to note that you want to provide indirect or diffused lights. This is primarily from the sun by utilizing shade curtains or white watts to maintain a photosynthetic photon flux density (PPFD) of anywhere between 120 to 200 micromoles.”

Stage 3: Root development — Stage three calls for increased light intensity. If growers are providing 70 to 120 micromoles of light with a target of anywhere from 8 to 12 moles per day, once those roots have initiated, Lopez advised to increase PPFD to between 200 and 400 micromoles.

Stage 4: Toning — In this stage, growers are preparing to tone liners for shipping or transplanting. Using 70 to 120 micromoles of supplemental lighting and maintaining a DLI greater than 12 moles of light is recommended.

“In some parts of the country, it’s going to be very difficult to maintain 12 moles of light because even with supplemental lighting, you may only be able to achieve 10,” Lopez says. “But as we tell most growers, the higher that DLI, the higher quality of crop you’re going to be able to produce and the faster you’ll be able to get that crop out of the greenhouse.”

At this point, growers can increase light intensity anywhere from 500 to 800 micromoles.

Supplemental lighting during plug production

To achieve a DLI of 8 to 12 moles, previous MSU research recommends providing 70 to 90 micromoles of supplemental lighting during stages three and four.

“We find that when you provide supplemental lighting for plugs, you’re going to reduce the production time of those plugs, you’re going to see an increase in stem diameter, which is going to produce a studier plug, and more branching,” Lopez says. “Oftentimes when these young plants are grown under high-pressure sodium lamps, because of the additional heat, you’re going to see that these young plants can also flower earlier upon transplant.”

HPS or LEDs?

“It’s very situational,” Runkle says, noting the final answer is often contingent upon many variables such as available electricity, cost to purchase and install, cost of electricity, hours of operation, lamp efficacy, fixture longevity and maintenance, light spectrum, uniformity and intensity. It is also noted that plants under HPS are typically 2 to 3° F warmer than under LEDs.

Beyond red & blue radiation

Charlie Garcia & Roberto Lopez, August 2020
photos courtesy of charlie garcia and roberto lopez

High-quality vegetable transplants for high-wire production are defined as having thick and straight stems, compact growth with short internodes, well-developed and deep-green leaves, and shortened production times. A minimum daily light integral (DLI) of 13 mol·m–2·d–1 or greater is required to achieve these desirable morphological traits.

However, in greenhouses located in northern latitudes, the DLI can average between 1 to 5 mol·m–2·d–1 during winter months. Under these low light intensities, plants have weak stems and large leaves, are leggy, flowers are aborted, and subsequently, fruit abortion can occur leading to economic losses.

Supplemental lighting is commonly used to increase the DLI within vegetable transplant greenhouses during light limited times of the year. High-pressure sodium (HPS) lamps have been the industry standard for greenhouse supplemental lighting. However, the availability of energy-efficient light-emitting diode (LED) fixtures for horticultural applications are a promising alternative.

What they found

Beyond reducing the time to produce a high-quality vegetable transplant, the radiation quality (color) of LED supplemental lighting influenced the morphology and physiology of high-wire vegetable transplants when the natural DLI was low (=7 mol·m–2·d–1).

From previous MSU research we have concluded that LED supplemental lighting must contribute to greater than 40% of the total DLI to elicit morphological responses. Given this, supplemental lighting radiation quality could be used to elicit desired high-wire vegetable transplant morphological features as they can differ depending on the intended use.

For instance, seedlings can be used as rootstocks, scions, or as non-grafted transplants. Grafted seedlings benefit from an extended hypocotyl [the part of the stem beneath the seed leaves (cotyledons) and directly above the root] length, since it helps to increase grafting success and hence survival rate, and reduce rooting from the scion after transplant. However, elongated hypocotyls are not desired for non-grafted seedlings, as it can lead to weak transplants and logistical challenges for shipping.

Thus, a grower producing non-grafted transplants might utilize supplemental lighting providing B25R95 radiation as tomatoes, peppers and cucumber were the most compact under this treatment. However, they should also consider that parameters such as leaf area and fresh weight were negatively impacted under B25R95 radiation.

Lastly, we have also determined that LED supplemental lighting providing B30G30R60 radiation produces comparable quality high-wire transplants to those grown under HPS lamps. Therefore, we can conclude that LED supplemental lighting is an alternative to the current industry standard for high-wire transplant production.