In the last decade, lighting for controlled environment agriculture has evolved rapidly thanks to improved energy efficiency, spectral tuning and fixture design. These improvements helped to drive the commercial adoption of light-emitting diode (LED) fixtures, especially in indoor vertical farms.

Today, growers can choose from a wide range of LED fixtures with different specifications. The selection depends on factors including the fixture cost, efficacy (the photon output per unit energy), the light spectrum, the form factor, and the light responses of crops.

While it is straightforward to compare the fixture cost and efficacy, it is often unclear how different commercial light spectra compare in crop production. To further complicate matters, fixture efficacy is related to its light spectrum. These are the reasons behind a recurring debate within controlled environment agriculture on broad-spectrum (white) LEDs versus red + blue LEDs.

White LEDs

Commercial white LEDs are blue LEDs with phosphor coatings that distribute most of that light at longer wavelengths. The remaining blue fraction varies depending on the phosphor material. Warm-white LEDs emit a relatively small blue fraction (e.g., 7%), whereas cool-white and daylight LEDs emit larger blue fractions (e.g., 20% and 30%, respectively). White LEDs emit blue, green, red, and sometimes far-red light, thereby covering a broad range of photosynthetically active radiation.

Some broad-spectrum LED fixtures include additional blue and/or red LEDs alongside white LEDs to create distinct broad spectra. White LEDs create a visually pleasant environment for workers to inspect crops, unlike red + blue LEDs, which poorly render colors of objects.

Red + blue LEDs

Red + blue LEDs are common in horticulture primarily because of their higher efficacy than white LEDs.

Red and blue light are also perceived as the “optimal” wavebands for plant growth because: 1) in the lab, chlorophylls mainly absorb red and blue light, but not green light; and 2) the McCree curve shows high quantum efficiency of photosynthesis (on an instantaneous basis) under red and blue light.

However, these do not consider the roles that green light plays in plant growth. Green light drives photosynthesis and controls plant shape. Our previous research (bit.ly/greenblueLEDs) showed that the inclusion of 33% green light did not affect lettuce dry weight under low blue light (0 or 20 µmol·m–2·s–1), although it decreased lettuce dry weight under high blue light (60 or 100 µmol·m–2·s–1).

A missing link

There is no census on whether white light or red + blue light is better to maximize crop yield in indoor growing systems. Proponents of white light stress the usefulness of green light and higher visual quality for workers. In contrast, proponents of red + blue light emphasize higher efficacy and perceived advantages in instantaneous photosynthesis.

Whole-plant growth, however, is the product of instantaneous photosynthesis and the ability of plants to capture light, which is influenced by the light spectrum. Therefore, efficacy aside, a missing link in this debate is how crop yield, not just instantaneous photosynthesis, compares under white light and red + blue light in indoor growing systems.

Studies

To address this, we compiled the results from three of our studies. The studies included side-by-side comparisons of hydroponic lettuce yield under warm-white light or red + blue light at various ratios.

We grew red-leaf lettuce ‘Rouxai’ indoors with a deep-flow hydroponic system at an air temperature of 68° F (in study 1) or 72° F (in studies 2 and 3). After sowing seeds in 1-inch rockwool cubes, we grew seedlings for 13, 11 and 11 days in studies 1, 2 and 3, respectively, and then transplanted them in 2-feet × 4-feet floating rafts (36 cells in studies 1 and 3 and 72 cells in study 2). We continued to grow the plants for 19, 14 and 19 days before harvests in studies 1, 2 and 3, respectively.

Plants began to receive different lighting treatments 4, 0 and 11 days after seed sow for 20 hours a day in studies 1, 2 and 3, respectively. All treatments delivered the same total photon flux density of 180 µmol·m–2·s–1. The only variable was the light spectrum. The relative spectral distributions of the warm-white, blue, red and far-red LEDs in these studies are shown in Figure 1. The warm-white LEDs emitted 7% blue, 29% green, 54% red and 10% far-red light.

For each lighting treatment, we calculated light use efficiency (LUE) of lettuce, which is the harvestable shoot fresh weight produced (in grams) per mole of light.

Results

In study 1, lettuce LUE under warm-white light was 16% lower than under 100% red light, but 28% higher than under 89% red + 11% blue light (Figure 2). Compared to warm-white light, lettuce LUE was 29% lower under 67% red + 33% blue light, and 50% lower under 44% red + 56% blue light. In other words, LUE decreased as the blue fraction increased.

In study 2, lettuce LUE was similar under warm-white light and 100% red light. Compared to warm-white light, lettuce LUE was 26% lower under 89% red + 11% blue light and 63% lower under 100% blue light.

In study 3, lettuce LUE under warm-white light was 33% higher than under 67% red + 33% blue light. In addition, we substituted far-red light for part or all of blue light in 67% red + 33% blue light. Compared to warm-white light, lettuce LUE was 20% lower, similar and 45% higher when 11%, 22% and 33% far-red light were included, respectively.

Lettuce growth responses were generally similar in the first two studies, albeit minor differences due to different growing periods, treatment durations and air temperatures. However, we used a different fertilizer in study 3 that stunted lettuce growth and intensified leaf redness.

In the first two studies, lettuce leaves under warm-white light had a mildly red tint. The red leaf coloration under warm-white light was between that under 100% red and 89% red + 11% blue light. To maximize crop yield and intensify leaf redness, red lettuce can be grown under a low blue fraction (e.g., 5–10%) and then several days (e.g., 5 days) before harvest, grown under a high blue fraction (e.g., 30%).

Take-home messages

Red + blue light is not superior to white light for plant growth. To the contrary, the LUE of red lettuce ‘Rouxai’ under warm-white light was at least 26% higher than under red + blue light when the blue fraction was at least 11%. It was 33–40% higher compared to red + blue light when the blue fraction was 33%.

The high crop yield under warm-white light can be attributed to its low blue fraction and inclusion of far-red light, both of which promote leaf expansion and thus light capture for photosynthesis.

Not all white LEDs are the same. Not all red + blue LEDs are the same. Without the LED specifications or comparative plant responses, it is not fair to say which is better. Compared to red + blue light with at least 11% blue light, the higher LUE under warm-white LEDs can offset their lower efficacy to some extent.

The efficacy of broad-spectrum LEDs can be increased by adding red, blue, and/or far-red LEDs to white LEDs, which can further narrow the efficiency gap between broad-spectrum LEDs and some red + blue LEDs.

In our next article, we will compare different broad spectra with and without supplemental red and blue LEDs.

Study author acknowledgements

We thank Osram, Osram Opto Semiconductors, MSU’s Project GREEEN, the USDA Specialty Crops Research Initiative, and companies that support vertical farming research at Michigan State University.

Dr. Qingwu (William) Meng (qwmeng@udel.edu) is an assistant professor of controlled environment horticulture in the Department of Plant and Soil Sciences at the University of Delaware. Dr. Erik Runkle (runkleer@msu.edu) is a professor of horticulture in the Department of Horticulture at Michigan State University.

Prospiant, formerly known under the brand as Rough Brothers, Nexus, and ThermoEnergy Solutions, has taken on a new identity with an updated mission — to embody health, wellness, long-term stability, financial strength and end-to-end expertise in all that it does. But, regardless of the rebrand, Prospiant remains an expert in its commercial greenhouse design and manufacturing roots, as well as the value it offers.

“The best aspect of a Prospiant greenhouse structure is quality,” says Group President, Mark Dunson. “Not only are the structures built-to-last by standing up to nearly any weather condition possible, but the greenhouse structures are designed to maximize agricultural productivity.”

Built with steel columns, trusses and concrete floors, structures at Prospiant are longstanding and carefully crafted based on growers’ needs. Cultivation goals, crop offerings and business objectives are all taken into consideration, as well as site location, climate and more. Other factors include the structures’ difference in size, complexity and required local ordinances, which are all discussed during the initial customer engagement process — a phase Dunson deems necessary.

“We think the first step is to have a rough plan and budget for the project or structure,” he says. “With that plan in mind, information gathering, and research will be productive.”

With a wide array of clientele, Dunson says Prospiant can work with growers during any phase of their buying journey — from the beginning stages of planning, architectural and engineering design, permit application and more, and even through the latter stages of expansion.

“We’ll provide tailored support that’s right for them wherever they are in their process. No customer is the same, [and] we really try to understand their needs and help them to make the optimal investment decisions.”

Like most items, structures from Prospiant can last a long time if taken care of. In this case, Dunson says structures can last up to 30 years with consistent inspection and maintenance of the structure itself, and supporting items. Overall, Dunson says structures from Prospiant are designed to maximize agriculture productivity — an important aspect of success in the industry.

“Agriculture is a competitive business and profit margins are often tight,” he says. “The difference between success and struggle can come from using the best possible cultivation tools to provide the edge for standing apart. Prospiant provides these tools by recognizing the unique situation of each and every customer. Our dedicated team develops greenhouses and integrated growing ecosystems for customers to reach their cultivation goals and to meet the demands of their customers and the market.”

Photo courtesy of Industries Harnois

Industries Harnois, a Canadian company manufacturing steel structures (greenhouses and fabric building), recently celebrated 55 years in the industry. Over that time, it has introduced new products, services and systems. One offering it has presented is its broad selection of greenhouse heating systems.

“At Harnois Greenhouses, we provide turnkey solutions integrating heating system, and have a team of experts that take into consideration different parameters to perform the best solution and approach,” Corenthin (Felix) Chassouant, sales director of North America says.

When it comes to heating, Harnois provides systems with hot water distribution or air heater, and heat generators that are adapted to various fuel types including natural gas, propane, biomass, fuel oil and electricity. When curating a heating system for growers, Félix says its team of experts analyze important factors such as budget, crops (biological), availability/cost of energy, efficiency and required space to implement the solution. They also consider the growers’ capabilities and experience using a natural gas, propane or biomass system for example.

These measures are taken into account because an efficient heating system is one of the most important steps for successful plant production, Félix says.

“Heating is very important because in some parts of the States, you cannot propagate year-round without a heating system. It will provide uniformity without releasing harmful materials,” he adds. “And a good heating system is a perfect way to provide optimal growing conditions such as temperature, humidity and in some cases, carbon dioxide.”

Besides the success that follows it, Félix recommends heating systems from Harnois because of its cohesive methods that work with other components of the company like the greenhouse design and model selection.

“What makes us different is our integrated solutions,” Félix says. “We have the experts, and our greenhouse heating solutions offer optimal performance. We have quality structures, engineers and a wide range of technologies to enhance production.”

Based in Kingsville, Ontario, Canada, Great Northern Hydroponics is a leading greenhouse operation best known for its tomato crops. Led by president Guido van het Hof, the company turned to LEDs for 16.5 acres of its 70-acre facility to reduce operating costs and improve production.

According to Van het Hof, Great Northern previously used high pressure sodium (HPS) lights and was one of the first adopters of supplemental lighting in Ontario. When LEDs became an option, he was curious about how broad-spectrum LEDs might help his operation. As a result, they began plant trials and research studies. The result was clear.

“We’ve spent a lot of time and energy on this work,” Van het Hof explains. “But over time, we’ve discovered that tomato crops perform beautifully under white light LEDs and come with higher yields and fruit weights as well. We’ve recorded 10% increases in fruit production and yield under Fluence’s broad-spectrum LEDs in comparison to previous harvests under other solutions. It’s a much, much better option.”

For Great Northern, there is also the added benefit of cost savings as the result of using LEDs. By retrofitting LEDs into 10-acres of glasshouse space, Van het Hof estimates that his business will save $150,000 Canadian (roughly $123,402 in the U.S.) in annual operating costs as a result of the switch. As part of the transition to LEDs, Great Northern can use fewer engines to fuel its entire facility. Before, it took three engines to power the facility. Now, it only takes two.

“That’s a really, really big deal for us,” he says. “We are always trying to be more efficient in how we work and this was a clear way of getting to where we want to be.” By working with Fluence, Great Northern was also able to qualify for the Independent Electricity System Operator’s (IESO) Save on Energy Retrofit program that Van het Hof says “drastically” reduced the cost of upgrading to LEDs.

As the business continues to grow, Van het Hof says that he plans to explore using LEDs to grow cucumbers, various peppers and strawberries. He also hopes that by using LEDs, Great Northern can produce increased tomato yields and keep yields consistent year-round. If that can happen – and Van het Hof is optimistic it will – then he says he expects profits to rise. The company’s retail partners, he says, are intrigued by the idea of being able to get the same volume of tomatoes year-round, thus keeping supply consistent.

“If we can do that, it would be a big win for us,” he explains. “It’s also something that is not possible without us making the switch to LEDs.”