Broad-Spectrum Lighting Returns Higher Cannabis Yields
“Before, we never really had lighting options. You just [grew with] HPS [high-pressure sodium] or metal halide, and you just grew with whatever spectrum that bulb came with,” explains Jason Sanders, the cultivation manager for TOCC. “Well, the beauty of LEDs is we now have full control over the light quality and the intensities, really allowing us to maximize plant performance.”
TOCC has conducted studies on light spectrum, “everything from broad white to high red and everything in between,” Sanders says, as well as on light intensity. In the most recent study conducted at the facility, researchers examined the impact of the amount of red light hitting the canopy.
The findings? Using a whiter light (meaning a light with a broader spectrum) throughout all phases of growth correlates to larger dry weight flower yields than LEDs with a higher red light content (typically referenced as pink light).
The study required each of TOCC’s three flowering rooms to be broken into four zones, each representing a different spectral treatment. The light treatments differed with varying fractions of red light. The R4 zone was set at 40% red light (meaning it had the broadest/whitest spectrum), the R6 zone at 60%, and the R8 zone at 80% red light. The treatments were deployed in a randomized complete block design across the three rooms to ensure statistical validity to the study.
Additionally, each zone had a variety of type 1 (THC-rich), type 2 (THC:CBD balanced) and type 3 (CBD-rich) cannabis cultivars, to identify cultivar-dependent spectral responses. Light intensity was normalized across spectral treatments applying high photosynthetic photon flux density (PPFD) at the top of the canopy.
The results of this study showed that the R4 treatment with the broadest white spectrum yielded the greatest floral bud dry mass, with TOCC seeing an average 17% increase in dry weight of type 1 flower compared to the control plants. For type 3 plants, the average increase was 11%, Sanders details.
Growers might see these results and determine that a broad white LED will yield the optimal results, but Dr. David Hawley, Ph.D., a senior scientist with Fluence, notes that there are other considerations to spectrum, including operating costs.
“Red diodes inherently are cheaper to run,” Hawley says. “They have really good electrical efficacy, converting electrical energy to photosynthetic output. When you can get more light out of a fixture for less money input, that's good. We always want to look at that balance: What's the best light from a pure plant performance perspective versus what light is the most economical for the grower? Because, at the end of the day, we understand the growers are going to be most concerned about their bottom line.”
That said, “with cannabis being such a high-value crop, the final quality has a major impact on the bottom line. Our research shows a broad-spectrum light so vastly outperforms the kind of plants that you get under a more red light, that … the energy savings between a broad white light and a pink light is so minor when calculating the value of the final product. The increase in revenue with the higher quality product quickly offsets the slightly higher operational costs,” Hawley says.
Sanders adds that in addition to looking at biomass yield, the study also looked at how the different amounts of red light affected secondary metabolite levels, but no significant differences were noticed amongst the different spectra.
Too Much Red Negatively Impacts Plant Quality
At the other end of the spectrum, the R8 and R9B lights, typically referred in the industry as pink light, had a negative impact on plant quality. “When we're growing at high PPFD, as we start going really high red, like with the R8, we're getting photo-bleaching of the flower,” Sanders says, adding that they “look like albino buds.”
As Hawley explains, “when you oversaturate photosystems in plants by bombarding them with narrow waveband of energy they readily absorb, like red light, you can essentially blow them apart. It's literally chlorophylls becoming so energized that their electrons become more reactive with oxygen and the whole molecule disintegrates.”
A broad-spectrum light doesn’t create this problem, as no one component of the photosynthetic machinery is being oversaturated. “When you have more red light, you're focusing so much energy into a narrow bandwidth, that chlorophyll tries, and fails, to absorb all of that energy causing it to self-destruct,” Hawley says. “With white light, you maintain your chlorophyll, overall plant photosynthesis stays high, yield stays high, and all is well.”
Questions Requiring More Study
While Hawley and Sanders are confident in results with the tested cultivars, one big question remains: How broadly can this spectrum and intensity to be applied?
“Should we tell everybody, ‘Hey, this is the best spectrum’ always?” Hawley asks. “The answer is dependent on the crop and the light intensity you are growing. Whether we look at tomato, lettuce, or cannabis, it's never the same spectrum for every single genetic variant out there. It's not how plants work. But what this research has shown is in cannabis production, a crop that takes high light levels exceeding 1,000 or more micromoles, broad white is best. For other crops that require lower light intensities, we are evaluating their responses under different light qualities.”
While Fluence and TOCC plan to replicate this study with different cultivars and conduct new studies looking at more exotic spectra, Hawley shares that “the best evidence that we have right now is Fluence's R4 spectrum is going to give you the best quality and best yield…. It's going to allow you to deploy your lights at an intensity far greater than you could achieve with other spectra, and that's going to lead you to the highest yields.”
Sanders says TOCC has already made the switch to exclusively using LEDs in all its grow rooms, from mother rooms to flowering. “Pretty much right after we started seeing the results in our flower rooms, [after] we started getting great yields, quality, plant performance, reduced electrical loads per light, we went ahead and made the full switch to LEDs in this facility.”
Learn more about this research on Fluence’s website.
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