Ask any grower how much light a cannabis plant needs. Most will say, "a lot." Now, try asking them how much light they would need if they wanted to increase the yield. Most likely, they'll respond, "as much as you can." But how do they actually know? There's a certain threshold beyond which more light won't do anything, and may even be harmful. Yet, with cannabis, it seems as if the more light you give, the better the yield will be. "Trust me, bro" is usually the source of that information. Anecdotal evidence abounds regarding the relationship between light intensity and cannabis yield. However, that may not be enough when a grower is dealing with razor-thin margins and running a costly, high-maintenance operation, leaving no room for "trust me, bro" assurances. Fortunately, thanks to increased efforts and freedom in cannabis research, a scientific basis for these anecdotes has emerged through the work of Wageningen University PhD Wannida Sae-Tang. In her recent paper, done in collaboration with Signify, she explored whether such anecdotes hold true and why it seems cannabis plants are so eager to absorb as much light as possible.
"At the time I started the experiment, growers were discussing how much light cannabis needs," Wannida explains. "At the plant physiology level, plants can only take in so much light. At a certain point, too much light can actually damage the plant. However, cannabis growers have historically tended to maximize light intensity, ranging between 600-2000 micromoles. At that time, I felt it was a bit excessive, but this is the normal light range used by medical cannabis growers."
Does it really need all that light?
Wannida undertook this research with one guiding question: Is it true that the cannabis plant needs all that light? "Initially, we didn't believe the cannabis plant would grow more if you increased the light intensity. However, our results proved our initial understanding wrong," she chuckles.
One of the main focuses of the research was to assess flower yields, as well as metabolites since that is the goal for every medical cannabis grower. "For our research, we couldn't measure every single metabolite, so we focused on major cannabinoids like THC, CBD, and their precursors," Wannida points out. "We also focused on a group of terpenoids." To the researchers' surprise, all these metrics increased as they increased the light intensity. "We found that not only did yield increase as plants were exposed to more light, but metabolites also increased. In other words, both quantity and quality improved with higher light intensity."
Throughout the research, the wavelengths used remained constant. "The majority of the light was red, with a bit of blue, and a small amount of green," Wannida explains.
Examining how different wavelengths affect growth was not part of this study, though some preliminary experiments had already been conducted. "We carried out preliminary experiments where we varied the amount of blue light, and at the same light intensity, we didn't see any effect. This seemed to indicate that whether you use high or low blue light, there's really no significant increase or decrease in terms of yield and metabolites. This was surprising for us, as many papers have claimed that high blue light can increase metabolites, which is also a belief held by many medical cannabis growers."
Increases all across the board
Wavelengths aside, the research results were quite clear: cannabis showed an exceptional response to high light intensity compared to other plants. "We observed that the photosynthetic rate of cannabis kept increasing as we provided more light. It didn't show any saturation, unlike a 'normal' plant, which reaches a saturation point."
But why does the cannabis plant behave this way? "My experiment didn't cover that," Wannida says. "But internally, we had some discussions. We now know for certain that high light intensity results in high yield. However, we can't say definitively why. There's a specific mechanism inside cannabis plants; they have a highly efficient system for specific enzymes. Understanding these inner mechanisms wouldn't only help us better understand cannabis plants, but we could also apply this knowledge to other crops. Perhaps, in the future, we might see genetically modified plants with similar mechanisms, like a tomato with high photosynthetic capacity that's better suited for specific geographic conditions."
While growers can be reasonably confident in the assumption that more light equals more yield, some caution is advised. "Even with LEDs, if you increase the light intensity significantly, it affects the temperature, so this needs to be controlled. Keep an eye on that, as well as your watering strategies, since more heat causes water to evaporate faster. Additionally, in our experiment, we provided extra CO2 to the plants. Even though we gave them more light, the primary source of photosynthesis is CO2. If a grower doesn't supplement with CO2, I assume the yield increase won't occur."