Supplemental lighting for cannabis production is a hot topic among many growers. More and more manufacturers are offering more and more lights. With so many choices, how do you know you’re making the right choice? Does it even matter? And for that matter, what does “supplemental light” even mean? Supplemental lighting specifically refers to adding in more light using an artificial light source, such as high-pressure sodium or other fixtures, when growing plants under a broad-spectrum light, such as the sun. Supplemental lights are commonly used in greenhouse production and are well understood for many crops. Cannabis, however, just hasn’t been legal long enough to build up a strong background of research. With federal legalization of hemp production, opportunities for well-established academic institutions to dive into hemp and cannabis research are opening up.
Cornell University is one of the institutions currently taking hemp and cannabis research to the next level. The Mattson lab, in conjunction with the GLASE group, recently carried out several different experiments. One of these experiments focused on the effects of supplemental lighting on hemp yields of both dry bud or total CBD and THC content after harvest. LED grow lights have become more prevalent in the cannabis industry; understanding how cannabis performs under these lights and how much of the difference is due to spectrum versus hardware is an important area of focus. The researchers wanted to dig into how different combinations of red and blue LEDs, and one white LED treatment, compared to an HPS treatment, as well as ceramic metal halide (CMH). Eight total supplemental light treatments were set up, and a single variety of hemp was grown side by side in a greenhouse environment.
So, what did they find? Interestingly, white LEDs performed just as well as the HPS, and better than some of the red and blue LED combos or CMH, when it came to dry bud weight. However, none of the lights came out as an overwhelming winner. This tells us that, while there might be some slight differences in results, good biomass yields can be obtained under any light type under an experienced grower. CBD and THC content of harvested buds was examined next. There has been a lot of discussion revolving around whether light type or spectrum could influence our overall THC or CBD content; this experiment offered the perfect chance to dig into that. Overall, CBD levels stayed between 8-12 percent for all light treatments. THC showed a similar indifference to light; plants were allowed to go “hot” and averaged around 0.47 percent THC regardless of light.
Moving from the academic field to the private sector, companies such as Fluence also invest in testing and proving the lighting science behind their products. On a recent webinar, Fluence took a deep dive into the effects of both UV-B and far-red light on three different cannabis cultivars. The researchers took one high-THC cultivar, one high-CBD cultivar, and one balanced cultivar. Some growers suspect that higher amounts of cannabinoids may offer some protection from high-UV environments; other fear that UV light may break down cannabinoids during production. Data were gathered on fresh and dry bud weight, along with cannabinoid and terpene content for all varieties under all treatments. To design each treatment, plants were provided with a baseline of 1100 µmol/m2/s of broad-spectrum light, and either UV-B or far-red light were added on top.
So, what did they find? First, UV-B was found to stress and harm plants across all treatments. Fresh and dry bud weight decreased as the amount of UV-B light increased. This could have been a result of how the treatments were applied; UV-B lights were applied for either two, four, or six hours at the end of the photoperiod. The results of leaf rolling, necrosis, and damage to pistils at even the lowest treatment show us that researchers have not yet found an appropriate application level for cannabis. THC, CBD, and terpenes were not affected by UV light, but they also did not seem to offer any protection. With UV light, the “poison is in the dose”; small amounts of UV light aid in sanitation without stressing plants out. Tread carefully with UV lights!
Next, far red light was added on top of the background broad spectrum light. Ultimately, far red light seemed to function just like additional broad-spectrum lighting; buds were larger, but when they added the same amount of broad-spectrum light to an additional treatment, the same increase occurred. Far red light did not affect cannabinoid content or terpenes in any of the cultivars. Given this information, the researcher concluded that there was no real reason to include extra far-red light instead of additional broad-spectrum light. However, just adding far-red light is not the end of the story. Changing ratios of red to far-red light provoke different shade-avoidance responses (stretching) or leaf expansion. Researchers could dive deeper into these effects by adding red light alongside far-red in different ratios, to find out if plant height or fan leaf size and expansion can be manipulated through lighting choices.
These results might seem disappointing – no exciting and unique responses were observed, no huge scientific breakthroughs have been made. However, science isn’t all about huge breakthroughs. Debunking myths and proving consistency is just as important. Between just these two studies, growers can feel confident that they can grow cannabis no matter what light they choose, as long as it is bright enough. Growers can see that there’s still a lot to learn about UV-B light, and caution is needed when buying into less tested or more expensive technology. More and more universities are ramping up and publishing research into hemp; the coming months are sure to bring even more exciting news. Keep your eyes open and your lights on!
In the Garden 