Different spectrum colors serve different purposes in the growth and development of the cannabis plant. Light, which is one of the most crucial factors in cannabis cultivation, comes in different colors. How does one know which spectrum to choose? Do we know how specific wavelengths shape our plants or affect their flower quality? Well – we know quite a bit, but a lot is still unknown. Here’s what we know so far.
Figure 1: Electromagnetic radiation
Effects of UV Light on Cannabis
There are certainly a lot of stories, myths, and rumors about UV. Valoya’s research efforts in this field combined with data from scientific literature should help growers better understand what UV does to cannabis.
The effect of UV on cannabinoids was first presented in an academic paper by Lydon et al. 1987. Lydon and his team concluded that UVB increased the floral THC concentration, however other cannabinoids were not affected by the UV exposure. Note that this paper talks about UVB (280-315nm) which is very intense radiation and typically not used in horticultural lighting.
In some horticultural light spectra, such as the Valoya NS1 and Valoya Solray385, the type of UV used is UVA (radiation between 315-400 nm). A little bit of UVA creates gentle stress that drives the plant to develop better defense mechanisms – cannabinoids.
However, a little too much and the plant’s growth is stunted and it underperforms. Valoya has tested UVA LED chips of different peak wavelengths to find the sweet spot where we get maximum cannabinoid output, without harming the plant’s natural processes.
According to our studies, we can optimally increase the floral and leaf cannabinoid concentrations in cannabis by using the 385 UVA wavelength. Note that, even though this is actual UVA light, that it is very mild and would not have adverse effects on humans exposed to it.
In simple terms, this balanced amount of high energy irradiation ‘toughens the plant up’ i.e. gets it to produce the precious cannabinoids which help it stay protected from abiotic stress. Growers use various other techniques such as light deprivation prior to harvest to achieve the same output. Another good outcome of high energy radiation is that some pathogens cannot withstand it.
To our understanding there are only two or three other LED manufacturers that use UV in their horticultural lighting. The reason most stay away from it is that it requires the luminaire to be made of highest quality materials in order to be able to produce UV light consistently and long-term.
Effects of Blue Light on Cannabis
Blue light has many similar effects as the UVA light. Blue photons however carry less energy compared UV, therefore the responses of blue can be milder than those of UV.
However, blue light has been shown to increase cannabinoid concentrations, especially THC, as demonstrated in the article by Magagnini et al. 2018. This was the first article to compare the differences between LEDs and HPS on cannabis growth and flower quality.
In general, blue light enhances compactness and makes plants bushier. This is good because very tall, stretched plants cannot support large flower formations. HPS typically grows those tall, slim plants due to complete absence of blue wavelengths in its spectrum.
Due to their bluish spectrum and the compactness this creates, traditional MH (metal halide) lamps have been popular for the vegetative phase. Chlorophylls absorb blue light efficiently and the plant can use the energy coming from blue photons in photosynthesis, making blue light very beneficial for the total yield output.
A typical, pink LED light is a combination of a high amount of red and a scarce amount of blue. Similarly to UV, blue LED chips are more costly. But blue light is crucial for good development of the cannabis plant.
Effects of Green Light on Cannabis
Not many studies can be found about the effect of green light on cannabis growth or metabolism. However, if one reads carefully, there are clues and data available even from the very early papers.
Mahlberg and Hemphill (1983) used colored filters in their study to alter the sunlight spectrum and study green light among others. They concluded that the green filter, which makes the environment green by cutting other wavelengths out, reduced the THC concentration significantly compared to the daylight control treatment.
It has been demonstrated that green color can reduce secondary metabolite activity with other species as well. For example, the addition of green to a light spectrum decreases anthocyanin concentration in lettuce (Zhang and Folta 2012).
If green light only reverses the biosynthesis of some secondary metabolites, then why put green light into a growth spectrum at all? Well, there are a couple of good reasons. One is that green penetrates leaf layers effectively. Conversely red and blue light is almost completely absorbed by the first leaf layer.
Green travels through the first, the second, and even through the third layers effectively (Figure 2). Lower leaf layers can utilize green light in photosynthesis and therefore produce yields as well.
Figure 2: The Solray385 spectrum above the plant (A), below one layer of leaves (B), below two layers of leaves (C) and below three layers of leaves (D).
The second good reason to include green and yellow wavelengths is because their addition to red and blue light results in a spectrum that appears white to human eyes. A full spectrum, containing all wavelengths creates a better working environment for people and makes it easier to detect pests, pathogens, and nutrient deficiencies. On top of that, it is nicer to work surrounded by white light, such as office space illumination, rather than the piercing pink link that most LEDs produce.
Effects of Red Light on Cannabis
How about red then? We know that red is used efficiently in photosynthesis, but is it good for anything else?
In many species the red to far-red ratio is one of the most crucial factors determining the plant morphology, flowering and even germination. When it comes to cannabis, the R:FR ratio has not been shown to influence the flowering time (Magagnini et al. 2018), but its effects on morphology can be demonstrated.