Not all artificial lighting is created equal. When it comes to cannabis cultivation, it has recently been discovered that exposing plants to different spectrums of light causes noticeable changes in plant structure, or morphology, and in cannabinoid and THC content. In Greece and elsewhere in Europe, medical cannabis can only be cultivated within climate controlled spaces, meaning either indoors with no access to sunlight or in a greenhouse. This is to adhere to the Good Manufacturing Practices (GMP) guidelines which mandate air filtration to the cultivation area, preventing dust, mold and other contaminants from reaching the plants. Clearly, artificial lighting is needed to grow indoors, and year-round greenhouse cultivation also requires supplemental lighting.
The emerging technology of Light Emitting Diodes (LED) has revolutionized our ability to build artificial light sources with specialized variations in spectral composition. Small diodes using different materials emit light with different wavelengths, and many different combinations of diodes can be used in one lamp, allowing new spectrums to be created. Traditional High Pressure Sodium (HPS) grow lights depend upon the atomic spectral emission lines from mercury and sodium; therefore the spectrum cannot be altered. The various spectrum options of LED lights have been tested in scientific studies, showing interesting results related to plant structure, and content of medicinal compounds.
Not only can LED lights be constructed with many different spectrums, they also draw on average about 20% less power to produce the same amount of light intensity as HPS. However due to the nature of the light source, which is made up of many small diodes, and the reflector properties, the light intensity experiences a larger drop off on the outer edges of the growing area than HPS. This means plants in that outer zone will be smaller and underdeveloped. LED lights produce much less waste heat than HPS, and therefore can be placed much closer to the plant canopy, increasing light uniformity, but making the effective growing area smaller. These reasons are why more LED light fixtures must typically be used in a cultivation facility to achieve the same overall coverage as HPS. Developers continue to work on innovative ways to increase light uniformity and the effective growing area of the individual LED lamp.
Photosynthetically Active Radiation (PAR) is light which has wavelengths between 400-700 nanometers. These are the wavelengths the plant uses for photosynthesis, which provides energy for growth and other life processes. The spectrum of an HPS lamp peaks near 600 nm, giving the light a yellow-orange color. It contains very little blue light (in the 400-550 nm range) or UV (100-400 nm). Light in the 500-650 nm range is thought to primarily effect growth rates and yield, however now there is increasing data showing that light in the blue and UV ranges causes enhanced production of cannabinoids and terpenes, as well as other plant properties. By altering the percentages of different wavelengths of PAR and adding varying amounts of different types of UV light, controlled scientific studies have explored the effects of these different spectrums on the cannabis plant.
In a recent study, “The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.”, plants grown under HPS, and two different LED spectrums were compared for morphological differences as well as for THC, CBD, and CBG content. Light intensity was kept uniform between the three different set-ups. Researchers found that high pressure sodium lights caused taller plant growth, while the LED spectrums resulted in plants which were shorter and had less space between branching nodes off of the main stem, causing the plants to have more branches and flowering sites overall. Although flower dry weight per plant was found to be slightly higher, about 15%, with the HPS lights, production of THC, CBD, and CBG per gram of this dry weight was significantly higher with the LED spectrums.
THC content was shown to be 26-38% less per gram of dried flower with HPS compared to the LED spectrums, which included 2 to 3 times more blue light, respectively, than the HPS. CBD and CBG were shown to be significantly increased with plant exposure to UV light, which was contained in one of the two LED spectrums tested. THC content was also observed to be higher with the LED spectrum containing UV light. Although the LED spectrum with UV had the smallest yields of flower dry weight per plant, this is more than compensated for by the large increase in production of these medicinal compounds within the resin glands of the plant. Although slightly more plant flower dry weight was achieved using the HPS, this flower was significantly less concentrated in the valuable therapeutic compounds which are sought after through the cultivation of cannabis. Also, the shorter, more “bush-like” plants created by the LEDs are easier to manage and maintain in indoor and greenhouse horticultural settings.
Overall, the development of LED lights and the many different spectrum options they offer opens the door to an exciting new appreciation of the specific effects of wavelength upon the production of pharmaceutical compounds derived from cannabis.
By J.R. Tremblay
 “The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.”, Magagnini G.a · Grassi G.a · Kotiranta S.b
aCouncil for Agricultural Research and Economics, Research Centre for Cereal and Industrial Crops (CREA-CI), Rovigo, Italy
bValoya Oy, Helsinki, Finland