Throughout history, humans have harvested cannabis for food, medicine and fiber. Archaeological evidence suggests it was utilized by humans 6,000-10,000 years ago. However, legal, commercial Cannabis production is a relatively new, rapidly growing, and highly profitable industry in Europe and North America.

Commercial cannabis production typically occurs indoors and is therefore incredibly reliant on environmental controls such as lighting for both vegetative growth (biomass) and budding (flowering). To maximize cannabis growth during the vegetative stage, it’s important to use high light intensity. To initiate budding, roper photoperiodicity control is necessary. The benefit of indoor grow operation is that artificial indoor lighting can provide a continuous and uniform cannabinoid yield for high-quality products. However, quality lighting systems can be costly and energy-intensive. Indoor cannabis production is known to be one of the most energy-intensive industries in the U.S.

Selection of lighting systems and light spectra are of utmost importance for maximizing success in large-scale Cannabis grow operations. However, many commercial growers in the Cannabis industry are still dependent on unreliable information, given the lack of scientific investigation in the U.S. in this field. For example, in the Cannabis industry, it is common practice to reduce the photoperiod to approximately 12 hours to initiate flowering. For other commonly grown flowering plants in the horticultural sector, flowering is initiated via night interruption (Blanchard & Runkle, 2010). Both methods initiate flowering, but the reducing photoperiod could lead to a reduction in plant yields. An improved understanding of Cannabis photobiology may improve the grower’s ability to maximize productivity while reducing costs.

What Are Photoreceptors?

How do plants perceive light? Photoreceptors. Photoreceptors are specialized cells that contain light-sensitive proteins. When photoreceptors are struck by sunlight, they absorb photons (units of light). Photoreceptors receive critical information about the environment from the photons, such as the strength and color composition of the light. The plant uses this information to modulate biological activity (ex. when and how to grow). The effect of light on plant morphology ( form and structure) is called photomorphogenesis.

Isn’t it just more light = more growth and less light = less growth? Yes and No. To understand the purpose of photoreceptors, we need to understand light and color. The electromagnetic spectrum encompasses all forms of light and energy. However, only a small amount of this energy can be seen by the human eye. We call that portion “visible light.” Visible light can be defined as light that contains wavelengths in the range of 400–700 nanometers (nm). This falls between the infrared and ultraviolet. The UV light is often further broken down into two categories UV-A (315-400nm) and UV-B (280-315nm). X-rays are also on the electromagnetic spectrum, but they fall outside the visible range( 0.01 nm – 10 nm), so of course, we cannot see them. The most significant part of the light spectrum for plants is PAR (400–700 nm), which falls within the visible light range. Over millions of years of growth and development, biochemistry in leaves has evolved to use different parts of the color spectrum for various purposes. Plant biologists, botanists, and horticulturalists have a good understanding of how plants use different color spectrum components during their growth cycles. Horticultural industry, growers tend to use different light spectra and intensities to manipulate plant morphology, secondary metabolism, and flowering. For example, blue and red light are crucial for photosynthesis to occur. Photosynthesis includes a set of reactions by which plant and phototrophic cells harvest, transfer, and store light energy in the carbon bonds of carbohydrates.

Most Cannabis growers understand that certain properties of light play a critical role in plant vegetative growth and reproduction (flowering), and secondary metabolite synthesis, and accumulation. However, recent research suggests that some artificial plant lighting systems, such as high-pressure sodium lamps or light-emitting diode (LED) lighting, will require improvements for large-scale Cannabis production to keep pace with market demand. By manipulating LED color-specific light spectra and stimulating specific plant photoreceptors, it’s possible to reduce operating costs while maximizing cannabis biomass and cannabinoid yield. This can include tetrahydrocannabinol (or Δ9-tetrahydrocannabinol) and cannabidiol for medicinal and recreational purposes.

Photoreceptors and Plant Growth

Red (~625–700 nm) and Far-Red (> 700 nm) Light

Red light impacts leaf and stem growth and leaf nutrient content, and it is required for chlorophyll production. In some plant species, flowering is induced when red light is delivered during the early part of the photoperiod and when far-red light is carried toward the end of the photoperiod. However, this may not be true for all Cannabis varieties or cultivar (genotypes). For example, the Cannabis genotype “G-170” is insensitive to changes in the Red: Far-Red ratio (Magagnini et al., 2018). Magagnini et al. (2018) found that a low R: FR ratio during a long photoperiod (18 h light, 6 h dark/vegetative stage) is very helpful to the full maturation of cuttings. These findings are contrary to popular belief in the cannabis industry.

Blue (~450–520 nm) and UV (< 400 nm) Light

Blue and UV light regulate various physiological and developmental processes, including water transpiration and CO2 exchange (Schwartz & Zeiger, 1984). Blue light mediates chlorophyll and chloroplast development (required for photosynthesis), enzyme synthesis, and plant density, and regulates responses to biotic environmental stresses. One study found that Cannabis plants are grown under blue light with a shorter photoperiod (12 h light:12 h dark/flowering stage) improved cannabinoid content (Magagnini et al., 2018). This same study suggested that a combination of UV-A (315-400nm) and blue wavelengths induce cannabigerol accumulation in Cannabis flowers. UV-B (280-315nm) is often considered more damaging to plant growth and development (compared to UV-A), and in large quantities, it is. However, small amounts of UV-B have essential benefits, such as promoting pest resistance, increasing flavonoid accumulation, and improving photosynthetic efficiency (Zoratti et al., 2014). UV-B light also elicits THC accumulation in both leaves and buds (Potter & Duncombe, 2012). Other studies have reported increased THC concentrations when cannabis plants were grown with 3h of daily supplemental UV-B radiation. This suggests that cannabinoids may play some role in UV protection.

Green (~520–560 nm) Light

Green light is often considered unavailable for plant growth since photosynthetic plant cells have limited absorbance for these wavelengths. Most “green” light is reflected off the surface of the leaf, which is why leaves appear green. However, for some plant species, there is evidence that small amounts of green light may be available for use in plant growth. Kim et al. (2005) found that a low percentage (≤ 24%) of green light enhanced plant growth, whereas > 25% green light inhibited plant growth. THC levels in cannabis plants are negatively affected by the presence of green light. So the small benefit of increased plant growth may not outweigh the costs (Magagnini et al., 2018).

LED vs. Metal Halide vs. High-Pressure Sodium

Light spectrum influences plant growth, flowering time, THC/CBD production, cannabinoid quality, and cannabinoid secondary metabolite production. One study found that Cannabis sativa flowers that were grown under LED lights produced 9.5% THC and that flowers grown under high-pressure sodium lights (HPS) contributed 15.4% THC. Other cannabinoids like CBD and cannabigerol show higher concentrations of THC when viewed under LED light treatments compared to HPS lights. A different study showed that combining 530-nm LED light, 440-nm LED light, 655-nm LED light, and metal halide lamps increased dry bud yield by 18–24% (compared to plants grown under a less specialized lighting system). Research found the same trends with cannabinoid and terpene concentrations (Hawley, 2018). Several studies have found that responses to light are cultivar/strain specific. Light recipes do not elicit the same reactions of plant growth, THC, CBD, and terpene production for all cultivars.

*Male and female Cannabis plants are substantially different in growth form and physiology. For this review, when I refer to Cannabis, I will be referring to female cannabis plants that produce flower buds, as opposed to hemp.


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