Light Spectrums

Now that summer is here and the sun is out, it’s a perfect time to talk about one of the stars of cannabis cultivation: light. Specifically, this week we’re bringing you an introduction to light cycles and the light spectrum’s impact on cannabis growth. For most of history, everybody grew their weed with the same exact light source: the sun. However, due to its illicit nature and uncontrollable growing environments outdoors, growing cannabis indoors became the go-to method and it has remained that way into the legalized market in large part due to the ability to control the environment - especially light. How and why are growers controlling light? To fully understand this, we first need to discuss the light spectrum, this being the full range of light waves, both visible and invisible, that help cannabis grow. All the colors we see around are simply different length ‘waves’ on the electromagnetic spectrum, measured in nanometers. If you’ve ever seen a rainbow or light shine through a prism and come out a rainbow of color, you know even seemingly white light features the full color spectrum inside. There are also infrared and ultraviolet lights, which are not visible to the human eye. Outside, the plants receive different types of light depending on time of day, season, and the location of the grow and as such, cannabis has natural responses to these cycles. Many plants - cannabis included - traditionally begin growing in the spring and flower later in the summer. The specific types of light that the plant receives at these times helps dictate its growth and indoor growers will mimic and modify these light schedules for optimal growth.


For example, red (600nm-700nm on the electromagnetic spectrum) and infrared light (700nm-1100nm) help plants get a sense of time and know when it’s day or night. Think how the sky is red at sunset. This is in part because red has the longest wavelength of visible light. Infrared light, which is not visible to the naked eye, lingers briefly even after the sun goes down. It is the ratio of red to infrared light that best helps plants understand when the sun is setting and rising. In the morning, the sky fills with infrared and red light as the sun rises which tips off the plant that it is daytime. As the length of the night gets longer, the plant begins to flower as it heads towards the winter months.


While the light we get from the sun features the full spectrum of light, different lights appear in different ratios depending on the time of day and the season. Blue light (400nm-500nm) is most prevalent in spring and encourages early stage growth with sprouts, seedlings, and vegetative-stage plants. Additionally, cannabis seedlings will primarily follow blue light to know which direction to grow in. It’s an important light source for sturdy leaves and strong branches and in high quantities, it creates plants with shorter and bushier leaves.


Finally, there is some emerging evidence that certain types of UV light might help increase THC and terpene production. There are several types of Ultraviolet light but we’re focusing on UV-A (315nm-400nm) and UV-B (280nm-315nm) light. These are the UV rays that give us suntans and sunburns and they each have an effect on cannabis as well. UV-A and UV-B light in too high of quantities is harmful and will damage the plant. However, UV-B is reported to increase THC content and increase flavor and effects according to some growers. One potential reason? Trichome production is the cannabis plants’ natural defense against UV rays. Check out our previous post on Trichomes for more information on that subject.


These types of natural light cycles and effects are easy for indoor growers to utilize and experiment with to grow plants to perfection.