Light-emitting diodes (LEDs) have triggered a groundswell of horticultural research over the past ten years. Unlike the high-intensity discharge (HID) lamps of old, LEDs allow us to easily isolate single wavebands of light photons — otherwise known as colors of light. That newfound control of the lighting spectrum lets us know how plants respond to light, and it debunks some long-standing myths about the best light for cannabis.
Greenlight is subject to the most misconceptions. While many growers believe cannabis plants don’t utilize green light for photosynthesis, they actually do, and green light is important for other processes as well.
The myth that plants don’t use green light stems from their green color: if the plants are reflecting green, they’re not using it. But the reality is not so simple, and only a small portion of green light is reflected to give plants their green appearance. Moreover, our human eyes sense green light more easily than other colors; not that much is reflected.
In this blog, we explore how green light affects cannabis and other plants, and how you can select lighting to maximize your growth.
Greenlight can be defined as light with a peak wavelength between 500 and 600 nanometers (nm). These green wavebands fill the gap between blue and red light in the visible light spectrum, and when combined with amber light, green creates full-spectrum, “white” light.
But you won’t find green LED fixtures on the market or see green LEDs contained within a full-spectrum fixture. While green LEDs are useful for research, they’re not practical for commercial cultivation. Due to the constraints of manufacturing technology, green LEDs lack the electrical efficiency of other colors of LEDs. White LEDs, on the other hand, efficiently deliver green light and fill out other parts of the light spectrum too.
So, instead of green LEDs, lighting engineers use “white” LEDs that produce green light and other wavelengths simultaneously. It’s important to note that although some manufacturers reference green light, they’re typically referring to the white LEDs within the lamp.
Many fixtures on the market favor a blue-red spectrum. Those fixtures exclude the middle wavelengths from the spectrum and rely on blue and red LEDs alone, creating an unnatural “purple” light. The design philosophy behind blue-red fixtures asserts that a heavily tuned “purple” spectrum is better for several reasons, as noted below. However, such a sharp departure from the natural spectrum of sunlight doesn’t yield the best results for cannabis.
One reason for the popularity of blue-red LED fixtures is efficiency. Blue- and red-emitting diodes produce more photons per input of electricity than other diodes. In the design of early LED fixtures, the constraints of LED technology influenced manufacturers’ beliefs about the“ideal spectrum.” So rather than designing a fixture based on the spectrum of the sun, they designed a spectrum based on the capabilities of LEDs. Now, LED technology is changing. White LEDs are more efficient than ever, allowing growers to reap the benefits of full-spectrum light while maintaining efficiency.
Another reason for green light’s exclusion from blue-red fixtures is a misinterpretation of laboratory studies. It’s well-documented that blue and red light are well-absorbed by plants and strongly drive photosynthesis. In fact, they’re the primary drivers of photosynthesis. The plant’s most abundant chlorophylls — chlorophyll a and chlorophyll b — absorbed and blue light.
In the lab, when a single leaf is exposed to blue light, its photosynthetic response is robust. Moreover, when chlorophyll a and b are extracted from the plant, they absorb blue and red light well. Greenlight, however, is poorly absorbed by extracted chlorophylls.
Yet the laboratory is an isolated situation, and these studies purposefully create an experimental setting that would never occur in the real world. While informative, they don’t tell us everything we need to know about how living plants use sunlight. In the real world, the function of green light is more complex — and more important than once thought.
Studies have shown again and again that cannabis plants use green light. But green light is different from red or blue. It behaves differently in the canopy, and different photoreceptors use it for photosynthesis — not just chlorophyll a and chlorophyll b.
In the canopy, green light that’s not reflected or absorbed by the upper leaves is transmitted, penetrating deeper into the lower leaves. And while it’s true that leaves reflect green light — and so appear green to our eyes — most of the green light is still used. Only 5-10% of green photons are lost to reflection; the rest are absorbed or transmitted.
Research by Heliospectra prompted the market’s adoption of broad-spectrum LED fixtures containing green wavelengths. Our early studies with white light and green-emitting LEDs confirmed what academics already knew: green light drives photosynthesis and plays other important roles.
The transmission of green into the lower canopy also decreases the senescence of the bottom leaves, and their sustained health lets them continue to help with photosynthesis. And green light drives photosynthesis directly too. In strong white light, green has been shown to drive photosynthesis more efficiently than red light. And the inclusion of 24% green light into a blue-red spectrum has been shown to enhance growth in lettuce. Without a doubt, the green light promotes growth.
And it’s reasonable to assume that cannabis plants use green light more than other crops. Cannabis is an extremely light-hungry plant with a dense canopy. Because green light penetrates through leaves better than other colors — and because green light efficiently drives photosynthesis in strong, white light — green wavelengths result in greater flower development.
Greenlight also signals plant morphology. It plays an important role in mediating a plant’s responses to blue light, and it influences leaf growth and early stem elongation. By sensing the ratio of light colors, plants can tell if they're in shade or if they’re receiving direct light, and how to position their leaves to maximize light absorption throughout the day. That’s because green light penetrates leaves more easily than other colors. When plants sense a higher green-to-blue ratio, they assume they’re shaded by other plants, and they’re more likely to elongate their stems to compete.
Many of the misconceptions about green light stem from how the human eye perceives green light. We easily perceive green light, but other colors, such as blue, are more difficult for us to see. Because so much of our environment (i.e., plant life) reflects green light, it's adaptive for us to perceive green most readily.
So, when indoor growers are scouting their plants for pests, a fixture with the green light in the spectrum (i.e., a white-light spectrum) becomes an advantage. Under a blue-red light, recognizing pathogens like powdery mildew and fungus gnats can be next to impossible. When a spectrum includes even a small proportion of green, grow techs have a much easier time uncovering issues before they scale into major problems.
It’s important to have a balanced spectrum for proper cannabis growth. Check out Mitra, for that perfect spectrum, designed by growers for growers.
Greenlight — often neglected by lighting engineers — is a natural component of sunlight and an important part of any electrical light spectrum. But less extreme spectral tuning presents an opportunity for increased efficiency and control over plant traits. While it’s best to have all wavebands of the spectrum included in some proportion, increasing blue, red, or far-red light can elicit desirable characteristics in cannabis — like increased cannabinoids and terpenes.
For more information about how cannabis uses light, check out Heliospectra’s Cannabis Lighting eBook. You’ll find a complete resource documenting how the quantity and quality of light affects cannabis throughout its lifecycle. Even better, you’ll find lighting tips to boost your crop outcomes.
Or, feel free to check out the other two parts of this three-part series:
 Bilodeau, S., et al. An Update on plant Photobiology and Implications for Cannabis Production. Frontiers inPlant Science. 2019.
 Runkle, E. GrowingPlants with Green Light. Greenhouse Product News. 2017.
 Terashima, I. et al. Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are green. Plant and Cell Physiology. 2009.
 Kim,H. et al. Green-Light Supplementation for enhanced Lettuce Growth Under Red- and Blue-Light Emitting Diodes. HortScience.2004.
This is part one of a three-part series documenting the effects of the light spectrum on cannabis. For even more information about cannabis photobiology, make sure to check out Heliospectra's Cannabis eBook.
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