marijuana light spectrum

Cannabis Cultivation: The Light Spectrum and Ways to Raise THC Levels

Creating an ideal environment for cannabis plants is only achievable by understanding the principles of nature – the light spectrum is a factor that cannot be ignored.


Most cannabis growers have multiple objectives in mind when planning an indoor grow. Drafting scenarios to achieve higher yields, increase THC levels, or simply to improve the overall health of a plant is an integral part of their hobby. This element of strategic planning involves the challenge to link knowledge of different scientific fields and to match those findings to a technical solution that helps to achieve predefined goals. Besides dedication and passion, it is the willingness to learn that differentiates good growers from future experts – so let us try to grow the royal way and learn what it takes to cultivate cannabis of exceptional quality. Today, we are looking at fundamentals of physics, and learn how the light spectrum affects the growth of a cannabis plant.


The sun emits energy in the form of solar radiation including gamma rays, x-rays, ultraviolet light, visible light, and even radio waves. Life on Earth is only possible because the ozone layer blocks this radiation, and reflects most of it back into space. This filtering process only allows wave lengths between 300nm and 1100nm to reach our plants and an even smaller portion of this light is visible to us. It is often referred to as the light spectrum, color spectrum or visible spectrum, and ranges from 380nm to about 750nm.

  • 180-280nm – UVC: Extremely harmful and luckily almost completely absorbed by the ozone layer
  • 280-315nm – UVB: Cause of sunburn and suspected to increase THC levels (!)
  • 315-400nm – UVA: Not absorbed by the atmosphere, commonly known as black light
  • 380-750nm – The visible light spectrum: Bands of wave lengths represent visible colours
  • 700nm-1mm – Infrared light: Not visible above 750nm but noticeable as heat on our skin


When shopping around for a grow light, you will likely come across the term “colour temperature”. This is essentially a way to describe the light appearance provided by a bulb, and is measured in Kelvin (K).

Colour temperature doesn’t mean the physical temperature of your light, but the degree of warmth or coolness of a light source—the “visual temperature”. When a light has a higher degree of Kelvin, it has a more blueish appearance. Thus, we call it a “cool” light. On the other hand, a bulb with a lower degree of Kelvin emits a “warmer”, reddish light.


In a strictly scientific sense, no. Colour temperature is normally used as a way to describe how the light produced by a lamp looks to the human eye. For some types of lights, such as LEDs or fluorescent lamps, it doesn’t describe a light’s spectral distribution or wavelength.

Without going too deep into physics here, the light from an incandescent bulb radiates light spanning the entire visible light spectrum. The white light from the bulb is the result of a mix of wavelengths (colours in the spectrum) “contained” in the light.

Other lights, such as LEDs or fluorescents, may emit light from a number of narrow wavelengths, with gaps or peaks within the spectrum. In other words, even if the light appears the same to the eye, it may be missing certain wavelengths (colours) that plants require for healthy growth.

Because LEDs tend to emit light in a very narrow colour spectrum, LED grow lights are usually outfitted as “full-spectrum” setups. They consist of a number of different-coloured LEDs that together cover most of the necessary spectrum for cannabis plants. These full-spectrum LEDs are comprised of different reds and blues, often mixed with additional white LEDs. Other, newer LEDs, such as COB lights, emit a light spectrum that more or less approximates natural sunlight; there’s no “gap” in the colour spectrum.


For vegging your cannabis plants, go with a cool light, one that emits a “daylight” colour with a high Kelvin of 6,000–6,500K. For flowering, a warm light with a reddish tone, about 2,800K, is optimal. You can also find grow lights with a “best of both worlds” colour temperature of about 3,500K, which you can use for both vegging and flowering.


Every organism living on Earth needs information what is going on around them to react to environmental changes, and ideally, get a slight advantage over other members of their species regarding natural selection and evolution. Interestingly, cannabis plants receive a lot of their information from the light they’re exposed to, and almost instantly react to different bands of wave lengths – a complex topic to fill books with, but let us focus on the basics first.

1. Vegetative Stage – “Blue” light for healthy leaves (range: 400-500nm; ideal: 460nm)

During the vegetative stage it is recommended to aim for as many leaves as possible, and to make sure plants stay rather compact, don’t stretch too much, and develop strong stems. Indoor growers tend to use metal halide bulbs, compact fluorescent lamps (CFL’s), or T5/T8 lighting fixtures with a blue band of light for the first few weeks to achieve these goals. When cannabis grows in nature, the high angle of the sun in spring and summer allows more “blue” wave lengths to penetrate through the atmosphere, a signal for cannabis plants to grow strong, large and healthy leaves.

2. Flowering Period – “Red” light for giant buds (range: 620-780nm; ideal: 660nm)

When cannabis plants enter the flowering period, highest yields can be achieved by exposing them to a light spectrum that contains lots of “red” wave lengths to promote budding. The rate of photosynthesis peaks when plants are subjected to “red” wave lengths of 660nm although latest NASA findings suggest that even “green” wave lengths, which are not associated as a major factor in photosynthesis, can also have an impact on how plants grow. Seeing a cannabis plant as simple photosynthesis factory is consequently a little hasty. But for now, choosing a lighting solution with a high degree of “red” in its spectrum remains the best way for growers to imitate the shallow angle of the sun in late summer and autumn.


Have you ever wondered why potent cannabis strains often originate from landraces that naturally grow in high altitude regions? There are experts who suspect ultraviolet light, especially a high exposure to UVB wave lengths (280-315nm), to be responsible for an increased THC production. The theory is based on the fact that a high elevation means lesser atmosphere between cannabis plants and the sun, leading to a higher exposure to UV rays. These ultraviolet wave lenghts knowingly damage our skin, and the human body reacts by producing melanin as protection – a cannabis plant assumingly does something similar – it produces more resin and THC as a form of natural sunscreen. It is too early to say if we are dealing with a theory or a cost-effective method to grow better cannabis but the concept seems plausible enough for hands-on experiments. UVB bulbs for reptiles only cost a few bucks; perhaps we should give it a try.

Seeking methods to increase THC production feels natural for growers – learn how the light spectrum can affect growth and potency of weed.

Demystifying Light Spectrum for Cannabis Growing

We use science to study cannabis growing and we’re passionate about helping growers to maximize yield and potency while reducing their energy costs. We have fun doing it too.

We’ve spent years in partnership with hundreds of cannabis growers, studying how marijuana plants grow and flourish under different lighting conditions. Many growers expressed their lack of confidence in alternative light spectrum and in particular LED. They felt more comfortable with the tried-and-true light spectrum from high pressure sodium and ceramic metal halide lamps.

At the same time, outdoor cultivators strongly believe the sun is the best source of light for marijuana growing.

We wanted to make sure we understood how cannabis grows under both artificial indoor grow-lighting and also the sun.

Then we focused our attention on traditional technology such as HPS, CMH, and fluorescent lamps. We compared top brands such as Gavita and Nanolux against each other, and against various brands of LED lamps, in flower. We also compared a broad range of fluorescent tubes used in clone and veg.

Using photo spectrometers to collect hundreds of spectral readings, we grouped the lamps that work well in cannabis together, and separated the ones that didn’t.

The results? We found what has been proven to be the correct spectrums for cannabis growing. As all growers know, cannabis plants prefer different light spectrums as they transition from clone to veg to flower. We identified which spectral mixes worked for each phase and engineered that in to the line of FGI LED lamps.

Our lamps include the spectrum grower like from HPS lamps like Gavita or Nanolux for flowering. For plants in the later veg phase we included spectrum from CMH lamps made by Gavita and others. In early veg and clone phases we include the spectrum found in florescent lighting. But in all cases we added light spectrum found in outdoor growing under the sun, because we believe it is true that our sun is the ultimate grow lamp.

Below are side-by-side spectral charts showing how each of these lighting types perform in terms of output of photosynthetic light, or PAR (photosynthetic active radiation is a measurement of the total amount of photosynthetic light each lamp produces, measured in µ/m²/s).

Figure A below represents total PAR from the sun. The white line on the graph indicates the McCree Action Curve which demonstrates which wavelengths of light plants use to make photosynthesis and grow.

Figure A; The sun and McCree Action Curve

Figures B below represent the PAR output of a Gavita double-ended HPS lamp as compared to the PAR output of the FGI Lightpanel 500 and 700. The white line indicates the McCree Action curve and shows how close each light is to replicating necessary light distribution of the sun.

Figure B; Gavita DE HPS lamp versus FGI LED Lightpanel 500/700 flower lamps

Figure C, below, represents the PAR output of a Gavita CMH lamp as compared to the PAR output of the FGI Lightpanel 500 and 700. The white line indicates the McCree Action curve and shows how close each light is to replicating necessary light distribution of the sun.

Figure C; Gavita CMH lamp versus FGI LED Lightpanel 500/700 veg mode lamps

Figures D, below, represents the PAR output of a typical T5HO fluorescent lamp as compared to the PAR output of the FGI Lightbar 100 and 185. The white line indicates the McCree Action curve and shows how close each light is to replicating necessary light distribution of the sun.

Figure D; Fluorescent T5HO lamp versus FGI LED Lightbar 100 / 185 veg lamps

In comparison to the sun, the FGI lamps have more preferred PAR output than HPS, CMH, or T5HO fluorescent lamps. The proof is in how the plants have responded. Healthier clone starts which receive the preferred blue light from the sun but also green, yellow, and red which are lacking in T5HO fixtures. Robust veg and mother plants which receive the full range of light spectrum of our sun, which is lacking in CMH lamps. And most important, big healthy flowers which tipping the scales increased dry weights than those produced under the more narrow HPS spectrum.

Demystifying Light Spectrum for Cannabis Growing We use science to study cannabis growing and we’re passionate about helping growers to maximize yield and potency while reducing their energy