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Best Light Spectrum To Grow Cannabis

Created by
JoshuaHolt JoshuaHolt
Added 2 October 2020

What Light Spectrum Is Best For Cannabis Grow

As you probably already know, light is a fundamental resource for most plants in nature. Cannabis plants love sunlight, but what is it about the sun that they enjoy most? This article goes into the light spectrum and discusses how you can use it to cultivate a beautiful ganja garden.

Cannabis needs plenty of light to produce big, juicy flowers, however it is not just about the amount of light. Specific spectrums of light can enhance plant development at different stages of growth, giving you even more control over your growing environment.

What Is Light Spectrum?

The light spectrum is the range of waves within the electromagnetic spectrum that is visible to us as human beings. Measured in nanometres (nm), the visible spectrum sits between 380nm to 750nm on the scale. This means the blue end (bottom) of the visible spectrum has a slightly shorter wavelength than the red side (top).

The sun radiates solar energy, of which a very small portion ends up reaching Earth. This is because the ozone layer absorbs and reflects back most of the harmful waves, allowing life to thrive. Waves that filter through the ozone layer are between 300nm - 1100nm, so the spectrum entering the atmosphere is wider than we are able to see.

The Electromagnetic Spectrum

Outside of the visible light spectrum we have gamma waves, x rays, ultraviolet waves, infrared, microwaves and radio waves. Ultraviolet waves and infrared waves sit at either end of the visible spectrum meaning they are included in the wavelengths that pass through the ozone. Both are beneficial for plants in some ways.

  • UVC (180 - 280nm) - Includes harmful ultraviolet waves that are blocked by the ozone layer. Causes severe burns.
  • UVB (280 - 315nm) - Can burn skin but only small amounts reach earth. Higher altitude areas will have higher levels of UVB.
  • UVA (315 - 400nm) - Always present in the atmosphere but not visible. 95% of ultraviolet waves entering through the ozone are UVA.
  • Visible Light (380 - 750nm) - The range of the electromagnetic spectrum that we are able to see as colour.
  • Infrared (700 - 1100nm) - Invisible to the human eye but can be felt as heat.

Here is a chart with an average breakdown of the visible spectrum. Bear in mind that the wavelengths cross over.

ColorWavelength (nm)
Red620 - 750
Orange590 - 620
Yellow565 - 590
Green520 - 565
Cyan500 - 520
Blue430 - 500
Violet380 - 430

Visible Light

Why Is Light Spectrum Important?

Cannabis plants need the right kind of light to grow to their full potential. Without the right spectrums of light, plants can underperform, growing small and producing low yields. Luckily, indoor growing techniques have allowed us to mimic the most important aspects of the sunlight received by plants outdoors.

Cool Blue Light In Spring Vs Warm Red Light In Autumn

Outdoor plants receive the full spectrum of light provided by the sun. In the spring, the spectrum of light that filters through the atmosphere results in a blueish tint. The blue spectra is what allows plants to develop strong foliage during the early stages of growth.

As the summer hits, the sun is higher in the sky and more light penetrates the atmosphere. By the time autumn arrives the sun begins to dip, when the red spectra is most prominent. Red wavelengths signal the end of summer so plants grow flowers in an effort to pollinate before winter.

We are still learning a great deal about how different colour wavelengths can affect a cannabis plant. Understanding how the sun provides plants with energy can help us grow better buds while making best use of our available resources.

Full spectrum LED grow by Trichsteppin from GrowDiaries

How Cannabis Plants React To Light

Cannabis plants can flourish well under blue and red lights provided they receive the right spectra within the colour band. By singling out these wavelengths, plants have easier access to the type of light they require. This is one of the reasons why indoor plants can be grown smaller than outdoor plants, because they do not need to stretch in search for the necessary spectra.

Blue and Red Spectrum LEDs

  • Blue (400 - 500nm) - Blue light is necessary for plants to grow strong leaves and branches during its vegetative cycle. Providing high amounts of blue results it short, bushy plants, which can be useful for growers trying to avoid tall plants with long branches. Recommended for seedlings, especially if you are growing indoors. 
  • Red/Infrared (620 - 750nm) - Red is important for expanding plant structure and promoting bud production. Naturally, plants use red light to recognise when it is day or night. Small amounts of infrared makes its way through the atmosphere during sunrise and sunset when wavelengths are longer. Cannabis plants grow taller under infrared until visible red becomes more direct.

Photoreceptors in a cannabis plant's leaves are constantly working to understand the light spectrum surrounding them. Leaves will turn to face the light during the day. When there is no light cannabis plants conserve their energy by 'relaxing' their leaves, making them droop slightly. Photoreceptors do not actually convert light into energy, which is more to do with photosynthesis.

How a cannabis plant reacts to light will also depend on the strain and where it originates from. Some genetics have been brought from high altitude areas where the light is stronger and there is a higher presence of UVB rays. It has been argued that exposure to more UVB can increase THC production, although this theory is inconclusive and needs to be researched further.

Best Spectrum To Grow Cannabis

It is hard to say exactly which part of the colour is spectrum is best but a large part of it has to do with the type of light your cannabis plants are grown under. Not all lights emit the same spectrum so choosing will depend on the results you are trying to achieve. The main thing is that your garden gets enough blue light during the vegetative cycle and plenty of red light for blooming.

Cool Temperature CFLs Can Be Used For Early Stages Of Cannabis Growth

You might be planning to grow indoors where you can have full control over the light. Or maybe you're thinking of trying some outdoor plants, much of which will be determined by the sun. Whichever you choose, make sure your plants get lots of light.

  • CFL (compact fluorescent light) lights are usually measured in Kelvin (K), which tells us the colour temperature of a bulb. Cooler, blueish lights have a higher Kelvin reading (6000 - 6400k), whereas a CFL that looks red will have a lower reading (2800k).
  • LED (light emitting diode) - LEDs emit a narrow spectrum of light, either in a small band of blue or red. Red, blue and white LEDs are often combined to create a 'full spectrum' emission that plants can understand for proper growth.
  • HPS (high pressure sodium) - Yellow/red light can be used throughout the grow cycle, but plants will grow taller with more spacing between internodes. Cannabis plants benefit from HPS lighting mostly in the flowering stage. 

If you are growing outdoors, you can expect plants to get larger than they would indoors because the spectrum is broader. Depending on where you live and the time of year, plants will respond to the light in different ways.

The colour spectrum of your grow light is important but if you want to produce decent harvests, the intensity is as necessary. The intensity of your light source plays a big role in plant development and needs to be powerful enough for plants to photosynthesise properly. If cannabis plants do not receive enough light, it doesn't matter what spectrum they receive, because they won't be able to function.


Knowing which light to give your cannabis plants is relatively simple once you understand how the electromagnetic spectrum works. Observing the energy of the sun across the seasons has given us the information and ability to grow weed to even higher standards.

If you were wondering about light spectrums, this article should have cleared things up a little! Happy Growing!

External References

The Electromagnetic Spectrum. - Keighley, H. & McKim, F. & Clark, A. & Harrison, M.. (1986).

This article was updated September 2020