light spectrum for cannabis plants

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.

Best Light Spectrum to Grow Autoflowers

When growing autoflowering plants indoor we need to simulate the environment they naturally grow in. The environment includes humidity, temperature, and the most essential of them all: light.

Either you prefer light bulbs or LEDs, you need the right spectrum in each stage for optimum growth.

1. What is Light Spectrum?

The light spectrum is the different colors (aka wavelengths) a source of light can emit. Light is measured in nanometers (nm) and each nanometer represents a band of light (a band of light is a section in the color spectrum). Humans can see a small part of the spectrum, between 380 to 780 nanometers, which means we can only see the colors ranging from violet to red.

Even though it appears white, the sun is a full spectrum light source and contains the whole spectral wavelength. That’s why rainbows happen. When raindrops refract light into individual wavelengths you can see all the colors (visible to humans) that make up sunlight.

In nature, cannabis plants grow under the sun, receiving the whole spectrum of wavelengths. This means we have to provide the maximum amount of wavelengths possible for the best development of our plant.

Although it’s not essential, it is considered good to provide the best light spectrum to encourage plant growth.

Remember this is not a rule, you can grow your plant from seed to harvest with any spectrum or amount of light but this can seriously affect your harvest.

2. Light Spectrums in Each Stage

During the vegetative stage, cannabis in the wild needs blue wavelengths to grow strong, big, and promote leaf growth. When growing indoors we aim to grow as many leaves as possible. With more leaves, there is more surface to absorb light, this way we ensure our plant develops a strong stem and branches preparing her for the flowering stage.

When entering the flowering stage, cannabis in the wild uses red wavelengths to promote bud formation. If we want to produce dense buds and increase yields we need to provide the plant red wavelengths, this will increase the rate of photosynthesis thus increasing bud formation.

• Tip: When experimenting with training, using “red” light will make plants grow taller, making it easier to train.

The general rules are “blue” light = shorter and stronger plants with more leaves, “red” light = taller and weaker (when compared to plants grown under “blue” lights) plants with fewer leaves.

3. Types of Light

Grow lights are usually divided into two kinds, light bulbs, and LEDs.

Light bulbs emit a limited scope of wavelengths meaning there are colors of the spectrum they don’t emit, therefore they are narrow-spectrum lights.

LEDs emit almost all the colors in the spectrum, they are composed of blue, red, and white diodes allowing them to emit all the wavelengths of the spectrum, therefore making them full-spectrum lights.

Light Bulbs: HPS, MH, CFL, and HID

Light bulbs come in four types: Metal Halide (CMH), High Pressure Sodium (HPS), Compact Fluorescent Lights (CFL) and High Intensity Discharge (HID).

MH bulbs are rated around 6500 Kelvin and are usually used in the vegetative stage because they emit a “blue” light (aka colder light).

HPS bulbs are rated at 2000 Kelvin are usually used in the flowering stage for their “red” light (aka warmer light).

CFLs are rated at around 5000 Kelvin, they emit a very weak “blue” light and are only used with seedlings and clones as they just need to develop roots for the first couple of days when the vegetative stage begins they should be transferred to a growing space with stronger lighting.

HID can cover from 3500 to 5000 Kelvin, they are similar to HPS but contains xenon inside, thus emitting a different spectrum and are more efficient while being more compact.

• Tip: You can also grow your plants under both CMH and HPS bulbs, this is called a mixed spectrum and will ultimately result in a better harvest.


• Cheaper than average LED lights

• You don’t need to change the whole fixture, just replace the bulb


• Electricity cost is higher if compared to LEDs.

• Need extra equipment for cooling.

• Requires changing bulbs (CMH to HPS) when entering the flowering stage.

Full Spectrum LEDs, and Infrared and UV Lights

All LEDs are full-spectrum lights unless advised, they may not contain all wavelengths but will emit the needed amount for each stage, unlike light bulbs, you won’t have to change anything other than your timer when you’re about to enter the flowering stage.

You can find full-spectrum LEDs that have UV and infrared diodes, if not, there are LEDs designed to emit those spectrums. They’re not that common in indoor growing but are said to benefit growers.

Just like everything else on the planet, cannabis plants are damaged by too much UV light but at the right amount. It can promote trichome production as the cannabis plants produce trichomes to protect itself from too much light.

As UV lights, infrared lights can harm your plants in excess but at the right amount, it can boost photosynthesis, affect yields, plant growth, and plant health in a positive way.


• Used on the entire life cycle.

• Longer lifetime than bulbs.


Note: It’s unusual to have UV and Infrared lights, they’re expensive and are NOT essential for growing good flowers. They’re recommended for more experienced growers who want to experiment with different spectrums.

4. Light Spectrums for Autoflowers

As most of you already know cannabis plants have two stages: the vegetative stage, and the flowering stage, needing a different light schedule and spectrums in each one of them.

Unlike photoperiodic, autoflowering plants start their life cycle in the flowering stage and depend solely on age to start producing buds. That’s why we recommend using a mixed spectrum of warm and cold light bulbs (CMH and HPS) or a full-spectrum LED during the whole lifecycle of your autoflower.

This way your plant will receive a complete spectrum of light guaranteeing the best flowers and biggest yield.

5. In Conclusion

The light spectrum is a small part of a much bigger system, there’s nothing as better light for your plant. But remember, the final product does NOT depend solely on your light.

Depending on which type you choose, either an LED or bulbs, you’ll have to adjust your growing environment accordingly to guarantee the best result possible.

When growing autoflowering plants indoor we need to simulate the environment they naturally grow in. The environment includes humidity, temperature, and the mos