Grow LED 1/3 – which light spectrum does Cannabis love?

The featured image shows the Greenception Series X Cannabis Grow LED.  

The cannabis plant is one of the most diverse plants on the planet. It can be consumed for the purpose of recreational relief or to cure and relieve various diseases and symptoms. The fibers of the plant can even be made into paper to write the American Declaration of Independence on it.

But at the beginning there is always a small cannabis seed in front of you or your breeder. There is still a lot to do before this tiny plant becomes a plant up to four meters high with resinous flowers. In this article series we will only deal with one of the most important and probably most complex of all components: the light. In this article series we introduce you to the entire complexity of this extensive topic, because light is by no means just light. For example, the plant sees light completely differently as we humans do. Literally.

PAR-Spektrum Cannabis

SL: The graph shows at which light wavelengths (colors) human beings can see the brightest.

PAR: The graph shows at which wavelengths of light (colors) plants can see the brightest.

The future of cannabis lighting is LED

Since we at Research Gardens have already fully arrived at the future and have the planet and your electricity bill in mind, there are only LEDs to marvel at in this post. On the one hand, the small semiconductors are very efficient in using electricity and, on the other hand, they can precisely map the types of light required by the plants. This means that, in contrast to conventional light sources, LEDs do not generate excessive waste heat and the right colored light shines precisely on the plants.

Sustainability is key

Even the German nature conservation association NABU recommends LEDs because they are mercury-free and can withstand frequent powering on and off for a very long time without any problems. When using an LED in your cannabis cultivation operation, you do not have to install any energy-guzzling cooling elements, because in contrast to conventional lamps, LEDs generate significantly lower temperatures and the dangerous 30-degree mark is only reached much later compared to HPS lamps.

Wavelengths and light colors as the key to the right light for optimal cannabis plants

When we talk about plant lighting, we should first analyze the exact needs of plants for light. For the aspect of light, we take solar radiation as a reference, because all plants have evolved to adapt to the light spectrum and the intensity of sunlight over millions of years and make optimal use of them with the help of special characteristics. Such features include, for example, the photosynthetic active mesophyll plant tissue, where the photoplastics generate energy from light.

Wavelengths over day

Source: GICON

It starts with a representation of the light spectrum generated by the sun with corresponding intensities at noon (upper graph) and evening (lower graph):

This light arrives somewhere on earth. A spectrum from less than 400nm light wavelength to over 800nm radiated by the sun is very clearly visible. The spectrum ranges from invisible UV light to blue, green and red to invisible infrared light. It is very clear to see that the light intensity differs greatly over the course of the day, due to the different angles of incidence of the sun over the course of the day. The intensities of individual wavelengths in comparison to one another also change over the course of the day. We keep these observations in mind.

But not all light that arrives on earth is used equally by the plant.

A scientist called McCree had already carried out investigations on this subject in the 1970s in order to assess at which wavelengths a leaf shows photosynthesis activity and how strongly photosynthesis is carried out. With the help of light filters, McCree irradiated the leaves with isolated wavelengths of the natural light spectrum and quantified (measured) the strength of photosynthesis by the amount of CO2 absorbed by the leaves for each individual light color.

CO2, carbon dioxide, is what a plant needs as a raw material for photosynthesis and with the help of the effectively processed amount of carbon dioxide it can be estimated to what extent a plant is photosynthesizing.

The result is the “photosynthetic action spectrum”, shown in second position in the graphic below. In parallel to the action spectrum, the two pigments in the leaf were examined, which were thought to be responsible for photosynthesis, in order to determine the so-called absorption spectrum: Chlorophyll A and B. To this end, it was measured which wavelengths were absorbed by the isolated pigments (chlorophyll A and B) and how much. Later, the pigment carotenoid was additionally discovered, which takes on regulating and also absorbing and thus energy-generating tasks in the context of photosynthesis.

peaks chlorophyll A and B sun radiation photosynthesis

It is easy to see that both the absorption and the photosynthetic action spectrum use blue and red components particularly efficiently.

Nice to know at this point: Because green light is used less efficiently for photosynthesis and cannot be absorbed to the same extent as red and blue light, a certain proportion of the green light is reflected and the leaves appear green to us human beings. Experts also call this observation the green gap.

Good to know: While the plant mainly “see” blue and red light, we humans have our peak in exactly the opposite direction, our eyes perceive green light particularly strongly and red and blue less so. This is where humans differ from plants regarding to light sensitivity.

PAR-Spektrum Cannabis

However, for some reasons, we should also take green light seriously when it comes to cannabis plant lighting:

In the past, many LED lamp manufacturers used the absorption spectrum as a basis for developing their own LED plant grow lights. A negligent mistake, as some analyzes of existing research show:

Photosynthese grünes Licht hilfreich Cannabis

This illustration shows very well that it makes a big difference whether one examines pigments in isolation or the whole plant. “Whole Leaf” in the diagram on the left shows very clearly that the plant absorbs 70% of light in the green spectrum despite of the efficiency of the isolated chloroplasts examinations.

The illustration on the right underpins this observation by showing that complex, multi tissue crop plants such as cannabis, beans or maize use green light almost as much as blue light for photosynthesis. In comparison, the green alga “Ulva”, which is only two cell layers thin, hardly uses the green spectrum photosynthetically. But its structure is also way much more like the isolated pigments were arranged in the first experiments to this topic.

This very detail is quite important: The isolated pigments and the very small thickness of the green algae. Because a big structural difference between more complex plants like cannabis and isolated pigments or the thin-layer ulva is the leaf thickness. Green light is especially absorbed by the deeper leaf layers, the mesophyll. This is done to prevent damage at the cellular level and to use the natural sunlight as efficiently as possible over the entire light spectrum for energy generation by means of photosynthesis. As I said, plants have worked continuously over the past 4 billion years to adapt to naturally occurring sunlight.

mesophyll stomata palisade schwammgebewe cannabis leaf blatt


PAR and MAR-Spectra

Red, green and blue light is not only absorbed by the plant with different degrees of efficiency, but is also used in a spectrally specific manner for certain functions within the plant’s development. Dietmar Prucker from the Weihenstephan University of Applied Sciences used a literature review to highlight the following influences of different wavelengths on plant growth:

  • UV-B (230-320nm) and UV-A (320-380nm):
    • reduced growth height, lower biomass, decreasing leaf volume
  • Blue (380 – 500nm)
    • Photosynthetic activity, previously unspecified influence on root growth
  • Green (500 – 600nm)
    • Photosynthesis in deep leaf layers via carotenoids
  • Red (600-700nm)
    • Photosynthese,Verminderung des Streckenwachstums (kompakter Wuchs), Verhinderung der Blütebildung bei Kurztagspflanzen (worunter Cannabis zählt)
  • Near Infrared (700-775nm)
    • Increased flower formation in short-day plants (opposite effect of light red), promotion of elongation and leaf surface growth

On the basis of these observations, the so-called PAR spectrum (photosynthetically active spectrum, parallels see above) and the MAR spectrum (morphologically active spectrum, morphologically = individual growth traits) have been classified for the light used for photosynthetically effects. In particular, the edge areas of the full spectrum, i.e. UV and infrared light / deep red, fall into the MAR range, while the range of visible light that is used photosynthetically (energy production) falls under the PAR range.

MAR spectrum PAR Spectrum differences

Effects of different light spectra on cannabis plants

There are still no 100% scientifically proven data on the subject of light spectra and cannabis. However, the field research carried out by our customers in the medical field, the cultivation of CBD or where it is already permitted for recreational use appear promising. We therefore allow ourselves to repeat the above list of the effects of the various light spectra specifically for cannabis:

  • UV-C (100 – 290nm):
    • Mostly filtered by the atmosphere
    • Leads to certain cell death by destroying DNA
      • It is therefore used as an infectious agent to destroy fungal spores and bacteria in cultivation facilities
  • UV-B (230 – 320nm):
    • If the radiation is too high, UV-B destroys DNA cells and slowly leads to cell death
    • Is largely filtered by the atmosphere, but reaches measurable radiation intensities at noon when the sun is at its highest point
    • Stimulates sunburn mechanisms: Cannabis plants protect themselves by producing cannabinoids such as THC in the trichomes.
      • Thus, the trichome content of the plant increases under UV-B. It is particularly interesting that studies have shown that UV stress significantly increases THC levels, whereas CBD levels only rise marginally with increasing UV radiation. In the case of CBD genetics, however, the terpene content increases noticeably when the plants are exposed to UV light.
  • UV-A (320 – 400nm):
    • At around 380nm, UV radiation is hardly filtered by the atmosphere and has a significantly less destructive effect on DNA. UV-A LEDs also have a significantly longer lifespan than UV-B and UV-C LEDs. This makes them the all-round carefree recommendation for all friends of increased active ingredient concentrations by triggering natural sunburn mechanisms, without having to fear that the plants will die of cell death.
    • A nice effect of UV-A radiation has also been shown in the rooting of cuttings, where the near-UV radiation ensures faster and more abundant root growth.
  • Blue (380 – 500nm)
    • Blue light, especially deep blue light around 450nm, has been shown to inhibit stretching when used with cannabis. Elongating plants, = particularly high internodial distances (distance between two side branches on main stem), can be brought under control with blue light. A high proportion of blue ensures compact growing plants and is therefore suitable for environments where the ceiling height is limited. Blue light is therefore often used when growing young plants or in the vegetative phase. Our customers on an industrial scale often carry out these phases on multi-storey shelves, where the compact growth is very advantageous.
    • As considered above, blue light is used particularly efficiently for photosynthesis. Overall, blue light leads to second high yields with the lowest possible power consumption.
  • Green (500 – 600nm)
    • Photosynthesis in deep leaf layers via carotenoids. After a brief hype of red-blue LEDs, cannabis plants in particular have shown their preference for sun-proof, white light. White light inevitably contains green and this green is converted into photosynthesis energy, especially in the deeper leaf layers. Dense canopies with tall plants benefit particularly from green (part of the white) light.
  • Red (600-700nm -> 660nm)
    • When the light is red, plants use light energy best for photosynthesis. A too rapid initiation of flowering is prevented by a high light red portion of the light, which also leads to a more compact growth. Rather simply put, light in the red area is one of the most energetic types of light, because its long wavelengths also generate more radiant heat than blue light.
    • This study also showed that a red-blue ratio of 7: 2 produced the highest cannabinoid values, higher than, for example, 6: 2 or 5: 2 red-blue ratio.
  • (Near)-Infrared (700 -800nm)
    • When the infrared light increases in the entire spectrum, the initiation of flowering is promoted, but also the extension and leaf surface growth due to the phenomenon of shadow escape. This can cause problems in tight plantings.
    • An interesting, if not yet widely documented fact about the use of infrared is the shortening of the flowering phase by up to a week. Thus, there are economic advantages to be found in the use of infrared light. In order to achieve these advantages, it is sufficient to let the infrared light burn on its own for about 15-30 minutes before switching on the entire lamp and after switching off the entire lamp.


As an interim conclusion and free advice for grow LED manufacturers, we can state the following key points at this point:

  • Cannabis plants use all types of light from UV to infrared.
  • Green light is used less efficiently than red and blue light, but much better than assumed some time ago
  • UV light and infrared modulate how a plant grows, thus influencing height growth, flower formation and, above all, increasing the active ingredient concentrations.

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