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Light Spectrum and Plant Growth

Light Spectrum and Plant Growth

Ever since NASA began experimenting with LEDs for growing plants in the 1980s we have known that different light spectrums have widely varied effects on plants. Some spectrums stimulate vegetative growth and others increase the yield in flowers and fruits. Other spectrums seem to have very little effect in plant growth. Thanks to the variable light spectrum available from LEDs we are finally starting to understand the relationship between light spectrum and plant growth.


How do we measure light?

Visible light is part of the larger electromagnetic scale which includes invisible spectrums such as radio waves and x rays. Each spectrum represents an electromagnetic frequency measured in nanometers (one billionth of a meter):

Do plants use all light spectrums produced by the sun?

Most indoor growers seem to believe that the best indoor grow lights would have the same light spectrum as the sun – a relatively full spectrum over the visible light frequencies. After all, plants evolved over millions of years to best convert light energy into carbohydrates and sugars. The most readily available light from the sun is in the middle spectrums which we see as green, yellow and orange. These are the primary frequencies that human eyes use. However, studies show that these are the least used light frequencies in plants. Most of the photosynthetic activity is in the blue and red frequencies.

The main reason for this counter-intuitive use of light by plants seems to be related to early forms of bacteria and the evolution of photosynthesis. Photosynthesis first evolved in bacteria over millions of years in the primordial sea. This evolved in bacteria long before the appearance of more complex leafy plants. These early photosynthetic bacteria extensively used the yellow, green and orange middle spectrums for photosynthesis which tended to filter out these light spectrums for plants evolving at lower levels in the ocean. As more complex plants evolved at lower levels they we left with only the non-filtered spectrums not used by bacteria – mostly in the red and green frequencies. The yellow, green and orange light is mostly reflected off the surface of the leaves and this is why photosynthesizing plants are green.

Do different light spectrums do different work in plants?

Not only do plants focus on specific light spectrums for photosynthesis but different light spectrums are used for different types of growth in plants. There are millions of photosynthetic receptors in a leaf of a green plant. Each receptor includes specialized pigments that absorb specific frequencies during photosynthesis. By measuring the amount of oxygen produced under various light spectrums we can measure the amount of photosynthetic activity under each light spectrum. This has produced a very detailed map of which light spectrum is related to which type of plant growth.



How do plants use different light spectrums?

Ultraviolet light (10nm-400nm)

Though overexposure to UV light is dangerous for the flora, small amounts of near-UV light can have beneficial effects. In many cases, UV light is a very important contributor for plant colors, tastes and aromas. This is an indication of near-UV light effect on metabolic processes. Studies show that 385 nm UV light promotes the accumulation of phenolic compounds, enhances antioxidant activity of plant extracts, but does not have any significant effect on growth processes.  UVB has also been demonstrated to elevate THC levels in Cannabis*

Blue light (430nm-450nm) 

This range of spectrum enables cryptochromes and phototropins to mediate plant responses such as phototropic curvature, inhibition of elongation growth, chloroplast movement, stomatal opening and seedling growth regulation. It affects chlorophyll formation, photosynthesis processes, and through the cryptochrome and phytochrome system, raises the photomorphogenetic response.

In more practical terms, these wavelengths encourage vegetative growth and are essential in lighting for seedlings and young plants during the vegetative stage of their growth cycle, especially when “stretching” must be reduced or eliminated.  It also stimulates the production of secondary pigments which can enhance colors and is known to also stimulate Terpene (i.e. fragrance) production.

Green light (500nm-550nm)

Most green light is reflected off the plant and plays a much smaller role in plant growth. However, there are some important aspects of light in this range so a certain amount of light in this spectrum range is beneficial. Green light is sometimes used as a tool for eliciting specific plant responses such as stomatal control, phototropism, photomorphogenic growth and environmental signaling. When combined with blue, red and far-red wavelengths, green light completes a comprehensive spectral treatment for understanding plant physiological activity. The function of green light is less well understood than the other spectrums, and there are only certain species of plants that require green light for normal growth. It’s effects appear to be very strain specific.

The pigments that can absorb green are found deeper in the leaf structure so it is thought that because green light reflects off of the Chlorophyll in leaf surfaces and thus is reflected deeper into the shaded areas of the canopy than Red and Blue which are readily absorbed, that green may actually be mostly absorbed through the undersides of the leaves as it bounces around in the shaded depths of the canopy.

Red light (640nm-680nm)

Red light affects phytochrome reversibility and is the most important for flowering and fruiting regulation. These wavelengths encourage stem growth, flowering and fruit production, and chlorophyll production.

The 660nm wavelength has a very strong photosynthetic action and also exhibits the highest action on red-absorbing phytochrome regulated germination, flowering and other processes. Most effective for light cycle extension or night interruption to induce flowering of long-day plants or to prevent flowering of short-day plants.

Far red (730nm)

Although the 730nm wavelength is outside the photosynthetically active range, it has the strongest action on the far-red absorbing form of phytochrome, converting it back to the red-absorbing form. It becomes necessary for plants requiring relatively low values of the phytochrome photoequilibrium to flower. Can be used at the end of each light cycle to promote flowering in short-day plants such as Cannabis.

Also, a higher ratio of far-red to red than found in sunlight can trigger the shade stretch response-where a plant when sensing it is shaded based on an elevated ratio of far-red to red- will stretch to try to elevate its canopy above its competitors.  This is why too much far-red is not advised if compact plants are desired, or in general. But small amounts or FR as provided by California LightWorks in our R/FR channel is very beneficial, and for this reason the ratio or R to FR is fixed on one channel in the 550 series.

Using Spectrum Control with Cannabis

The exact way that plants use light is very specific to individual plant species and their natural environment. Evolution has produced a huge variety of plant strategies for growth and it is impossible to over generalize light responses. However, we do have a lot of practical experience with indoor cannabis growth results. Below are some general strategies and recommendations based on years of practical experiments with indoor lighting.

The most common question we receive from growers in regards to spectrum control in cannabis cultivation is “What is the optimum Spectrum mix for Cannabis?” And the answer is it depends on what YOUR priorities are. Different spectrum mixes promote different plant morphology in different growth stages, and there simply isn’t one ideal. And that is the main benefit of LED’s over HID, the ability to use varying spectrum to design the plant for what you want from it.

There are basically 5 (or possibly more) different aspects to the end product in Cannabis that establish its value, and different people want different things.

1) Flower weight (ie. Overall flower yield)

2) Flower density (ie. Resin content and oil/wax ratio)

3) Flower cosmetic appeal (colors, structure, as well as density)

4) Fragrance (Strength i.e. terpene concentration and fragrance complexity)

5) Potency (THC and CBD levels)

What must be understood here is there is NO IDEAL SPECTRUM that will optimize ALL of these aspects of the final product simultaneously. Each can be individually optimized but there will be tradeoffs.

Goals of the Commercial grower:

What followers are SOME of the typical goals the average commercial grower might consider most important:

1) Some growers may want Maximum OIL yield for edibles etc. and the cosmetic aspects and fragrance of the flowers are not important. Potency is extremely important here.

2) Some may want maximum oil yield for top-shelf extracts, shatter etc…, where flower cosmetics are unimportant, but resin yield, resin quality (oil/wax ratio) and fragrance are very important. Potency is also important and often lab measured.

3)      Some may want maximum Flower yield (weight) period.  There numerous factors that play into this such as Resin content vs. flower matter (fiber), wax vs. oil, etc…, but these people only care about total flower yield by weight. With the market getting more and more competitive, this mindset will struggle to compete.

4)     Because of the significant differential in price between top-shelf flower and lower quality or outdoor flower, (2x or more) most commercial growers are currently looking to maximize top-quality flower yield, ie. flower with high shelf-appeal, i.e. excellent cosmetics, fragrance, and density. Potency is important and often tested but typically considered strain specific and not considered that dependent on cultivation techniques.

So all these examples will have potentially DIFFERENT ideal spectrum mixes, and while those ideal spectrum mixes are not fully known, we can get you close.  And please note, any fixed spectrum light source like HPS or MH will never have the ability to accomplish the ideal in any of these areas. That will require variable spectrum control.

Also please note: The single most important element in Cannabis yield is shaping of the plant BEFORE peak flower production such that only flower sites see light. This cannot be stressed enough. The best light and the best nutrients will not effect yield as much as insuring that only flowers sites and select sun leaves see light, and that all flowers left on the plant get enough light. And proper design / layout and mounting heights of the lighting system to minimize plant shading and create consistent lighting levels is critical to this process.

Growth stages of Cannabis:

There are also generally 4 growth stages in cannabis that have different spectrum requirements.

  • Vegetation– In Vegetation (VEG) stage, rapid, healthy overall plant and root growth is desired, and in general most growers desire maximum growth but with shorter compact plants with short inter-nodal spacing preferred.
  • Pre-flower – Pre-flower is the period from when the 12/12 flower cycle is first initiated, to roughly the end of the second week (in an 8-week flower), or until the small flowers are prevalent and the rapid growth stretch slows. Again, for most growers, the desire in this stage is to maximize SIZE, while limiting stretch.
  • Flower – The peak Flower period is generally from week 3-7 and is the time when the plant (stem / leaf) growth stops and all the plant energy focuses on flower production. Maximum flower matter size and good structure is generally the goal here.
  • Ripen or Finish – The Ripen period is generally from week 7 to finish (in an 8-week flower) where the Flower growth, (i.e. size) slows and plant energy refocuses on resin and terpene production. This is the period where the flower acquires a significant portion of it’s density, ie. resin content. This transition is not clearly defined, and some strains have big increases in resin production during this period, and others not as much.

Optimizing spectrum for ideal results

So understanding that enhancing each aspect of plant growth can be a tradeoff, and with the basics of our scientific understanding of Spectrum and Plant Morphology, we can now attempt to come up with some starting points for spectrum mixes for various end results. Please understand, these are starting points and you will need to experiment to reach the ideal for your environment, strain, and desired results.

Goal #1 above, Maximum OIL content for processed edibles, etc.

In this example, our goal is to maximize resin and really THC/CBD yield overall. This includes both flower AND leaves, stems, etc. So a good starting point in terms of Spectrum programs would be:

Veg: Obviously plant SIZE is the big driver at this point so a spectrum with full red and blue is important. In effect we are mimicking the sun, but with LEDs historically our best results in VEG are found with a RED/BLUE mix of around 60/40

Pre-flower & Flower: In this case where flower structure is not important, only resin yield, a higher blue component (ie. closer to sun) can be used than in the other approaches. A good starting point would be 70/30 RED/BLUE but possibly even more blue.

Ripening:  Because we are already running extra blue in flower, no changes are probably necessary in this stage.

UVB: UVB supplementation is highly desirable in this approach because it can increase THC levels by as much as 30%. SO UVB should be supplemented for the last 5 weeks of flower minimum.

Goal #2 –  Resin for Extracts, shatter, etc.

In this example our goals are similar to Goal 1 above except there is a greater focus on Fragrance.  SO we can follow example 1 above except that in the ripen stage we will decrease the red a little more, to raise the Blue/Red ratio to stimulate terpene production more. Say 65/35.

UVB: UVB should be utilized all the way through the flower in this case because not only do we want to increase TCH in resin, but also terpene production and other pigments all the way through flower.

Goal # 3 – Maximum Flower yield

Pure flower matter yield can be favored by running fairly high red levels all the way through, a good starting point would be 80/20.  This is the kind of growth pattern seen with HPS.

Goal#4 – Maximum Top-shelf flower yield.

This type of end product is the approach where having the ability to vary spectrum in all the different growth periods is most important, and where Hybrid Spectrum LED systems (individual Red/Blue/White control) significantly out perform all other types of lighting systems.

So a good starting point for this type of grow would be:

VEG: Depending on the inter-node spacing desired, decrease R/B ratio for shorter internodes, General recommendation: 60/40 for short tight internodes. This is the ratio found in the CLW VEG spectrum mix.

Pre-flower: To again reduce stretching, R/B ratio can be increased to 70/30 for the first 2 weeks of flower, or 75/25 for taller plants. Extra deep blue will stimulate additional pigments during this critical growth period enhancing flower colors and fragrance.

Flower: In this stage we want to maximize flower SIZE, so we will increase the Red/Blue ratio to 80/20. This is ratio that is found in the California LightWorks Full Cycle spectrum mix, or with the 550 series full on.  Even higher Red ratios (by lowering the blue) can be used to further promote flower matter, but there can be a sacrifice in resin, fragrance, and secondary pigments. There is always a tradeoff between flower mass and resin (density) /cosmetic quality. We do not advise an R/B ratio above 90/10, and for no more than a week or two in the middle of peak flower, or it will impact resin and fragrance.  And too low, (for example 60/40) during this critical period will promote excess leaf content in the flowers and a fluffier structure akin to outdoor flower.

Ripen: Here we look to again enhance resin and terpenes (fragrance) so we suggest lowering the R/B ratio back down to 70/30 or even 60/40 for the last 2 weeks. At this point the higher blue ratio will not alter the flower structure or promote excess bud leaves, because flower growth is winding down, and transitioning to resin production. Results in this phase of growth are very strain specific and can be influenced by nutrient changes as well, so you are encouraged to try small changes each harvest to slowly dial in your ideal.

UVB: IN this case UVB can be very important and it can be supplemented either the last 4-5 weeks, or even throughout the entire flower period to stimulate pigments and terpenes and most importantly THC. Note, UVB supplementation does NOT increase CBD levels.

By using this 4-stage spectrum control approach you can truly optimize the cosmetics, fragrance, density, and color, i.e. shelf-appeal of your flower with little or no sacrifice in yield as compared to HPS or other fixed spectrum systems.


So in conclusion, it can not be stressed enough that these recommendations are only starting points, because the all the results are strain specific and can also vary with other factors such as temperature, shading, and nutrients.

Experimentation with additional changes such as varying the white (ie. green) levels, or gradating the changes over time instead of just switching them are encouraged, but we do suggest that you carefully document all changes and limit them to 5% change in any spectrum per growth phase, and only one change total per harvest. Too many changes in one cycle and you will not know what did what. So remember, ONE CHANGE PER HARVEST.

Also, there have been suggestions and a Dawn / Dusk type of ramp up and down to simulate the slow changes in the sun have value, but we have not seen solid university data in this regard to date. But these types of changes are easily accomplished with the SolarSystem, 550 controller.

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