How Light Spectrum Affects Plant Growth

How Light Spectrum Affects Plant Growth

Since the 1980s, scientists have known that diverse spectrums of light have broadly differed impacts on a range of plants. However, on account of the variable spectrum available from today’s cutting-edge Light Emitting Diodes (LEDs) we are finally able to truly understand the delicate relationship between light and plant growth.

How is light measured?

Visible light is but a small part of the electromagnetic scale which includes visible and invisible light energy such as radio waves. Each range of color serves to illustrate an electromagnetic frequency. Light waves are measured on a scale of nanometers (one billionth of a meter):

The Electromagnetic Spectrum //Photo Credit Fcat Standard 

Do varied light spectrums produce different effects in plants?

While many growers hold the belief that the most efficient grow lights should have the same light spectrum as the sun. After all, plants have evolved with the light which is most readily available, our sun. Sunlight falls in the middle spectrums which are seen as green, yellow and orange. These are the predominant frequencies used by the human eye. However, different light spectrums can be used to manipulate different types of growth in plants and many studies have shown that these many in fact be the least used light frequencies in most plants.  

Ultraviolet light (10nm-400nm)

An excess of UV light exposure can prove dangerous for many flora, but small amounts of UV light prove to be advantageous. In many cases, studies show that 385 nm UV light is beneficial for plant colors, tastes and aromas, but does not have any significant impact on growth. UVB has also been reported to increase levels of THC in Cannabis.

Blue light (430nm-450nm)  

This range of light encourages seedling and vegetative growth and is vital for plants during the vegetative stage of their growth cycle. This is especially when trying to reduce “stretching” and ensure a good yield.  It also triggers the enhancement of colors and is known to also stimulate Terpene (i.e. fragrance) production.

Green light (500nm-550nm)

Green light has a small role in plant growth, as most of it is reflected off of the leaves.The role of green light in plant development is less well understood than the most other light waves, and there are only specific species of plants that need green light in order to carry out normal growth. The necessity of green like seems to be extremely strain particular.

Red light (640nm-680nm)

Red light wavelengths energize stem development, blooming and natural fruit generation, as well as chlorophyll creation. The 660 nm wavelength has very prevalent photosynthetic activity and is the best option for night interruption when you want to prevent short-day-plants from flowering or encourage long-day plants. Red light is also very effective for light cycle extension.

Far red (730nm)

Although this is not with the functional range of most plants, converting it to a red-absorbing form can make it vital for plants which require lower exposure to phytochrome photo equilibrium in order to flower. It may additionally be supplemented toward the finish of every light cycle in order to encourage flowering within short-day plants, including Cannabis.

Growth stages of plants:

There are typically four separate stages in plants, of which have unique spectrum requirements:


  • Vegetation – In Vegetation (commonly known as veg.) stage occurs shortly after seedlings appear. At this stage overall rapid, hearty plant and root growth is desired, and in general, most growers desire to get maximum height and size possible at this stage.

Female and male plants at the pre-flower stage // Photo Credit CGW

  • Pre-flower – Pre-flower is considered the period of time from when the flower cycle first begins, until about the end of the second week, or until small flowers become apparent and the quick vegetative growing and stretch both slow. Again, for many, the desire at this point is to increase size, while minimizing potential stretch.


Flower – The prime Flowering period is around the 3-7 week marker and is the time at which most plant growth stops and the plant begins to refocus energy on flower production. Escalating flower size and improving overall structure is typically the key at this point.


  • Ripen or Finish – Ripening typically occurs from week 7 until all Flower growth nearly stops. At this point the plant energy will refocus on terpene production and resin. This is the time that the flower produces a majority of its density and resin content. This transition is difficult to define, as some strains tend to have large increases in resin production during this period, while are less significant

“What is the ideal Spectrum mixture?”  

Evolution and practice has produced a huge variety of strategies, making it impossible to generalize specific light responses to all plants. The truth is the answer wholly depends on what your personal growing priorities are. Different types of light promote different plant morphology in different stages of growth, and there simply isn’t one ideal.  


Meeting Goals of the Commercial grower:

Veg: As plant size is the driving factor in this example a spectrum consisting entirely of red and blue is important. In effect we are attempting to mimic the sun, but with LEDs. The best results in veg stage are typically found when using a RED/BLUE mix of 60/40.

Pre-flower & Flower: In this case, flower structure is unimportant, only resin yield matters. A higher blue mixture can then be implemented. A recommended ratio would be 70/30 RED/BLUE, while higher blue may be beneficial.

Ripening: Since extra blue is already in use in the flowering stage, no changes will be necessary in this stage. However, UVB supplementation is highly recommended, as reportedly it can raise THC levels by as much as up to 30%. With this in mind, supplement UVB for the remaining five weeks of flowering at least.

2) Growers in search of maximum resin/oil production for use in high-quality extracts and shatter etc…, where the aesthetics of the flower are unimportant, but things like the  yield, quality and smell of the resin are key. Our goals will be similar to one first example, except targeting more for fragrance. For example one can be followed until the ripen stage where it will then be beneficial to decrease the red a bit more, to raise the Blue/Red ratio in order to stimulate the production of terpenes.

UVB: This time UVB should be utilized all the way through the flower because not only do we want to increase THC in resin, but also an increase in terpene production and also the production of other pigments the throughout the entirety of flowering.

3)  Some growers may want the most Flower yield (weight) period. With the market getting more and more competitive, pure flower yield can be increased simply by lighting with relatively high red levels the entire way through growing. A good recommendation would be 80/20.  

4) Due to the incredible difference in price between top-shelf herb and that of less appealing or outdoor buds, many commercial growers are presently looking to enhance top-quality yield. In other words, resulting flower with shelf-appeal; excellent appearance, scent, and density. Potency is also key and often lab tested.

All of these will need varying spectrum mixes in order to be ideal, however a good starting point for this type of grow would be:

VEG: Taking into consideration the inter-node spacing desired, 60/40 is recommended for short tight internodes, while R/B ratio can be increased for less tight internodes.

Pre-flower: Again, in order to minimize stretch, R/B ratio can be increased to 70/30 for the first two weeks of flower. Added deep blue light can help to stimulate the pigments to grow more during this vital period of growth, enhancing the fragrance and color of the flowers in the process.

Flower: Now, maximizing flower size is key, so the Red/Blue ratio should be raised to 80/20. In the California LightWorks Full Cycle spectrum mix you will find this red/blue balance. Even more red can be added to further enhance flower production, but there can be a loss in resin, perfume, and secondary colors. Using R/B ratio above 90/10 is never advised as it negatively will impact resin and fragrance quality.  

Ripen: Here trying again to enhance resin and terpenes, we suggest lowering the R/B ratio back down to 70/30 or 60/40 for the remaining two weeks.  

UVB: UVB can be incredible beneficial and it may be supplemented within the last 4-5 weeks, or throughout the entire growing to enhance pigments, increase terpenes and most importantly raise THC. However, UVB supplementation can not increase levels of CBD.

By implementing a four-stage spectrum control technique the aesthetics, perfume, density, pigment, and overall shelf-appeal of the cannabis flower can be optimized with little to no sacrifice in the amount of yield.

Be aware, multiple lighting changes within a single cycle can produce the opposite of the effect you're hoping for and damage your plants. So remember ONE CHANGE PER HARVEST.


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  • Zunammie Keren
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