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When growing cannabis, or any other plant, the light spectrum plays an important part in the growth cycle. The colour and type of light you use will influence the plant growth.

What is the light spectrum?

When we see natural light, it might appear white to us, but it is actually made up of many different colours. You can see these many colours when you put light through a prism or when looking at a rainbow.  The visible light spectrum falls between 380 nm to 750 nm (nano meters). This is the spectrum the human eye can see, but plants are able to use an even broader spectrum of light for photosynthesis. Outside of the visible light spectrum you have UV (Ultra Violet), X Rays and Gamma Rays and on the other end you have Infrared, Microwaves, and Radio waves. These all make up part of the light spectrum.

How Plants See Light

Plants tell the difference between light based on its electromagnetic wavelengths. Plants can see light, like ultraviolet (UV) that is not visible to humans. Plants use light to help them tell what time of the day it is and based on the length of daylight, they can then tell what season it is. To do this need to be able to sense brightness and intensity of light, light exposure length and the location of the light source.  A common phenomenon in plants called Phototropism sometimes occurs where the plant will actually bend towards the light source. Some evidence shows that blue light waves help the plant locate the light source. Plants are amazing in the way they see and use light.

 

How does the light spectrum affect growth?

Each different colour spectrum affects the plant in different ways like how it germinates, grows and even how it forms buds! In outdoor growing we do not have the option of changing the colour spectrum of sunlight, the light spectrum slowly changes through the seasons. From spring to autumn, the light that shines on your plants will determine how they grow. Plants are triggered to enter vegetative and then flowering stage as the seasons change. With Indoor growing you have complete control over the spectrum. The sun’s rays are replaced by a grow light with a specific light spectrum for plant growth. Your cannabis plant constantly changes its growth pattern to maximise the amount of light it receives. You will actually be able to notice the physical changes in the plant for example the plant will turn its leaf blades to face the light as the sun moves across the sky, and then will pull all its leaves downwards at night to save energy and protect itself. In indoor growing you can sometimes notice how when the light is off, the plants are slightly droopier compared to when the lights are on. Each different colour light from blue to red to ultraviolet, effects your cannabis plants growth differently.

The two main colours on the light spectrum is blue and red light. However there has been some research done on how green light affects the plants growth. Plants can be grown under any colour light like red, blue or yellow, but they will not flourish to their full potential. A plant needs a good mixture of different spectrums of light.

Blue Light

Blue light falls between 40nm and 500nm on the light spectrum. The most ideal being 460nm. Blue light is used mostly in the vegetative stage of growth. In nature, the high angle of the sun in spring and summer allows more blue wave lengths to penetrate the atmosphere. This signals the cannabis plant to grow strong, healthy leaves. In the vegetative stage, having blue light will encourage your plant to have lots of leafy growth, a strong stem and keep young plants from growing to tall and lanky. Plants that are grown with only blue light tend to be short and thick with dark green leaves.

Red Light

Red light falls between the 600nm - 700nm range on the light spectrum. The brightest light occurs around 660nm on the spectrum and is the most efficient and ideal red wavelength for the plant to absorb. In photosynthesis, plants are best able to make energy out of red light. Many plants can actually grow even if they only receive red light, although they won't grow as big and healthy as they would under a full spectrum light.

In the germination and seedling stage, before the seed makes it to the surface, its roots go down into the soil and the seed grows upwards in the direction of greater levels of red light. Blue light usually does not make it underground, but the seedling can sense red coming from the surface and will grow in that direction.  

Grow lights with a spectrum containing lots of red light are most often used in the flowering stage. The red light spectrum helps encourage plants to grow tall and helps promote bud formation. The plant will be triggered into the flowering stage quicker under red light than blue light.  

When the cannabis plant enters into the flowering stage, highest yields can be achieved by providing them with a light spectrum with lots of red wavelengths.  

Unlike humans, plants can also sense far-red (infrared) light. The ratio between red light to far-red light also influences how leaf growth and stem elongation. If there are higher levels of far-red light, the stems will start to stretch up and grow taller towards the light until it gets to a ratio with more red and feels that it is in directs sunlight again.

Green Light

Green light falls between the 500nm - 600nm range of the light spectrum. Over the last decade there has been a lot of research done different light spectrums and how they affect plant growth. Green light has been shown to be involved in seedling and vegetative growth, the initiation of the flowering stage, stem growth, CO2 and water usage and plant health in general. Nearly all grow lights contain some amount of green but adding too much green could potentially harm your yields. Enhanced growth from green light is something that can still be explored.

UVA / UVB / Ultraviolet Light

UVA / UVB / Ultraviolet Light falls in the 280nm - 400nm range of the light spectrum. When we think of the term UV-A/UV-B we usually associate it with protecting our skin outdoors. We use sunscreen to protect our skin from getting damaged by the UV rays produced by the sun.

When we are talking about plants, the UV-A (315 to 400 nm) and UV-B (280 to 315 nm) are sometimes classified as part of the blue light spectrum. In the past few years, we have now discovered that the UV spectrum has its own unique effects on plants.

There are some growers that believe providing UV-B light to plants may increase the THC levels in cannabis buds. This theory is based on the fact that just like how the human body starts producing melanin as protection against the sun, so a plant does something similar. They start to produce more resin and THC as a form of natural sunscreen.  

  

What If You Can Only Pick One Spectrum?

Research shows that rather than providing your plant with more of a certain spectrum of light, its best to provide a full spectrum so that the plant gets the energy it needs in all different wavelengths. The more a light can imitate sunlight, the better.

If you can only buy one light and have to choose between one spectrum or the other, it is generally recommended to use the flowering light spectrum, which contains more red light, for the entire grow. This will result in better yields. However, some growers claim to have successfully grown buds in the blue light spectrum.

 

How is Light Measured?

When talking about light measurement we are describing qualities like colour, intensity, and the amount of energy it can deliver to the plant. There are two common terms that you have most likely come across but may not fully understand. These are PAR and Lumens. A great expression in the cannabis growing community is "Lumens are for humans, PAR is for plants".

 

What are Lumens?

Lumens are the amount of light a bulb provides. Lumens measure "Luminous flux" or in simpler terms, the amount of light you as a human can see and how bright it is to the human eye. One lumen is equal to the light given off by one candle, 1ft away. Lumens it not the best way to measure light as it doesn't measure light exactly how the plant sees it but it can give us a good general idea of how bright a light is to the plant. There is an expression in the cannabis growing community that "Lumens are for humans, PAR is for plants". What they mean is that as a grower you should be more concerned about how much PAR a light gives off, rather than it's levels of lumens. Lumens should not however be ignored. Plants need to be exposed to enough light to ensure that they have a healthy grow cycle.

 

What is PAR?

PAR stands for Photosynthetically Active Radiation. PAR refers to the light in the 400 to 700nm range which is the spectra of light that plants can use for photosynthesis. When people talk about the 'amount of PAR a grow light gives off' they are talking about how much light in the PAR range that a grow light is giving off. Grow lights radiate lots of different wavelengths but only some of these wavelengths will be absorbed by your plant to consume for growth. The wavelengths that remain will reflect away from the plants, so these will not be useful. Having more PAR will result in more growth and healthier plants.  

 

How is PAR Measured?

PAR is measured as PPFD (Photosynthetic Photon Flux Density). This is the amount of light in the PAR range that the grow light is giving off. It is measured as micromoles per square meter per second (μmol/m²/s).

To measure the exact PAR output of your grow light, you will need a PAR meter.

Plants are more efficient at producing energy from light in some parts of the PAR spectrum than others. We know that plants are most efficient at photosynthesis when using the blue and red light range. You will most likely see graphs like this one when people are talking about PAR. These type of graphs measures how well each type of chlorophyll in plants can absorb energy

from light at different parts of the PAR spectrum. 

Light Bulb Colour Temperature

Colour temperature is normally used to describe how the light produced by the lamp looks to the human eye. The "whiteness" of a light is measure by its colour temperature in units of Kelvin (K). Lights that have a lower Kelvin rating have a yellowish tint and light with a higher Kelvin rating has a bluish tint. You may have heard people referring to light as either "cool" or "warm". This describes the colours of the spectrum going from violet to red. Cool colours (violet, blue, green) are on the lower end of the light spectrum, while warm colour (yellow, orange, red) are on the higher end. Natural daylight has a colour temperature of 5000 K to 5800 K. The way we perceive natural light can vary based upon the weather and season. Colour temperature should not be confused with light spectrum colours which do not change.

 

Some Last Notes About Light and Cannabis

As mentioned before, nothing beats natural sunlight. A cannabis plant grown outdoors can grow huge in the right sunny conditions. When growing indoors you are not able to use the sun and so your next best option is to find a good quality grow light to take its place. Using a grow light with a fuller spectrum of light will have a greater effect on healthy growth and bigger yield. It is up to you to decide which type of lighting you need for your unique situation but I hope this article could 'shed some light' on how the light spectrum affects your cannabis plants so you are able to keep learning as you grow.