Grow Tip: Understanding The Light Spectrum
3 min

Grow Tip: Understanding The Light Spectrum

3 min
Growing Seedshop
An in-depth understanding of the light spectrum is an aspect of cannabis growing that separates the pros from the mediocre, giving growers another tool to push their cannabis to the limits.

When it comes to cannabis and light, it is important to know that not all light is made equal. Light can be an extremely deep and complex subject area, and can easily descend into mind-boggling science and statistics. Without even getting into types of lights, there are all sorts of factors that can influence a cannabis grow. One such factor is the light spectrum, which we will be looking at today.

First thing first, understanding the light spectrum is quite advanced growing, and it is not going to help you improve your grow much if you have not already mastered the basics, such as watering and nutrition. So if you are a novice, don’t worry too much about this yet, just focus on looking after your bud.


This in itself can be quite complicated, but to put it as plainly as possible, the light spectrum is the visible portion of electromagnetic radiation, also known as light. It is split up into bands of wavelengths, with each band representing a different colour.


Cannabis, like most plants, has evolved to turn the radiation from the light spectrum into energy; however, it is more efficient at using some bands of light than others. The sun is the ultimate source of light for plants and outputs strong radiation in every visible band of light. Indoor grow lights try to mimic this output, but some do it better than others. This is why it is important to make sure your lights are outputting the right bands of the light spectrum.


While cannabis is able to harness the power of the full spectrum to one degree or another, it is much more efficient at turning certain bands of light into energy during different parts of its life cycle.

Vegetative Growth

During the vegetative stage, it is the blue band of light that is best utilised by cannabis, encouraging healthy growth. The optimal reading for blue light in this phase is 460nm – nm being a measurement of light wavelengths. A bulb with a strong 460nm output will encourage a vegetative cannabis plant to grow with big healthy leaves and bush out. This mimics the sun during the Spring and Summer months, when the Earth is exposed to more of the blue spectrum.


During flowering, the needs of the cannabis plant change. During this time, a red light with a wavelength measurement of 660nm is best suited to your cannabis’ needs. This mimics the low hanging sun of Winter and Autumn, when more of the red part of the spectrum reaches the Earth. Cannabis has adapted to make full use of this change in light, and a red light of this wavelength will encourage the plant to focus its energy on healthy budding, boosting its efforts – which let’s face it, is what every grower wants to achieve.


It’s a good question. Why does cannabis respond better to blue and red light at various points throughout its life? Well, it is likely to do with evolution. As mentioned, different parts of the spectrum naturally fluctuate throughout the year as the Earth moves around the sun and the seasons change. Cannabis has adapted to work with the available resources, developing two different types of chlorophyll that are highly sensitive to either red or blue light. There are other pigments within the plant that help process the other spectrums, and while they are important for a range of functions, they are not nearly as influential as these two types of chlorophyll.


So, what does this mean for the grower? Well, it means it is best to use a light that has a strong blue output during vegetation, and one with a strong red output for flowering – if you really want to push your plants. However, it is very important to note that this doesn't mean a cannabis plant should be exposed to only blue and red light. As mentioned, a cannabis plant is able to use various parts of the spectrum, and will require full spectrum to really thrive – you just want the blue and red aspects to be stronger during the different phases of life. As such, dual spectrum bulbs that neglect the rest will not be suitable.

So as you can see, for the advanced grower, light spectrum, and the wavelengths your bulbs are putting out are very important. Fortunately, most bulbs will have details on their light spectrum, allowing you to easily assess their worth. Of course, there are other aspects to light other than light spectrum – a 1000W full spectrum light is going to be very different from a 100W full spectrum light. And once again, this is not an essential foundation of growing cannabis; even if you don’t have bulbs that excel in these colour bands, you will still be able to grow potent bud. This is just a way to pushing things even further.

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