How the Light Spectrum Affects Your Cannabis Crop

How the Light Spectrum Affects Cannabis - WeedSeedShop

Growing cannabis is an adventure into physics and biology as much as it is an adventure into botany! Understanding a little bit about how the light spectrum affects cannabis plants can help you create the best growing environment for your goals. In this article, you’ll learn all about how different coloured light changes the behaviour of cannabis plants.

To the regular eye, all light seems more or less the same colour. However, when we look a little closer, we see that actually, light is made up of a lot of colours. All of the colours in the spectrum in fact.

This is perfectly demonstrated when you shine a light through a prism or even observe a rainbow after a storm. What we see is that white light contains all the colours of the rainbow. Now what does this have to do with your cannabis crop?

Research tells us that the colour of light can alter the way that a plant chooses to grow. In general, this doesn’t apply to plants that are grown outdoors. On the other hand, indoor growers have maximum control over the colour of light they choose to use – but changing it up is never without reason.

There’s no hard and fast rule about light spectrum when it comes to growing weed. One is not better than the other. They all have varying effects, and for this reason growers might mix up the colour of light at different stages throughout their grow. Depending on what your goals are for growing weed, changing up the light spectrum could help you get there.

Why do cannabis plants respond to different light spectrums?

How the Light Spectrum Affects Cannabis - WeedSeedShop

Outside the world of indoor growing, cannabis plants receive their light from the sun. And contrary to what your eyes might tell you, the colour of sunlight changes depending on what time of year it is.

The plant’s sole responsibility is to take all of the information its immediate environment presents it with, and make the most of it. In fact, it has spent thousands of years perfecting this biological process.

During spring and summer, the sun has a very direct path through the sky, causing more of the blue spectrum of light to make it to Earth. This causes a cannabis plant to grow many leaves and shorter stems. It is essentially trying to spread itself out. On the other hand, red spectrum light is the main agenda getting closer to winter. This causes a plant to flower and start creating buds.

Weed responds to the change in sunlight with a certain kind of light sensor in its leaves and stems called photoreceptors.

These photoreceptors act as their “eyes” so that they know whether it’s morning or night time! The same way that you might gravitate towards the smell of French fries, knowing that there’s food there, plants gravitate in the direction of certain colours within the light spectrum. Through their photoreceptors, they essentially receive instructions about what to do next!

So, when you are growing your own cannabis plants indoors, it’s your job to be the sun and give them the right instructions!

Growing with blue light

Plants that are grown under blue light tend to have shorter stems and big, healthy leaves. Some growers choose a blue light during the seedling and vegetation stage because it encourages node growth, which can lead to a potentially bigger yield.

If you are growing in a very limited space, blue light spectrum globes, such as Metal Halides are perfect because they stop your plant from getting too tall. Having said that, plants that have many nodes in a small space can be harder to manipulate for techniques such as Low Stress Training.

Cannabis plants need blue light to grow. It’s what helps them distinguish the sky from the ground. When your plant is just a baby seedling, its root system and above-ground system work much the same way. Until, of course, one starts growing towards the sky (blue light) and starts acting more like a stem than a root.

All lights have some degree of blue light in them: Even red lights contain some of the blue spectrum because it is imperative for plant growth.

Growing with red lights

Growers usually employ red grow lights (such as HPS globes) once the vegetation stage is over. Some growers even use red lights to help flip the plants from veg to flower. Red light encourages stretching and tall plant growth. This helps a plant enter the flowering stage, producing the buds that growers are so desperately after.

In the natural environment, the suns starts to emit a reddish kind of light when it is approaching the winter. This is because it is generally much lower in the sky. This is the cue for plants to start flowering and creating buds, because they know that they probably will not survive a snowy winter.

Having said that, it isn’t unusual for growers to use red lights from the seedling stage. Remember – they still contain some of the blue light necessary for growing. Rather than encouraging a short, leafy plant, a red light will give a grower a tall plant with much more space between nodes.

For the purpose of training techniques such as low stress training (LST), this can be an advantage. More space between nodes means more room for bending and manipulating the stems.

Growing with green lights

There is still a debate about the benefit of growing weed under green lights.

NASA was involved in some research about the behaviour of plants grown under green light, although their study used lettuce, not cannabis.

Interestingly, plants that were given predominantly green light had a tendency to grow slower. But the lettuce plants that were given a combination of red, blue and green lights only had a far better yield than full spectrum lights. But don’t forget folks, we’re talking about lettuce – not cannabis! (Although both can be used in salads, right?)

LED lights usually don’t contain any of the green colour spectrum, whereas fluorescent lights do. We know that green light affects vegetative development, CO2 usage, plant height and even stem growth, but there’s a lot we don’t know about it yet.

Changing the light spectrum doesn’t necessarily increase yield

How the Light Spectrum Affects Cannabis - WeedSeedShop

It’s an old wives’ tale that the colour spectrum alone can change the yields you get at the end of the day. Although having a blue light encourages many nodes, it will require some more training techniques from a grower to get bigger yields. And, well – we don’t know enough about green light to say that it definitely increases yield.

The best way to yield more from your cannabis plants is to give them more light. Whichever spectrum you are using, giving plants more of it (i.e. longer hours) will generally tend to encourage more bud production. Lights are a great way to manipulate the way your plants grow, to suit your grow space and what you plan to do with them.

Nothing says that one colour spectrum is more efficient or beneficial than another. Each grower has their own system, which probably comes from years of trying many different things – and we encourage you to do the same! To achieve the best final product, it takes a lot of patience and the willingness to see how plants behave under different conditions. Enjoy the journey!


Leave a Comment

Your email address will not be published. Required fields are marked *


  • Author_profiles-WSS-Emma Ryte

    Emma Ryte

    Born in Germany, lived in England my childhood years and spent my high school era in the US. My parents are basically hippies and that is why I have had the possibility to speak freely and explore my passion for the cannabis plant. My love for writing followed soon after. Writing about this subject has taught me so many things about the use, cultivation, health benefits and industrial options, and I love that I can share this knowledge with you! I visit Amsterdam as often as I can and I love the vibe there when it comes to weed, it lets me try new things and learn about the newest developments in the industry. The one person I would love to smoke some with would have to be Stevie Wonder. I love relaxing to his music while high and I would love to meet him!
    More about this author
Scroll to Top