A different color spectrum can change how cannabis germinates, grows, and even makes buds! Although we don’t have a lot of options to change the spectrum of sunlight when growing outdoors, we have almost complete control of the spectrum when growing cannabis indoors since we’re using grow lights!
So what’s the “best” light spectrum for growing cannabis? Unfortunately, that’s a big question since each spectrum has its own unique effects and some are more useful to you than others, but let me share everything we know so you can decide what’s best for your goals and situation.
There is no “best” light spectrum for growing cannabis, but you can choose the best one for your goals and grow room!
So why do cannabis plants react to the spectrum of light? When growing outdoors under the light of the sun, the spectrum of light received actually gives a cannabis plant a lot of information about what’s going on in the world around them.
As just one example, during the spring and summer, more of the blue spectrum of light makes it to earth because the sun takes a more direct path through the sky.
In the summer, a plant responds to this bright direct light (with lots of blue) as a cue to grow vegetatively with lots of big leaves and short stems. The plant tries to spread out as much as it can and increase its leaf mass.
Blue light – Grow lights with a higher ratio of blue light are often used in the vegetative stage as they tend to make cannabis plants grow short and squat, with big healthy leaves.
As fall approaches the sun starts hanging lower in the sky. As a result, more of the light that reaches the plant falls within the red spectrum. More light in the red part of the spectrum is a sign to the plant that summer is coming to an end and it’s time to get in gear before winter. As a cannabis plant gets exposed to a higher ratio of red light, it reacts by growing longer stems with more space between the leaves, getting as tall as possible for the best position to start making buds and pollinate via the wind!
Yellow/Red light – Grow lights with higher amounts of red are often used in the flowering stage to encourage plants to grow tall (stretch) and to help promote budding. Plants may switch to the flowering stage faster under red light than under blue.
By keeping track of the ratio of colors in the spectrum of light, a cannabis plant is able to “know” a little bit about what’s going on around them which helps plants grow in the best way possible! As an indoor grower, it’s your job to be the sun and tell your plant what to do!
If you’re trying to keep plants short with lots of leafy growth, use growth lights that give off light in more of the blue spectrum during the beginning of your plant’s life. When it’s time to switch the plant to the flowering stage, you might switch the lights to something that has more of the reds and oranges that cannabis likes during the flowering stage.
But you don’t have to follow the general rules! I personally use an HPS bulb from seed to harvest, which gives off primarily red/yellow, because I like how young plants grow under them – I find it easier to train the plants when they have a little bit more space between nodes! But if I were in a very height-limited space, I might consider trying to give more blue light to help keep stems naturally shorter.
How big a difference does spectrum make to plant growth?
As long as a cannabis plant is getting bright light with at least some red and blue, it will grow normally, so any reputable plant grow light you get on the market will do the job and get you to harvest with top-quality buds!
Although certainly not essential to healthy plant growth it is considered a good form to choose lights in the proper light spectrum to encourage the growth you want in the vegetative and flowering stages of the marijuana plant. If you have a tool, why not use it?
However, keep in mind that different strains are affected differently by the color spectrum of the light. Some plants are barely affected at all while others might react much more strongly. It’s important to remember that the changes caused by the light spectrum are relatively small (such as stems tending to grow longer) and many other factors drive some of the same changes.
For example, if your plant isn’t getting enough light, it will grow long and lanky even if the light has lots of blue because it’s “reaching” for more light.
On the flip side, if you give your plant high levels of light with lots of yellow, it tends to stay short because it’s getting so much light that there’s enough blue and no need to get taller. Since almost all grow lights give off a spectrum of light that is suitable for healthy cannabis growth, in many ways the amount of light has a much bigger effect on your plant than the spectrum.
The best way to increase yields is to give your plant more light altogether!
In other words, giving your plant more light is going to give you a bigger return on yields than simply changing the spectrum. So if it’s possible to give your plant more light (up to a point), that is going to increase your yields even if you’re adding light in the “wrong” spectrum. Even bright white fluorescents that dip into the green spectrum will produce great plant growth.