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The Light Spectrum Matters When Growing Cannabis

Updated August 20, 2022

When growing cannabis, or any other plant, the light spectrum plays a key role in the growth cycle. Factors such as the color and the type of light that you use will influence plant growth, including the quality of growth.

It is possible to use a single light throughout the entire growing process, but this will lead to less-than-stellar results. Instead, you should learn a bit more about the light spectrum and which types of lights to use at each stage in the growth cycle.

A Quick Overview: When to Use Each

Overall, there is general guidance about when to use each light spectrum color during cannabis growth. This gives you a quick overview of when to use each type of light.

Blue for Vegetative

Most people use blue lights during the vegetative stage. This will encourage the cannabis plant to be short and squat and to have large, healthy leaves.

Indoor growers will typically use compact fluorescent lamps, T5/T8 lighting fixtures, or metal halide bulbs during the first several weeks. This encourages the plants to stay compact.

Red for Flowering

Most people use red or yellow grow lights that have higher ratios of red during the flowering stage. This helps encourage the plants to promote budding and grow tall. It is also possible for plants to get into the flowering stage more quickly if you use red light compared to blue light.

Other Notes

You can also find lights that combine various elements of the electromagnetic light spectrum or get creative. As long as your chosen grow light provides sufficient light, you will be able to grow your cannabis successfully. The chosen light may just affect the size of the plant, its speed of growth, and the quality and quantity of the yield.

Keeping that information in mind, take a closer look at each of the colors in the light spectrum and how they affect cannabis growth.

A chart representing the frequency, wavelength, and photon energy at each point in the light spectrum.

When to Use Red Lights

Red lights sit on the electromagnetic spectrum between 600 and 700 nanometers. You will find the brightest shade of red at approximately 660 nm. This is also the most efficient wavelength of red light for your plant to absorb. Red light tends to be more intense in nature during the fall, as the sun begins sitting lower in the sky. As such, red light signals the cannabis plant that summer is almost done, and winter is approaching. This results in the growth of longer stems that have additional space between the leaves. It also results in elongation, so the plant can make buds and use the wind to pollinate.

When you use red grow lights, this signals your cannabis plants that it is summer. The result is encouraging your plants to grow tall. The red spectrum of lights, including infrared and far red, also promotes budding. Red light does this by activating the phytochrome pigments. Keep in mind that low red light intensities have the potential to inhibit flowering in plants with short days if they interrupt darkness. Cannabis falls into this category.

If you grow your cannabis plant under just red light, this could lead to more elongation than you want. As such, you should take care to include at least some blue light in addition to the red.

There are also considerations within the red spectrum, including your ratio of red light to the far-red light. This will influence stem elongation as well as leaf growth, with larger quantities of far-red light lengthening the stem. If the lights you use do not have enough far-red light, the overall growth of your cannabis plant may be very minimal.

Due to the promotion of budding, you should avoid using red lights unless you are ready for your plants to start budding. Instead, wait to make the switch to red until your cannabis plant shows indications that it is entering the flowering stage. This change should actually help speed up the flowering stage, giving you fully grown cannabis more quickly.

When to Use Blue Lights

Blue light has a wavelength of between 400 and 500 nm, and the chlorophyll a pigment absorbs it. During photosynthesis, both red and green light are more productive than blue. In nature, more blue light will reach the plants in the spring and summer as the sun sits more directly in the sky. In summer, the plants see blue lights as their cue to begin vegetative growth.

Blue grow lights will help your cannabis grow to be stocky and short. These lights are also cooler, which can help boost the production of trichomes and terpenes when the plants enter the flowering stage.

Many people say that the flavor of crops harvested after ending growth under purple light is better. That comes from the increased production of terpenes and flavonoids, which are directly related to the flavor.

Because of this, you should ideally switch to blue lights when it gets closer to harvest time. For the best results, combine the lights with a slightly cooler grow temperature. This can lead to cannabis crops that feature purple hues for visual appeal as well as quality production.

You should always make sure to have at least some blue light during your cannabis growth. This will help prevent leaf shrinkage and stem elongation. Blue light also regulates the opening of the leaves’ stomatal pores on their undersides. These must be open in order to absorb carbon dioxide.

The normal photoperiodic lighting intensity of blue lights should not affect flowering, whether for short-day or long-day plants. Higher than average intensities, however, can inhibit flowering for cannabis and other short-day plants.

When to Use Green Lights

The green light spectrum helps encourage plant growth overall. Importantly, this part of the light spectrum does a better job of penetrating into the plant compared to red and blue lights.

Ideally, you will want to incorporate some green-spectrum light into your grow lighting to get the benefits of this color. If you cannot afford a light that only has the green spectrum, then opt for a fluorescent light that includes a small amount. Luckily, only a few LED lights will not have any green spectrum in them. Every other light should have at least some.

It is also important to never overdo the green light spectrum at the expense of other lights. While green lights are incredibly helpful for plant growth, your cannabis plant requires some of the other colors even more to thrive.

When to Use Yellow Lights

Yellow lighting is a strong choice when it comes to promoting long stems and encouraging germination in your cannabis plants.

Yellow lights are actually very versatile, as they also work well during the flowering stage. At that point, they can slightly increase bud production.

If you mostly keep your cannabis plants under HPS lighting, you can expect them to feature gold or yellow hues, possibly even within the buds. This is just a result of the lighting and should not affect flavor, aroma, or quality.

When to Use Purple Lights

Most of the LED grow lights on the market for cannabis will emit a somewhat purple hue. This part of the color spectrum helps encourage the growth of fatter buds, which is always ideal.

Growers should strongly consider opting for a mixed spectrum light, such as one including purple. This will improve the light’s penetration into the plant, providing it with more energy. That increased energy, in turn, results in larger yields and stronger growth.

Non-visible Parts of the Light Spectrum

Keep in mind that the visible light spectrum, including all of the colors mentioned above, are between 380 and 750 nm. Other than this, there is also UVC light, which is 180 to 280 nm. This light is very harmful, and the ozone layer almost fully absorbs it. You do not want to expose your cannabis plant to UVC light.

UVB light has a wavelength of 280 to 315 nm. It is the light that causes sunburn, and it may be beneficial to your cannabis growth. UVA light has wavelengths of 315 to 400 nm and is also called black light. Infrared light has a wavelength of 700 nm to 1 mm and is not visible (past 750 nm), but we can feel its heat on our skin.

UVB Light and THC Levels

Some growers have found that including UVB lights in their grow space will encourage the production of THC. Remember that this is part of the light spectrum that is not visible. This is still a theory, but many experts believe that it is the case.

The idea comes from the fact that many potent strains of cannabis are originally from areas with higher altitudes. Higher elevations typically have less atmosphere separating the cannabis plant from the sun, which results in greater exposure to UV rays.

Remember how UV light damages our skin and how the body protects it by producing melanin? Experts believe that something similar happens with cannabis. The plant seems to produce more THC and resin to work as its own version of natural sunscreen.

This Is Still Not Proven

Keep in mind that the connection between UVB light and THC levels is not yet established. It is still a plausible theory that growers may want to take advantage of. Just use caution as this has still not been proven to be helpful or effective.

Understanding the Science of the Light Spectrum for Cannabis Growth

For those who want an even deeper dive, you can work to understand why the light spectrum influences cannabis growth. Remember that light is just one portion of the electromagnetic spectrum, with others including infrared light, gamma rays, and X-rays.


Plants rely on photomorphogenesis to react to the spectrum of light. This process is fully separate from photosynthesis. This is related to the way that plants adjust their growth patterns as a way to maximize the light they get.

Cannabis plants use their senses to determine the light spectrum and use that information to determine the optimal way to grow. Plants have photoreceptors, which are light sensors. These are similar to the human senses in that they provide the plants with information about the available light spectrum. Think of it as similar to how we use our nose to tell us where food is.

Plants also have cryptochromes, which are blue light receptors. These are used to help set the internal clock of the plant. Phytochromes are unique plant molecules that are primed with red light exposure. From there, it can absorb far-red light, which is nearly infrared. Phytochromes regulate germination, leaf and stem growth, as well as flowering. It may also be related to helping plants sense temperature.


You should also be aware of phototropism, which is when plants bend towards a source of light. On a related note, scientists have found evidence that wavelengths featuring blue light make it easier for plants to find the light source.

What That Means for Your Chosen Light

This means that when you choose a color of the light spectrum for your plant, it will be able to sense the color via its photoreceptors. It will then use the information about what that light spectrum means in nature to determine how to grow, whether that promotes budding, longer stems, or something else.


Cannabis plants, like all other plants, rely on the light spectrum for cues about their growth. Blue light is particularly useful during the vegetative growth phase, while red light is particularly useful during the flowering stage. For the best results, you will also want to supplement these with the other colors on the visible light spectrum.

If the entire idea of changing around your lights is too complicated or you cannot afford to invest in multiple grow lights, then you can still grow cannabis with certain lights that contain all the necessary color spectrums. However, these will not produce as optimal results as at least using separate lights that focus on blue and then red light.

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