best lights for autoflowering

Best Light Spectrum to Grow Autoflowers

When growing autoflowering plants indoors we need to simulate the environment they naturally grow in. The environment includes humidity, temperature, and the most essential of them all: light.

When growing indoors, the best light spectrum is one that comes as close as possible to the sun’s spectrum, so basically, a full spectrum.

Either you prefer light bulbs or LEDs, you need the right light spectrum in each stage for optimum growth.

1. What is Light Spectrum?

The light spectrum is the different colors (aka wavelength) a source of light can emit. Light is measured in nanometers (nm) and each nanometer represents a band of light (a band of light is a section in the color spectrum). Humans can see a small part of the spectrum, between 380 to 780 nanometers, which means we can only see the colors ranging from violet to red.

Even though it appears white, the sun is a full spectrum light source and contains the whole spectral wavelength. That’s why rainbows happen. When raindrops refract light into individual wavelengths you can see all the colors (visible to humans) that makeup sunlight.

In nature, cannabis plants grow under the sun, receiving the whole spectrum of wavelengths. This means we have to provide the maximum amount of wavelength possible for the best development of our cannabis plant.

Although it’s not essential, it is considered good to provide the best light spectrum to encourage plant growth.

Remember this is not a rule, you can grow your cannabis plant from seed to harvest with any light spectrum or amount of light but this can seriously affect your harvest.

2. Light Spectrums in Each Stage

Best light spectrum for growing plants

During the vegetative stage, cannabis in the wild needs blue wavelengths to grow strong, big, and promote leaf growth. When growing indoors we aim to grow as many leaves as possible. With more leaves, there is more surface to absorb light, this way we ensure our plant develops a strong stem and branches preparing her for the flowering stage.

Best light spectrum for blooming plants

When entering the flowering stage, cannabis in the wild uses red wavelengths to promote bud formation. If we want to produce dense buds and increase yields we need to provide the plant red wavelengths, this will increase the rate of photosynthesis thus increasing bud formation.

• Tip: When experimenting with training, using “red” light will make plants grow taller, making it easier to train.

The general rules are “blue” light = shorter and stronger plants with more leaves, “red” light = taller and weaker (when compared to plants grown under “blue” lights) plants with fewer leaves.

3. Types of Light

Cannabis grow lights are usually divided into two kinds, light bulbs, and LEDs.

Light bulbs emit a limited scope of wavelength meaning there are colors of the light spectrum they don’t emit, therefore they are narrow-spectrum lights.

LEDs emit almost all the colors in the spectrum, they are composed of blue, red, and white diodes allowing them to emit all the wavelengths of the spectrum, therefore making them full-spectrum lights.

LEDs vs Light Bulbs

LEDs Bulbs
Expensive Cheap
More efficient Consume more electricity
Take less space Need more space
Runs cooler Emit a lot of heat

Light Bulbs: HPS, MH, CFL, and HID

Light bulbs come in four types: Metal Halide (CMH), High Pressure Sodium (HPS), Compact Fluorescent Lights (CFL) and High Intensity Discharge (HID).

MH bulbs are rated around 6500 Kelvin and are usually used for cannabis in the vegetative stage because they emit a “blue” light (aka colder light).

HPS bulbs are rated at 2000 Kelvin are usually used for cannabis in the flowering stage for their “red” light (aka warmer light).

CFLs are rated at around 5000 Kelvin, they emit a very weak “blue” light and are only used with cannabis seedlings and clones as they just need to develop roots for the first couple of days when the vegetative stage begins they should be transferred to a growing space with stronger lighting.

HID can cover from 3500 to 5000 Kelvin, they are similar to HPS but contains xenon inside, thus emitting a different light spectrum and are more efficient while being more compact.

• Tip: You can also grow your plants under both CMH and HPS bulbs, this is called a mixed light spectrum and will ultimately result in a better harvest.


• Cheaper than average LED lights

• You don’t need to change the whole fixture, just replace the bulb


• Electricity cost is higher if compared to LEDs.

• Need extra equipment for cooling.

• Requires changing bulbs (CMH to HPS) when entering the flowering stage.

Full Spectrum LEDs, and Infrared and UV Lights

All LEDs are full-spectrum lights unless advised, they may not contain all wavelengths but will emit the needed amount for each stage of cannabis plant growth, unlike light bulbs, you won’t have to change anything other than your timer when you’re about to enter the flowering stage.

You can find full-spectrum LEDs that have UV and infrared diodes, if not, there are LEDs designed to emit those spectrums. They’re not that common in cannabis indoor growing but are said to benefit growers.

Just like everything else on the planet, cannabis plants are damaged by too much UV light but at the right amount. It can promote trichome production as the cannabis plants produce trichomes to protect themselves from too much light.

As UV lights, infrared lights can harm your plants in excess but at the right amount, it can boost photosynthesis, affect yields, plant growth, and plant health in a positive way.


• Used on the entire life cycle.

• Longer lifetime than bulbs.


Note: It’s unusual to have UV and Infrared lights, they’re expensive and are NOT essential for growing good flowers. They’re recommended for more experienced growers who want to experiment with different spectrums.

4. The Best Autoflower Light Spectrum

As most of you already know cannabis plants have two stages: the vegetative stage, and the flowering stage, needing a different light schedule and spectrums in each one of them.

Unlike photoperiodic cannabis plants, autoflowers enter the flowering stage automatically and depend solely on age to start producing buds. That’s why it’s essential to choose your cannabis light spectrum carefully, we recommend using a mixed light spectrum of warm and cold light bulbs (CMH and HPS) or a full-spectrum LED during the whole lifecycle of your autoflower.

This way your plant will receive a complete light spectrum, guaranteeing the best flowers and biggest yield.

5. Light Spectrum FAQs

What are “blurples”?

Blurples are LED fixtures that emit a purple light, in general, these lights claim to be full-spectrum but actually only contain blue and red diodes that emit specific wavelengths.

These lights can be used to grow cannabis but you need to have in mind that they do NOT emit a full spectrum, they’re just emitting a small part of the blue and red light spectrum so make sure you research before buying your new LED.

Which wavelengths are essential for cannabis?

When looking for a light fixture, you should make sure that the 440nm and 660nm wavelengths are at peak, meaning that your lights should emit more red and blue than the others but should definitely provide all of them. If you don’t know or the manufacturer doesn’t specify it, don’t doubt to send them an email, most respectable companies will provide this information.

What is cool and warm light?

When talking about lights, “cool” or “warm” refers to the light spectrum, which goes from violet to red. So, cool colors are on the lower end of the light spectrum and include colors such as violet, blue and green while warm colors are on the higher end and include yellow, orange, and red.

What’s the best light spectrum for seedlings?

As you may know, cannabis seedlings are in the vegetative stage so they will react better to blue or cool lighting. If there’s not enough blue light your seedling may end up stretching too much so even despite not needing a lot of light power, you should provide the right spectrum. If you don’t want to use high-intensity lights or you cannot dim down your LED, you can use 50w CFL bulbs for seedlings and clones until they’ve grown enough to be transferred to a more powerful light fixture.

6. In Conclusion

The light spectrum for weed is a small part of a much bigger system, there’s nothing as better light for your plant. But remember, the final product does NOT depend solely on your light.

Depending on which type you choose, either an LED or bulbs, you’ll have to adjust your growing environment accordingly to guarantee the best result possible.

This post was most recently updated on December 29, 2020.

What is the best light spectrum for cannabis and what spectrum is better for each phase of a cannabis plant's life cycle.