Best Light Spectrum to Grow Autoflowers

by in Grow Light Advice February 27, 2018

Providing different spectrums in each stage may influence the quality of your harvest.

When growing auto-flowering 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.

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

1. What is Light Spectrum?

The light spectrum is the different colors (aka wavelengths) 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. Thats why rainbows happen. When raindrops refract light into individual wavelengths you can see all the colors (visible to humans) that makeup sunlight.

visible spectrum

Humans can see only a part of the spectrum, between 380 to 780 nanometers (from violet to red).

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

Although its 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 plant from seed to harvest with any spectrum or amount of light but this can seriously affect your harvest.

2. Light Spectrums in Each Stage

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. 

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.

light spectrum and its benefits for cannabis growth

Light Spectrum and its benefits for Cannabis growth.

•  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

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

Light bulbs emit a limited scope of wavelengths meaning there are colors of the spectrum they dont 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.


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 in the vegetative stage because they emit a blue light (aka colder light).

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

vegetative stage

Light Spectrum: use blue wavelengths for the vegetative stage, and red for the flowering stage.

CFLs are rated at around 5000 Kelvin, they emit a very weak blue light and are only used with 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 spectrum and are more efficient while being more compact.

differen light bulbs

Light Bulbs: HPS, MH, CFL, and HID.

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


•  Cheaper than average LED lights

•  You dont 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, unlike light bulbs, you wont have to change anything other than your timer when youre 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. Theyre not that common in indoor growing but are said to benefit growers.

Quatumn Board LED Grow Light 100w

LEDs – Light Emitting Diodes. Many manufacturers now produce outstanding LEDs that almost mimic the suns full spectrum.

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 itself 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.

•  Energy-saving.

•  Longer lifetime than bulbs.


•  Higher cost.

Note: Its unusual to have UV and Infrared lights, theyre expensive and are NOT essential for growing good flowers. Theyre recommended for more experienced growers who want to experiment with different spectrums.

4. Light Spectrums for Autoflowers

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, autoflowering plants start their life cycle in the flowering stage and depend solely on age to start producing buds. Thats why we recommend using a mixed 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 spectrum of light guaranteeing the best flowers and biggest yield. 

5. In Conclusion

The light spectrum is a small part of a much bigger system, theres 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, youll have to adjust your growing environment accordingly to guarantee the best result possible.

Leave a Reply

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