What is Light Spectrum?

The light spectrum is the range of waves within the electromagnetic spectrum that is visible to us as humans. Measured in nanometres (nm), the visible spectrum sits in between 380nm to 750nm on the scale. This means the blue side (bottom) of the visible spectrum has a slightly shorter wavelength than the red end (top).

The sun emits solar energy, of which a very small portion winds up getting to Earth. This is because the ozone layer absorbs and reflects back the majority of the damaging waves, allowing life to flourish. Waves that filter through the ozone layer are between 300nm - 1100nm, so the spectrum entering the atmosphere is wider than we have the ability to see.

Outside of the visible light spectrum we have gamma waves, x rays, ultraviolet waves, infrared, microwaves and radio waves. Ultraviolet waves and infrared waves sit at either end of the visible spectrum meaning they are included in the wavelengths that travel through the ozone. Both are helpful for plants in some ways.

• UVC (180 - 280nm) - Includes harmful ultraviolet waves that are blocked by the ozone layer. Causes severe burns.
• UVB (280 - 315nm) - Can burn skin however only small amounts reach earth. Higher elevation places will have higher levels of UVB.
• UVA (315 - 400nm) - Always present in the atmosphere yet not visible. 95% of ultraviolet waves entering through the ozone are UVA.
• Visible Light (380 - 750nm) - The range of the electromagnetic spectrum that we can see as colour.
• Infrared (700 - 1100nm) - Invisible to the human eye but can be felt as heat.

Why Light Spectrum is Very Important

Marijuana plants need the right kind of light to grow to their full capacity. Without the right spectrums of light, plants can underperform, growing small and generating low yields. Luckily, indoor growing methods have enabled us to imitate the most important elements of the sunlight received by plants outdoors.

Outdoor plants get the full spectrum of light offered by the sun. In the spring, the spectrum of light that infiltrates the atmosphere produces a bluish color. The blue spectra is what allows plants to develop strong foliage during the early stages of growth.

As the summer arrives, the sun is higher in the sky and more light penetrates the atmosphere. By the time fall comes the sun begins to dip, when the red spectra is most pronounced. Red wavelengths indicate the end of summer so plants grow flowers in an attempt to pollinate before winter.

How Marijuana Plants React to Light

Marijuana plants can flourish well under blue and red lights provided they get the right spectra within the colour band. By singling out these wavelengths, plants have easier access to the type of light they require. This is among the reasons why indoor plants can be grown smaller than outdoor plants, because they do not need to expand in search for the required spectra.

• Blue (400 - 500nm) - Blue light is required for plants to grow strong leaves and branches during its vegetative cycle. Supplying large quantities of blue leads to short, bushy plants, which can be helpful for growers trying to prevent tall plants with long branches. Recommended for seedlings, especially if you are growing inside.
• Red/Infrared (620 - 750nm) - Red is essential for expanding plant structure and promoting bud production. Naturally, plants utilize red light to identify when it is day or night. Small amounts of infrared make its way through the atmosphere during sunrise and sunset when wavelengths are longer. Cannabis plants grow taller under infrared until visible red becomes more direct.

Photoreceptors in a marijuana plant's leaves are continuously working to understand the light spectrum surrounding them. Leaves will turn to face the light during the day. When there is no light cannabis plants conserve their energy by 'relaxing' their leaves, making them sag slightly. Photoreceptors do not actually turn light into energy, which is more to do with photosynthesis.

How a marijuana plant responds to light will also depend on the strain and where it originates from. Some genetics have been brought from high elevation areas where the light is stronger and there is a greater presence of UVB rays. It has been suggested that exposure to more UVB can increase THC production, although this theory is inconclusive and needs to be investigated further.

Best Spectrum to Grow Marijuana

It is tough to say exactly which part of the colour spectrum is best but a big part of it relates to the type of light your cannabis plants are grown under. Not all lights produce the same spectrum so choosing will depend on the outcomes you are trying to achieve. The main thing is that your garden has sufficient blue light throughout the vegetative cycle and enough red light for flowering.

You may be intending to grow inside where you can have complete control over the light. Or maybe you're considering trying some outdoor plants, a lot of which will be determined by the sun. Whichever you pick, be sure your plants have a lot of light.

• CFL (compact fluorescent light) lights are usually measured in Kelvin (K), which tells us the colour temperature of a bulb. Cooler, blueish lights have a higher Kelvin reading (6000 - 6400k), whereas a CFL that looks red will have a lower reading (2800k).
• LED (light emitting diode) - LEDs produce a narrow spectrum of light, either in a small band of blue or red. Red, blue and white LEDs are often merged to create a 'full spectrum' emission that plants can understand for proper growth.
• HPS (high pressure sodium) - Yellow/red light can be utilized throughout the grow cycle, but plants will grow taller with more spacing between internodes. Cannabis plants benefit from HPS lighting mostly in the flowering stage.

If you are cultivating outdoors, you can expect plants to grow larger than they would indoors since the spectrum is wider. Depending on where you live and the time of year, plants will react to the light in different ways.

The colour spectrum of your grow light is important however if you want to produce decent harvests, the intensity is also necessary. The intensity of your light source plays a big role in plant growth and needs to be powerful enough for plants to photosynthesise correctly. If marijuana plants do not receive sufficient light, it does not matter what spectrum they get, because they won't have the ability to function.