What is Light Spectrum?

The light spectrum is the range of waves within the electromagnetic spectrum that is visible to us as human beings. 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 ends up getting to Earth. This is because the ozone layer absorbs and reflects back the majority of the damaging waves, allowing life to thrive. Waves that infiltrate the ozone layer are in 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 pass through the ozone. Both are beneficial for plants in some ways.

• UVC (180 - 280nm) - Includes harmful ultraviolet waves that are blocked by the ozone layer. Causes major burns.
• UVB (280 - 315nm) - Can burn skin but only small amounts reach earth. Higher altitude areas will have higher levels of UVB.
• UVA (315 - 400nm) - Always present in the atmosphere but not visible. 95% of ultraviolet waves entering through the ozone are UVA.
• Visible Light (380 - 750nm) - The range of the electromagnetic spectrum that we are able to 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 potential. Without the right spectrums of light, plants can underperform, growing small and producing reduced yields. Fortunately, indoor growing methods have allowed us to simulate the most vital aspects of the sunlight received by plants outdoors.

Outdoor plants receive the full spectrum of light supplied by the sun. In the spring, the spectrum of light that infiltrates the atmosphere creates a bluish tint. The blue spectra is what enables plants to produce 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 autumn arrives the sun starts to dip, when the red spectra is most prominent. Red wavelengths indicate the end of summer so plants grow flowers in an effort to pollinate before winter.

How Marijuana Plants React to Light

Cannabis plants can thrive 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 need. This is among the reasons why indoor plants can be grown smaller than outdoor plants, because they do not have to stretch in search for the required spectra.

• Blue (400 - 500nm) - Blue light is needed for plants to grow strong leaves and branches during its vegetative cycle. Offering high quantities of blue leads to short, bushy plants, which can be helpful for growers trying to avoid tall plants with long branches. Recommended for seedlings, especially if you are growing indoors.
• Red/Infrared (620 - 750nm) - Red is important for expanding plant structure and promoting bud production. Naturally, plants use 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 trying to understand the light spectrum surrounding them. Leaves will turn to face the light during the day. When there is no light cannabis plants preserve their energy by 'relaxing' their leaves, making them droop slightly. Photoreceptors do not really turn light into energy, which is more to do with photosynthesis.

How a cannabis plant reacts to light will also depend on the strain and where it originates from. Some genetics have been brought from high altitude areas where the light is more powerful and there is a greater presence of UVB rays. It has been argued that exposure to more UVB can boost THC production, although this theory is inconclusive and needs to be investigated more.

Best Spectrum to Grow Marijuana

It is hard to say precisely 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 emit the same spectrum so choosing will depend on the outcomes you are trying to accomplish. The key is that your garden gets enough blue light during the vegetative cycle and enough red light for flowering.

You might be planning to grow inside where you can have full control over the light. Or maybe you're thinking of trying some outdoor plants, much of which will be determined by the sun. Whichever you pick, make sure your plants get lots 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 give off a narrow spectrum of light, either in a small band of blue or red. Red, blue and white LEDs are often blended to produce a 'full spectrum' emission that plants can comprehend for appropriate growth.
• HPS (high pressure sodium) - Yellow/red light can be used 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 get larger than they would indoors because the spectrum is wider. Depending on where you live and the time of year, plants will respond to the light in different ways.

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