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 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 side (top).

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

Beyond 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 severe burns.
• UVB (280 - 315nm) - Can burn skin yet 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 penetrating through the ozone are UVA.
• Visible Light (380 - 750nm) - The range of the electromagnetic spectrum that we can see as color.
• Infrared (700 - 1100nm) - Invisible to the human eye but can be felt as heat.

Why Light Spectrum Is Very Important

Cannabis 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 producing reduced yields. Luckily, indoor growing techniques have enabled us to simulate the most vital aspects of the sunlight received by plants outdoors.

Outdoor plants receive 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 enables plants to develop strong foliage during the early stages of growth.

As the summer hits, the sun is higher in the sky and more light penetrates the atmosphere. By the time autumn 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 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 color band. By singling out these wavelengths, plants have easier access to the type of light they need. This is one of the reasons why indoor plants can be grown smaller than outdoor plants, since they do not need to expand in search for the required spectra.

• Blue (400 - 500nm) - Blue light is necessary for plants to grow strong leaves and branches throughout its vegetative cycle. Supplying high amounts of blue leads to short, bushy plants, which can be useful for growers trying to avoid tall plants with long branches. Suggested 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 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 cannabis plant's leaves are constantly trying to understand the light spectrum surrounding them. Leaves will turn to face the light during the day. When there is no light marijuana plants conserve their energy by 'relaxing' their leaves, making them droop a little. 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 stems from. Some genetics have been brought from high elevation areas where the light is more powerful and there is a higher presence of UVB rays. It has been argued that exposure to more UVB can increase THC production, although this theory is inconclusive and needs to be researched more.

Ideal Spectrum to Grow Marijuana

It is tough to say exactly which part of the color spectrum is best but a big part of it has to do with the kind of light your marijuana plants are grown under. Not all lights emit the same spectrum so choosing will depend on the outcomes you are trying to achieve. The key is that your garden has enough blue light during the vegetative cycle and sufficient red light for flowering.

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

• CFL (compact fluorescent light) lights are typically measured in Kelvin (K), which tells us the color 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 emit a narrow spectrum of light, either in a small band of blue or red. Red, blue and white LEDs are frequently 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 mainly in the flowering stage.

If you are cultivating outdoors, you can anticipate plants to get larger than they would indoors because the spectrum is broader. Depending on where you live and the time of year, plants will respond to the light in different ways.

The color spectrum of your grow light is important but if you want to produce good harvests, the intensity is also important. The intensity of your light source plays a big role in plant development and has to be powerful enough for plants to photosynthesise correctly. If marijuana plants do not get sufficient light, it doesn't matter what spectrum they receive, because they will not be able to function.