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 end (bottom) of the visible spectrum has a slightly shorter wavelength than the red end (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 thrive. Waves that infiltrate the ozone layer are in 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 penetrate the ozone. Both are useful 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 but only small amounts reach earth. Higher altitude places 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 Important

Marijuana plants require the right type of light to grow to their full capacity. Without the right spectrums of light, plants can underperform, growing small and producing low yields. Luckily, indoor growing techniques have enabled us to mimic the most important elements of the sunlight received by plants outdoors.

Outdoor plants receive the full spectrum of light provided 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 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 arrives the sun starts to dip, when the red spectra is most prominent. Red wavelengths signal the end of summer so plants grow flowers in an effort to pollinate before winter.

How Cannabis Plants React to Light

Marijuana plants can thrive well under blue and red lights provided they receive the right spectra within the colour band. By singling out these wavelengths, plants have better access to the kind of light they need. This is one of the reasons why indoor plants can be grown smaller than outdoor plants, since they do not have to expand in search for the necessary spectra.

• Blue (400 - 500nm) - Blue light is needed 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. Suggested for seedlings, especially if you are growing indoors.
• Red/Infrared (620 - 750nm) - Red is essential for expanding plant structure and promoting bud production. Naturally, plants utilize red light to recognise 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 constantly trying to comprehend 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 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 comes from. Some genetics have been brought from high altitude areas where the light is more powerful and there is a higher presence of UVB rays. It has been suggested that exposure to more UVB can enhance THC production, although this theory is inconclusive and needs to be investigated further.

Finest Spectrum to Grow Cannabis

It is tough to say exactly which part of the colour spectrum is best but a big part of it has to do with the type 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 accomplish. The main thing is that your garden receives enough blue light during the vegetative cycle and sufficient red light for flowering.

You might be planning to grow indoors where you can have full control over the light. Or maybe you're considering trying some outdoor plants, much of which will be determined by the sun. Whichever you choose, make sure your plants receive plenty of light.

• CFL (compact fluorescent light) lights are normally 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 commonly merged to produce 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. Marijuana plants benefit from HPS lighting mostly in the flowering stage.

If you are cultivating outdoors, you can anticipate plants to grow larger than they would indoors because 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 wish to produce good harvests, the intensity is also necessary. The intensity of your source of light plays a major part in plant development and has to be powerful enough for plants to photosynthesise correctly. If marijuana plants do not get adequate light, it doesn't matter what spectrum they get, because they will not have the ability to function.