If you could increase your plant's THC content by up to 28%, would you do it? Of course, you would! Here's how...

Add UV light.

That's it! In fact, you don't even need to add UV light to your grow for the entire growth period. Learn why in a minute or two. 

UV light's affect on resin production in cannabis has been well documented in scientific studies and home gardens. Many growers have been supplementing with UV light, specifically UV-B (UV-A has been shown to help, too), and they are noticing obvious results in THC production.

The average amount a grower might increase their plant's THC concentration is 5% (for example, from 16% THC to 21% THC) or a 23% increase. However, as mentioned, a 28% increase in THC production is possible depending on genetics, environment, etc. 

What is UV light?

Image: Mouser Electronics

UV light is electromagnetic radiation that falls between visible light and x-rays. UV light is made up of three different types:


  • 315-400 nm
  • long-wave UV light not absorbed by the ozone layer
  • relatively harmless to humans
  • increases THC production in cannabis but not as much as UV-B light
  • assists in reversing plant DNA damage caused by UV-B light
  • stimulates the production of...
    • flavonoids (contribute to cannabis' flavor and smell)
    • anthocyanin (purple color in some cannabis strains)
  • useful, though not the single best UV type for the cannabis garden


  • 280-315 nm
  • medium-wave UV light mostly absorbed by the ozone layer
  • harmful to humans over long term; skin and eye protection is advised;
  • increases the amount of THC production in cannabis
  • there is a direct correlation between UV-B stimulation and THC production
  • very useful and the best UV type for the cannabis garden


  • 100-280 nm
  • short-wave UV light completely absorbed by the ozone layer and atmosphere
  • very harmful to humans
  • used as a sterilizing agent in hospitals
  • not a concern in the cannabis garden

In sum, we are only concerned with UV-B and UV-A radiation for cannabis.

Why does UV light increase THC?

UV-B light is damaging to plants. Therefore, the plant's natural response is to produce compounds to protect itself. It can't get up and move into shade (it's a plant, after all) however, it can produce it's own sun block in the form of THC. In fact, it is one of nature's best sun blockers!

How does UV light increase THC?

A chain-reaction is started during cannabis' flowering phase when the plant is exposed to UV radiationbeginning with an increased production of malonyl-CoA and phenylalanine. Malonyl-CoA is used by cannabis to create Ovitol, which is used to make THC.

Evolution of a UV defense system

THC production differs in space and in time due to differences in UV radiation. Latitude, elevation, and the time of day affect how much UV light plants receive.

Areas on the planet where UV light is strongest exhibit the highest concentrations of THC. Cannabis strains that grow in low latitudes—below the 30th parallelcontain higher concentrations of THC, while areas above the 30th parallel larger amounts of CBD relative to THC.

Image: High Times

UV light also increases in elevation. The chart bellow shows that there is 30% more UV radiation at 6,000 ft and 45% more at 9,000 ft. Areas along the northern 30th parallel—including Nepal, Afghanistan, and Morocco—are all famous for their excellent hash. These are all regions at high elevations.


Image: Vitamin D Wiki

Lastly, there is more UV light emitted during the middle of summer than their is during the later in the year due to the angle of the sun. Cannabis that is ripened outdoors in August has been shown to contain more THC than plants harvested in October.

Using evolution to your garden's advantage

We can take the wisdom of nature and apply it to our home gardens...

Grower research has shown that UV-B light is most effective when applied at about two weeks prior to harvest. This is because the flowers have already started forming trichomes (little stalk-like resin glands) and are about to start filling with with THC, terpenes, and other cannabinoids. When UV light is applied at this growth stage, the plants will produce more THC in an attempt to defend itself from the harmful UV rays. 

Increased THC production will only occur on the flowers that are exposed to the UV light. Therefore, you will likely see an increase in THC production on the flowers closest to the light and a smaller increase in THC production lower down on the plant (~1%-2% less THC). Since UV light's do not get very warm you may consider adding additional UV lights on the sides or between plants to get these tough spaces. Though you could be better off growing a SCROG to ensure an even canopy that is fully exposed to the light (grow lights and UV light).

How much UV do my plants need? About 5-10% of the total wattage. So, if you are growing with 1000 watts, you will need about 50 to 100 watts of UV light. 

Getting started with UV-B supplementation

We recommend using LED grow lights to supplement with UV light. Up until the time of this writing, there has not been an UV-B LED grow bar made for increasing THC in cannabis. However, now we can say there is...

The Reflex UV-B Supplemental LED Grow Light by Cirrus LED Systems was specifically developed for home grows and commercial applications.

We are offering you a chance to pre-order a Reflex UV-B Supplemental LED Grow Light for a reserve price of only $30. 


Buy Now


Reflex UVB LED Grow Light


Reflex UVB LED Grow Light