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Which is better: UVA or UVB?

UV light for plants are split into 3 categories based on wavelength:

  • Ultraviolet A (UVA) is from 320-400nm and comprises about 3% of the photons in natural sunlight that make it through Earth's atmosphere. UVA lights for plants do not damage DNA.
  • Ultraviolet B (UVB) is from 290-320nm and makes up less than 0.15% -- less than 1/5th of 1% -- of total natural sunlight. UVB light is energetic enough to cause damage to DNA, including inducing cancer in animals. Luckily for us, the Earth's ozone layer blocks almost all of the sun's UVB light.
  • Ultraviolet C (UVC) is from 100-290nm and is almost completely filtered out by Earth's atmosphere, so is not a component of natural sunlight. UVC light is energetic enough that it is used for sterilization purposes-- it kills living cells.

Plants respond to exposure to both UVA and UVB light; different plants respond in different ways, but in general studies have shown increased production of antioxidants / flavonoids and other natural sunscreen compounds in a process called photomorphogenesis.

In Cannabis plants, exposure to UVA vs UVB light increases production of THC and CBD. There is some confusion caused by a 1987 study by John Lydon, Alan Teramura and C. Benjamin Coffman titled "UV-B Radiation Effects on Photosynthesis, Growth and Cannabinoid Production of two Cannabis sativa Chemotypes". The study grew Cannabis plants and exposed some to UVB light and others to no UV light at all, finding increased THC concentrations for the plants exposed to UVB compared to the plants not exposed to UV lights for plants at all.

Some people have interpreted this to mean that only UVB light increases the production of THC in Cannabis plants, but this study does not demonstrate that. First, the study was not designed to test exposing Cannabis plants to UVA light- only UVB or no UV at all, so nothing can be concluded from the study about whether UVA affects THC production in Cannabis. Secondly, the study's setup was flawed and really only tested whether exposing Cannabis plants to both UVB and UVA increased THC production compared to no UV light at all, because the cellulose acetate filter they used in an effort to eliminate UVA light from their broad-spectrum UV source was later shown by a different study to allow UVA light to pass through. So the original study only showed that exposure to both UVA and UVB light increased THC production compared to no UV, but not whether UVA or UVB (or the combination) was responsible.

From our own research grows, Black Dog LED has demonstrated that UVA light alone can increase THC and CBD production in Cannabis plants. The combination of UVA and UVB light (from a standard "reptile bulb" fluorescent light) also increases THC and CBD production, but the inclusion of UVB in the light has noticeable detrimental effects on plant growth compared to only UVA.

This is why we've engineered the Black Dog LED Phyto-Genesis Spectrum™ to only include UVA light, without any UVB wavelengths. The UVA still increases production of secondary metabolites such as THC, CBD, terpenes and flavonoids but without the negative effects of UVB light.

10 thoughts on “Which is better: UVA or UVB?”

  • reza

    hi i have just UVA light.
    can i use them for my cannabis to get high THC?
    and also can you explain to me how many hours they should be on a day in flowering thanks.

    Reply
    • Noah

      Hi Reza,
      Yes, you can use a UVA light to increase the active compounds in Cannabis. However, I would need to know more about the UVA light you have and the primary light you are using for flowering. One of the keys with UV is to balance it with the other spectra to create an efficient growing spectrum. If it isn't too strong you should be able to run UVA for the 12 hours your flowering lights are on. There is also a chance it won't be strong enough to have any effect as well, but again, I cannot say without knowing more about the light.

      Happy gardening,
      Black Dog LED

      Reply
      • Nickolus A Wiebel
        Nickolus A Wiebel July 5, 2018 at 11:06 am

        I realize this post is years old but I'm wondering if a zoo-med reptisun 10.0 HO UVB bulb is either good or bad concerning cannabis grown in a terrarium with a Reptisun hood containing white and red LED and blue LED along with the UVB bulb.

        Reply
        • Amanda Herman

          Unfortunately, we do not know the output or spectrum so it would be impossible for us to say. Please note, using UVB for extended periods of time can be harmful to your plants and self.

          Reply
  • Brandon

    Is there any specific uva lights you guys would recommend on the market?

    Reply
    • Noah

      Yes, Black Dog LED. Other than that, one of the other major grow light manufacturers claims UV and we have tested and shown the have none, so buyer beware.

      Reply
  • Brian

    What is the proper ratio from your experience? I see natural sunlight is comprised of 3% UVA. PS I met you at end of necann Boston 2018, wish we talked more.

    Reply
    • Kevin

      From our experimentation, having about 3.5-4% UVA and 96-96.5% PAR and IR light is about the right ratio for maximizing quality and canopy penetration without overly stressing the plants from too much UVA.

      Reply
  • Rob

    Can i use a uv light for constant light on my plants for veg and flower

    Reply
    • Kevin

      Yes, you can use UV constantly, as long as it is the appropriate amount of UV light. We've found that 3-4% UV photons is about ideal and build that into our lights.

      Reply
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