I just put in a 10K bulb (finisher bulb) as it adds lots of UV. What's your opinion on using the 10K bulb during veg?

By Ian Ashdown | Last updated: January 24, 2022

grow light over a cannabis grow

This is short question with a long answer.

It may be true that a 10K finisher lamp adds “lots of UV,” but it is important to ask what this means. Several academic studies have shown exposure to UV-B radiation promotes the production of THC in cannabis sativa. A 1987 study reported the THC content of floral tissue increased by 30 per cent when the daily UV-B dose over 70 days was increased by 13 times.

More recently, an unpublished study by Seoul Semiconductor demonstrated that five hours of exposure to UV-B in the final three weeks prior to harvest increased THC content by nine per cent (interestingly, total terpene content was increased by 48 per cent).

This, however, is for UV-B radiation, which ranges in wavelength from 280 nm to 315 nm. There do not appear to be any academic studies regarding the effects of UV-A radiation, with wavelengths of 315 nm to 400 nm, on cannabis. This is unfortunate, as most of the commercially-available finisher lamps produce plenty of UV-A radiation but basically no UV-B radiation. There is plenty of anecdotal evidence available on the internet claiming finisher lamps are effective in increasing THC content. However, “lots of UV” comes with considerable risks.

Exposure to UV-A can enhance photosynthesis, but excess UV-A can inhibit it. Depending on the species, UV-A exposure may enhance or inhibit the production of terpenoids — it depends on the species. For cannabis, we simply do not know. Perhaps more important is the risk of eye damage from both UV-B and UV-A radiation exposure — you can literally sunburn your retinae, a painful condition called photokeratitis that is not noticed until several hours after exposure. UV-blocking sunglasses are available and should be worn when working with finisher lamps. As for the question of whether finisher lamps are useful during veg… well, the jury is still out.

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Plant Growth Lighting

Written by Ian Ashdown | Research Engineer, Consultant, Owner of Sun Tracker Technologies

Profile Picture of Ian Ashdown

A research engineer, Ashdown brings almost 40 years experience in lighting design, research and development, as well as software engineering. He owns and runs Sun Tracker Technologies as well as Byheart Consulting.

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