Supplemental UV light questions

What are peoples thoughts on supplemental UV light added during late flower when using led lights? I have read mixed information regarding benefits- some say uvb has almost no benefit and you want uva, but i know high intensity UV can cause some undesired effects on plants.

From what i have gathered, it can boost trichome production but seems best used only in the last couple of weeks.

Does anyone add additional UV in small amounts throughout the entirety of flower? Just wondering if it is actually worth it and what kind of differences folks have observed when rerunning genetics that they are familiar with.

I run UVa/b through my entire flower cycle, including pre flower.

UV is 100% a stressful input.

However, used correctly UV has so many benefits its insane. Microbial, and pest control are some really nice side effects of adding supplemental UV.

in my experience running a high UV garden takes some adjustments in style, specifically around pruning and defoliation. This is sound logic in my mind, and proven through experience, however I have little scientific proof to validate my claim.

With higher quality, higher penetrating light, with supplemental out of spectrum radiation(UV/FR) you want a denser canopy of old growth foliage with the highest amount of photoreceptors. This gives the plant the ability to “sense/see” it’s environment, which in turn allows for faster metabolic activity through out the cryptochrome and phytochrome systems, as well as other metabolic pathways. There are also some feed and water advantages to having a greater amount of foliage that play into the metabolism of the plant in a system we really can’t control. In essence giving the plant more control over itself.

To give you and idea of my UV light schedule, I start around 2 hours of UV light at “mid day” day 1 of flower. They are still immature plants. My UV period will grow into all 12hrs for the last 3 weeks of flower, dependant on genetic sensitivity.


I had read before you should not use moe than 3 hours of uv per day cycle. I have no direct experience with UV and Im truly asking for your experience.

I have been wanting to test UV as underlighting for the lower canopy once I get everything settled with the build. I have ready the many benefits of UV lighting and under canopy lighting.

Add a single supplemental bar to your garden and experiment. I litterally just told you everything I do basically above. Nothing is set in stone because each strain and chemotype responds differently. You are going to regret you didn’t get enough bars for your entire space :wink: but it’s a great way to test it out


We all had UV blasters as kids and had no idea

Burn the retinas out of the mites like we did as kids!


Well, im going to try some supplemental UVA at 2 hr/day to be extra cautious and do the full 12hr in my last 2 weeks. Ill report back on the topic in a couple months when i get results. Running the same cut as my last run, so i should be able to compare them easily.

I am on my 4th cycle using the GC ROI 720’s with UV

I was told not to use before the 4th week after the flip and to introduce it starting with 1hr and increase an additional hour every 4 days until reaching no more than 6 hours total.

Because I was running a new strain, whether it helped or not I do not know, but there outcome was good

Next cycle with a strain I have run for several years, I decided to not listen. Big mistake, I added the UV the first week after the flip and increased until by the 4th week I was running the UV’s 12 hours a day… stressed the phuck out of the plants. That is what I get for not listening to sage advice

Third cycle with a tried and true strain that already produced heavy yielding frosty buds was all it took to make be a believer. I didn’t start the UV’s until the fourth week and increased to a total of 5 hours and that seemed to be spot on. The yield increased by about 25% and the trichome production to the already frosty buds was off the hook.

Getting ready to switch out another area to the GC’s


Are you saying with UV supplementation you defoliate less?

Yes. Less defoliation, less targeting older growth. Lollipop early flower. If you defoliate like normal, your going to fry your plants. The actions a plant takes, are triggered or revolve around light. All of them. The plant needs to be able to sense this light, and sense it everywhere.


*Other lights off just to show coverage.

Well the coverage could be better, I opted to just start with a couple pucks of UVA. Should still be able to gauge its impact. Starting with 1 hour at first day of week 4 since flip. Lets see how this goes.


I know a lot of people suggest that you don’t run your UV untill the end of flower… Im of the opposite opinion.

End of flower, very little of the work the plant is doing is going into producing new resin. The flower is simply bulking, which it would do to produce food for its offspring, and the resin you do have is starting to cure and degrade, and with the addition of high intensity UV light, more than likely at a faster rate.

We can detect THC-A and THC relatively easily with UV light because the chemicals fluoresce the light into the 400+ nm range. The plant does this when it’s alive to protect itself from the harmful ultra violet rays. This is the response that will give you the resin production you are looking for… However, the plant must “feel/see” the ultra violet radiation at an appropriate time in its life cycle and it’s metabolic rate must be sustained as it is a heavy process for the plant.

This phenomenon is also why UV garden tend to finish earlier than garden without supplemental lighting

Providing the ultra violet light at the end of the flower cycle is NOT the best time. I would even argue that the last 2 weeks of your flower cycle the UV light could be turned off(I appreciate the antimicrobial effects in late flower protecting my fruit)


Liking the results so far, still have a month to go and they look more frosty than usual


1-2 weeks left. Cant wait to extract these.


Amazing photos!

Beautifully sugared.

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I know I have said this countless times here and elsewhere. I feel like a broken record and like I’m trying to plug a hole in a dam with my thumb, LOL.

In fact, this topic, and the so-called entourage effect, are two my biggest pet peeves as far as unproven claims that everyone assumes are fact go in the cannabis world.

A quick search of my username and UV-B will lead to many of my posts on this topic.

Show me well designed, controlled, peer-reviewed studies using the scientific method that found UV-B provides a statistical increase in total THC. Not including the 40-ish year old Lydon study which was so flawed it’s a waste of paper. IIRC, I only know of a single PhD thesis that found a minor benefit from UV-B in increasing THC but not THC-A.

Also, please explain what would be the photobiological chain of events that causes UV-B irradiation to increase THC. (It’s not ‘sunburn’ protection or a stress response like many people like to suggest.)

I can show at least three recent studies finding no statistical increase in total THC from UV-B irradiation. Granted, two of them have a few issues (like PPFD and UV-A that’s too low relative to UV-B). And one of those studies found an increase of THC by a minor amount (no effect on THC-A), meaning the net effect on total THC was insignificant.

I implore anyone reading to think critically about this topic and then do a research review to see what has been published on this topic. The band wagon says UV-B increases total THC based on unscientific one-off grows in someone’s home without controls, statistical analyses, or any sort of research grade plant growth chamber. Without irradiance monitoring and control (accurately and precisely quantifying UV-B requires a spectroradiometer), precision environmental control, >95% min/avg UV-B uniformity over the all test plants, and repetition.



You simply will not get the result you’re looking for from UV-A (and I highly doubt from UV-B, either, but I’m hoping to be proven wrong about UV-B someday.)

Well i am just here reporting my own personal results when growing the same clones under all of the same conditions with the exception of the addition of UV-a.

Humidity, canopy average ppfd, water, feeding, etc have been kept the same.

It isnt perfect, but from what i have seen i am happier with these results.


This tend to be EVERYONE’S real world reaction, again though just in my experience. also in my own totally non scientific way, think that you really need to increase the red side if you are increasing the “blue” side. Aka FR.

Also. @Ralf With UV it seems that the morphology changes to a point where you are getting a smaller biomass and “tighter” morphology. Which may be why some of us are experiencing a higher total TAC/g, and why the flowers APPEAR to have “more” resin, and I’m really unsure if your serious about the plant and UV but the metabolic pathway would be/start at cryptochrome.


That’s great. What you’re doing is the basis of science. Hypothesize, test, report. It’s awesome you and others like my buddy @vortal test and share what you see. :+1: :metal:

I’m sorry if what I wrote seemed to target you or anyone in this thread. That wasn’t my intention. I promise. I value everyone’s input and I’m not trying to say people are doing things incorrectly.

But it’s really important to differentiate between what you’re doing and using the scientific method. That’s not to say you’re doing anything wrong, only that we can’t give the same weight to what you’re doing as we do a well designed and controlled study. Nor could we form a theory based on uncontrolled studies. But again, there is great value in what you’re doing - it’s often how scientists are able to create hypotheses (from people in the field reporting things they see).

I’ve been growing and studying photobiological effects for decades, and for nearly that same period I’ve been fighting against the blanket claim that UV-B increases total THC based on the Lydon study from 1987.

Regarding UV-A, I’m not surprised you’re seeing improved plant health and quality. Nor that you’re happy with the results so far. UV-A is very important for plants and far too many growers don’t use enough.

@Vortal is correct that UV-A is important for many reasons, but sadly, increased THC is just not one of them (when we’re talking about effect of the UV-A spectrum on THC production vs. adding more photosynthetic photons that increase photosynthesis, stomatal aperture diameter, and other morphological and photochemical responses).

If you’re only goal for adding UV is to increase THC (not from increased photosynthesis, but from the affect of UV spectra), then UV-A isn’t going to help.


That’s very true. Excellent point.

UV-B reduces plant growth and leaf size, along with reducing photosynthetic rate, biomass yield, and flower size. Which tends to artificially increase the leaf and bract trichrome density (trichs per mm2), because reduced biomass without a change in trichrome quantity means increased trichrome density. Plus, there may be an increase in trichrome quantity from UV-B, which would further increase trichrome density, and therefore TAC/g. But that’s not the same thing as increasing THC within individual trichormes (i.e., THC biosynthesis).