UVA and UVB, best practices

I’ve been trying to add UVA and UVB in my grows with limited success. I keep over doing it.

My first grow adding a HLG UVA to my HLG600 totally fried my plants. Previously my practice was to lower the light and intensity to obtain the desired DLI, but doing that with UVA on 12 hours a day totally fried the pistils and was probably the worst smoke I ever grew. The UVA was obviously too close.

I’ve read most the threads on UVA and UVB, but it takes lots of work trying to put together the best practices. I’d like to put it all in one place so if you use UV light I’d like you to answer the following questions best you can so that we can all learn from your best practices.

The first question that needs answering is how close should the UVA light be above the canopy?

Second, what is the coverage area of UVA fixtures. Does one fixture cover a 4x4, 5x5 area or something else.

When introducing UVA to your grow how do you do it? 12/12 when you flip to flower? Gradually over a number of days/weeks? Do you use it 12 hours a day or something else?

Now as to UVB. I know the general rule of thumb is 3 hours a day for last 3-4 weeks of flower, but all the other questions remain.

How close should a UVB fixture be above the canopy?

What is the coverage area of a 2-foot UVB T-5 fixture?

When introducing UVB, do you slowly build up to three hours a night? When do you begin to introduce it? Do you do something else like using it longer or shorter hours, or putting it on rotating schedule? Anything else?

When you use UVA and UVB on the same grow do you use them at the same time? Do you change up the light schedule in anyway to accommodate for use of both UVA and UVB? Any other suggestions using UVA and UVB together?

Thanks for your contributions.

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Ive used UVA and UVB. Not successfully lol. That grow hermied but the plants came out alright. You can use UVA from the beginning in veg when they sprout all the way to last 3 weeks of flower. At that time i switch to UVB. When i start my UVA on 12/12 i just try to maintain a 2ft distance and right to 12/12 no easing into it. UVB i was the same way. I didnt ramp it up over a couple days just full blast off the back and i didnt experience any light burn. Most of what i learned about UVA and UVB was from hellraiser and @dbrn32. You might find looking into hellraisers thread called hellraiser grows zkittlez and banana kush in coco helpful

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Im not sure you’d want to use UVA and UVB at the same time? Id wait for others to chime in tho


Ive ran the hlg uva30-xl with no ill affects. Hlg recommends 18" and 1 is good for a 5x5, some people run two in a 4x4. Only key point is it is recommended to use for 12 hours a day only whether your plants are in flower OR VEG. As far as uvb i have not used. Another thing i wanna add is that by adding uva, it is not magically going to make avg weed to A+ weed. It is a minimal improvement of approximately 1% thc more. With slightly more frost. Hopefully something in this helps.


@BigCat420 you had quoted my setup above…the copy paste thingy us covering my name.

I still use the UVA and UVB.

I use the hlg 30 UVA in my veg and flower tents. Both are 4x4 tents.

The UVA in the veg I run across and underneath a hlg 300L Bspec… In the middle of the 300L there’s no LEDs and much less heat.
12hr/day at the level of my main lights… Slightly lower as it’s underneath the 300L by about 1/2 inch or less. I start after veg stage starts not during germination or seedling stages.

In the flower tent the uva30 is attached to my hlg 600 Rspec. 12 hours per day through the grow… Like nature.

The UVB is 2 x 12" T5 uvb florescent… Wish they had leds… Set at the same height as my main lights… One on either side of the 600Rspec.
For 2-3 hours per day in the middle of the 12hr Photoperiod tapering it on about a week after the light flip to 12/12 starting at an hour… Then tapering it down over the last few weeks shutting it off when things look close.

That last part is mainly due to checking the buds more often for ripeness…and forgetting to turn off the uvb before opening up the tent. Gladly I always use uvb glasses in the tents just in case… Plus long sleeves.

Uvb is bad stuff… Be careful.

Another thing I have noticed is since I don’t scrog mine… Yet… I keep saying one day… I veg for 8 weeks using LST sometimes a bit rougher… To get an even canopy and deep coverage… And lollipop prune underneath.

Anway…if I rotate my plants throughout the week as I feed (twice a week to 20% runoff) and during a smaller watering (Recharge and Tribus Original) in the middle. Rotating the plants position in the tent not the plants themselves… I pick a front for each plant. I get the best results.

I usually have 4 plants max but have tried a fifth smaller plant… A runt… Which i left in the center… All I can say is uva and b can mess up the buds. If I find a pic if one I’ll post it.

I use the uvb at this point, more for the fact that we cannot escape it in nature… So plants benefit to some extent.

Plus I do hope the uvb will help keep pests down if not maybe it’ll super power them and a good battle will ensue one day at plant check time.


Seems like @Fiefie has you covered. Sorry for covering your name i was just taking screenshots of info i found useful a while back and had no clue who posted it all this time lol! Good info posted above hopefully it all works out for you

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I’ve never used UVA, but I have 3 of the California Lightworks UVB fixtures. I add them for the last 2 to 3 weeks of flower. I start at 15 minutes per day in the middle of the lights on period, and increase by 15 minutes every other day or so until I reach 2 hours. California Lightworks says you can run them longer, but I’ve never gone past 2 hours because I’ve seen some minor leaf crisping going on.
Just make sure you follow safety warnings, the UVB is very harmful to our eyes and skin. I keep a 15watt light bulb plugged into the 2nd outlet on my UVB timers to make it obvious to me when they’re on so I don’t accidentally open the tents or closet without thinking about it.


Like @Poseidon1 I am also using the HLG 30 XL but mounted to the bottom center on an HLG Scorpion Diablo. This is my first run with UVA and so far things are looking good. No bud burn and I added for the last 3 weeks of plants grow cycle. I ran for 18 hours around 18” from the tops of the main colas.


this might help


I only use UVA. I use it the last 3 weeks of flower. The first week I use it 1 hour a day. 2nd week 2 hours a day and the last week before the chop, 4 hours per day.
Here is a pic of a GFOG I recently harvested using this method. My height is about 24" from top of canopy.

Last pic is about 3 months after harvest. Been in a grove bag for 3 months at 61%.


Adding uva, which I do, requires lower d temps. With white leds like hlg has you can be up to 80 and have no ill effects due to the leaf temp being same as room( leaf temp can be up to 85). But with uv(a or b) you up the temp on the leaf by 10-15 degrees. This is why bulb users keep temps down alps. I keep my room under 75. Closer to 70. I have 9 uva hlg in an 8x8. I keep them 4-5 inches closer than the boards. They will turn pistils red/brown if they are touching but not a few inches away
Are you running CO2? Here’s 1 plant and 1 bud.
Uva def makes a difference. UVB does damage, so I don’t use it.

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I use uva after week 1 on with the lights. So 12 hours a day.

I run a kind 330w with seperate uva ir far red switch. I turn it on 25 percent first week. 50 2nd. 75 3rd. 100 last weeks. So far no bad effects either. I keep 6 to 10 in hes from top of plant


What do you think about the spider farms sf1000d for a 2x2 tent

My light is said to be UV, however, the actual specs state: 430nm

So I’m curious to know what everyone thinks, is this UVA or is it just the low end of Blue?

The reason I ask is to know if it would be ok to use for the entire grow or just time it for the last 3 weeks ?

What light did you buy? Let’s see the specs, although I 430nm is generally not considered uv.

The company that made them went a different direction and stopped building them so I got it on sale when they cleared their Wearhouse of old inventory

cirrus Reflex UVB Bar LED
led driver LDP-105r062
Spectrum Blue 430nm
Type Constant power design, in constant current mode
Input Voltage (Vac) 90 ~ 305
Output Power (W) 105
Min Output Voltage (Vdc) 20
Typical Output Voltage (Vdc) -
Max Output Voltage (Vdc) 62
Output Current (A) 0.30~3.00
Full-Power Current Range (A) 1.69~3.00
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Deleted coment

I wrote a response earlier that I deleted because I assumed your light could not be UVB because as far as I knew, all UVB lights used florescent fixtures. Then I looked up the light on Amazon where the manufacturer claims to have the only LED UVB light on the market. Being the first I guess they felt they could charge for it, but the Cirrus light company, as far as I can tell, is known for making LED advertising signs. I could find no additional information about Cirrus grow lights or your UVA/UVB all-in-one light. I was looking for their recommendation for using the light.

Based on input from forum members and manufacturers information, I use my UVA LED light through most the flowering stage up until I switch to UVB lighting in the last three weeks of flower. I start conservative and build up to 8 hours a day for UVA. I start out at 1 hour/day when switching to UVB as soon as the pistils start turning brown and receding which is usually three weeks from finishing. I’m careful to keep the UV lights at least 18 inches above the plants.

I’m not sure how a dual uva/uvb light would best be used. I suggest contacting manufacturer and asking.

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