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You are here: Home / Grow Lights / Tips & Advice

The Effect Of UV Light On Plants (Black Lights For Weed?)

Last updated September 15, 2023 By Steven 34 Comments

how ultraviolet light affects plantsPlants, like humans, are living organisms.

And just like us they require nutrients and the right conditions to flourish. While plants obviously need water toย survive, light is their main source of energy.

The natural light we enjoy here on earth comes from the sun, a blazing mass of fire that produces enough energy to maintain all life forms on this planet.

The light from the sun is composed of packets of energy called photons; it is this energy that plants utilize to make their food supply.

The light from the sun is made up ofย varying wavelengths. Plants use most of this spectrum, some colors far more than others, but they do not make use of ultraviolet and infrared light.

Does this mean that UV light has no effect whatsoever on plants?

Quite the opposite. Varying levels of UV light bring about distinct characteristics in crops. Unfortunately, most of them are negative.

First we will cover the effects of UV light on plants in general and then we will cover the effect of UV on cannabis specifically.

Contents

  • 1 How Does UV Light Affect Plants?
    • 1.1 What Is UV Light?
    • 1.2 Impact of UV Light on Microbes
    • 1.3 Ultraviolet Light Causes DNA Damage
    • 1.4 UV Light Leads to UV-Resistant Crops
    • 1.5 UV Light For Growing Marijuana
  • 2 Do Plants Need UV Light?
  • 3 How To Provide UVA/UVB Lights For Plants
    • 3.1 UVB LED Grow Light: Is It Worth It?
  • 4 How Does Black Light Affect Plant Growth?
    • 4.1 Can Plants Grow Under Black Light?
    • 4.2 Do Black Lights Help Plants Grow?
    • 4.3 Black Light For Growing Weed?
    • 4.4 Do I Need A Black Light In My Grow Room?
    • 4.5 Should I Use A Black Light During Flowering?
    • 4.6 Will Reptile Lights Work For Plants
  • 5 Effect Of UV Light On Plants: Final Thoughts

 

How Does UV Light Affect Plants?

Before we get into the effects of UV light on plants, let’s briefly talk about what exactly is meant by ultraviolet light.

 

What Is UV Light?

what is uv light

Ultraviolet light is invisible to the naked eye and is the shortest wavelength in the spectrum, lying between 100 to 400 nm (nano meters). Before UV light reaches the earthโ€™s surface, most of it is absorbed by the stratosphere.

The earth’s atmosphere is well-adapted to absorb all UV-C radiation, but UV-A and UV-B light still reaches the earth’s surface. Luckily, this light is not too harmful at the levels that reach us.

It is UV-R light that is most damaging to life forms. Thankfully, only 7-9% of it is able to reach the biosphere.

For this reason, under normal conditions, UV light does not have a substantial impact on plant growth. The exact effects of UV light have been evaluated under laboratory conditions, however.

 

Impact of UV Light on Microbes

microbe under ultraviolet light

Microscopic organisms such as bacteria play an important role in a plantโ€™s life, both good and bad. Some bacteria, such as the ones that cause wilt and rust, may induce diseases in plants.ย Others, such as the nitrogen-fixing bacteria, may play a vital role in growth and in repairing damage.

Ultraviolet light is detrimental to these microbes and may result in their death. Many scientists have tried using UV light to kill pathogens.

The problem is always the same:ย while ultraviolet light kills off germs, it also destroys beneficial and symbiotic microbes that play an active role in the healthy growth of a plant.

When UV light kills of these organisms, it causesย changes in the composition of materials that the plant needs to make its food supply. For example, ultraviolet light can cause retardation in plants, if it kills of nitrogen-fixing bacteria, because it reduces the amount of usable nitrogen.

 

Ultraviolet Light Causes DNA Damage

dna damage

It has been widely documented, that UV-R light is highly damaging to life forms, especially their DNA, lipids and proteins.

When DNA is damaged, genetic material retards and either results in mutation or cell apoptosis, where the cell engulfs itself to protect itself from damage. This can cause cancer, among other things and it is a big reason people ask: are LED grow lights bad for your skin?

DNA damage, however, may not always be a negative; mutations in plants are the evolutionary forces that lead to greater diversity and often stronger organisms more suited to surviving.

For example, plants are able to make use of blue light and UV-A to push toward controlled apoptosis. This ensures that nutrients are not wasted and organs that have grown old are eliminated so new organs can be formed. And reptiles need UVA and UVB to survive.

 

UV Light Leads to UV-Resistant Crops

crops under sun

With the world heading toward a possible climatic crisis, many researchers have started worrying about the impact of UV light on crops. Considering how thin the ozone layer has become, it is highly likely that in the near future, more DNA-damaging UV radiation will make it through the atmosphere down to the earth’s surface.

This does not necessarily have to be bad news, though. In controlled laboratory experiments, researchers found that crops that were exposed to more UV light actually started producing molecules to block it.

This means that these crops are able to survive in harsher climates and drier regions. Moreover, the plants then used ultraviolet light to their advantage to curb mold and other diseases that were festering in the soil.

This new research might be critical in the future as global warming raises temperatures, the ozone layer is further depleted and more light penetrates down to the earth’s surface.

And, not only did plants become more resistant to harmful light and microbes in those experiments, they also changed their shapes: they became shorter and thicker, which helps reduce water loss.

While UV light is generally harmful to plants, it can still be used to bring about positive effects. A final such effect comes in the growing of cannabis.

 

UV Light For Growing Marijuana

growing marijuana

Ultraviolet light causes the production of resin, and with it THC and CBD, in order toย protect the marijuana plant from harmful UV rays. Thus, adding UV light to LED grow lights results in an increase in THC in the resulting buds.

There is no question that at its core, UV light is harmful to plants. But in harming plants, it actually causes them to develop protective mechanisms that make them stronger going forward.

With weed, this results in an increase in THC and CBD. For this reason, feeding your marijuana plants low levels of supplemental UV light will actually help them and generally result in better crops, as is the case with cannabis.

 

Do Plants Need UV Light?

No, plants do not need UV light. It actually causes them harm. But in causing harm, it forces plants to protect themselves, which can result in a positive for our needs.

Cannabis is the best example. UV light forces it to create more resin to protect itself, which means higher THC and CBD levels. For that reason, many marijuana growers look to add UV light to the final few weeks of the grow, when it has the most effect on the final product.

But what is the best way to give your plants ultraviolet light?

 

How To Provide UVA/UVB Lights For Plants

fluorescent uv light for plants

A lot of LED grow lightsย have UV diodes these days, but they only have UV-A light. That’s becauseย UV-B LED diodes are incredibly expensive and are only included on very high priced fixtures.

Is it worth it to pay for UV-B? Is UVB good for plants?

There is a prevalent belief that only UV-B light is beneficial to THC and CBD production, but this is based on a poorly run study that did not, in fact, prove this. Black Dog LED did their own research and found that UV-A light also increases production of THC and CBD.

For that reason, any LED grow light that has UV diodes will work just fine to give your plants some UV light.

Nevertheless, I know some people will insist their plants need UV-B light. But using LEDs is not the way to do it.

 

UVB LED Grow Light: Is It Worth It?

Short answer: no.

As mentioned above, UVB diodes are extremely costly.

Fixtures that do include UVB light do so by attaching a UVB fluorescent bulb to their fixture. They are, in essence, an LED grow light with an additional UVB bulb.

And you pay for this addition. There are only two of these lights on the market: the Amare Solar Eclipse 500, which costs $1075 and the California Lightworks SolarSystem 1100 with UVB, which costs $1799.

Personally, I find this a bit gimmicky and it is never worth the increased cost.

Then there is the Cirrus UVB bar. It is the only fixture that uses actual UVB diodes. And it uses only those diodes. It is a pure UVB LED grow light, meaning it functions as supplemental lighting only.

The problem is: it costs $499. For a supplemental light!

Honestly, the benefits from adding UV-B light are not worth paying several hundred (let alone over $1000) dollars.

The only way adding UVB makes financial sense is to get a regular T5 fluorescent fixture and put a T5 fluorescent UVB bulb in it.

These bulbs cost only a little more than a standard fluorescent bulb and they also emit UV-A light, in addition to UV-B.

Here is a good option in two different sizes (these are just the bulbs; you can get any standard T5 fluorescent fixture like this one for them).

AgroMax 2 Foot Pure UV T5 Fluorescent Grow Light Bulb
AgroMax 2 Foot Pure UV T5 Fluorescent Grow Light Bulb
$10.97
View On Amazon
We earn a commission if you make a purchase, at no additional cost to you.
11/12/2024 07:01 pm GMT
Lasso Brag
AgroMax 4 Foot Pure UV T5 Fluorescent Grow Light Bulb
AgroMax 4 Foot Pure UV T5 Fluorescent Grow Light Bulb
$13.97
View On Amazon
We earn a commission if you make a purchase, at no additional cost to you.
11/12/2024 06:57 pm GMT
Lasso Brag
Lasso Brag

 

How Does Black Light Affect Plant Growth?

black light bulb

Many people ask me about black lights and reptile lights. They want to know if those can be used to supply UV light to their plants.

Black lights emit UV-A light only, so they affect plants the same way any other source of UV-A light does (which was covered above). They are a fairly weak source, however.

Below are answers to the most common questions I get.

 

Can Plants Grow Under Black Light?

No, most plants can not grow under a black light, if the black light is the only light source. If other light is present, they can grow under the black light, assuming it is not too strong or close to the plants. The black light itself does not do anything to help growth, though.

 

Do Black Lights Help Plants Grow?

Black lights do not help plants grow. They can help out in the ways described above, as in boosting production of THC and CBD in cannabis, but they do not aid growth at all.

 

Black Light For Growing Weed?

While a black light will, as mentioned, boost THC and CBD production, it will not grow weed on its own. You can use one as supplemental lighting, but your cannabis plant will not grow without an actual grow light or sunlight.

 

Do I Need A Black Light In My Grow Room?

No, you do not need a black light in your grow room. If you are growing marijuana, the addition of UV light can boost THC and CBD production, but it is not necessary for plants. If you do add ultraviolet light, it would be better to use a grow bulb like the AgroMax bulbs linked to above, since those emit both UVA and UVB light.

To boost the production of THC and CBD, you would only add ultraviolet light during a specific part of the grow cycle (see next question).

 

Should I Use A Black Light During Flowering?

If you are adding a black light to your grow for the purposes of boosting the production of CBD and THC, then you’ll want to use that light only during the final few weeks of the flowering stage of growth.

 

Will Reptile Lights Work For Plants

Reptile lights will work in the same way that black lights or other ultraviolet light will work. They will not help the plant grow, but will activate their defense mechanisms. This leads to, for example, the production of trichomes in marijuana.

The main difference between reptile lights and black lights is that most reptile lights emit UV-B light (there are also UVA reptile lights, but reptiles need UVB more), while black lights emit UV-A light.

 

Effect Of UV Light On Plants: Final Thoughts

Hopefully you now have a better idea what effect UV light has on plants and whether it makes sense for you to add ultraviolet light to your grow. And if you decide to do so, you should also have an idea of the best way to do that.

Now it’s your turn. Do you have any additional questions concerning ultraviolet light for plants, or specifically for marijuana? If so, please ask them in the comments below and I will be happy to add them to this article.

 

Sources

  • http://www.ehow.com/facts_5572031_effects-ultraviolet-light-plant-growth.html
  • http://www.uv4plants.org/wp-content/uploads/2014/10/UV-web-opt.pdf
  • https://www.ncbi.nlm.nih.gov/pmc/articles/PMC160223/pdf/041353.pdf

Reader Interactions

Comments

  1. Jeff Wegmann says

    January 28, 2019 at 19:22

    hi there, as far as exposure to the supplemental uvb regiment, is 6 hours on recommended for 32w uv florescent bulb per day in a 12 12 flowering cycle thank you Jeff

    Reply
    • Steven says

      January 29, 2019 at 01:11

      6 hours is fine. The usual recommendation is somewhere between 3 and 6 hours.

      Reply
      • Bobby says

        August 22, 2020 at 21:08

        What size does it need to be for black light usage

        Reply
  2. fotis says

    February 15, 2019 at 09:42

    thanks a lot for the info ! I have a t8 18w blacklight and i have a 2×2 grow space .what is the recomended height for the t8? Also it needs to be facing the plant ? primary light is 150w diy led quantum board with lm561b 20 cm above canopy

    Reply
    • Steven says

      February 16, 2019 at 00:49

      Just hang it high enough that the light covers all of the plants. I’m not sure what you mean by “facing the plant”. If it is in a reflector, then yes, you should direct the light at the plants.

      Reply
      • fotis athanasiou says

        February 18, 2019 at 17:03

        Sorry I meant if it has to be very close to the plants like the normal fluorescent to penetrate or higher up with my other led lights. Thanks for your reply ๐Ÿ™‚

        Reply
        • Steven says

          February 19, 2019 at 01:59

          No, it doesn’t need to be close. Hang it higher up with the LEDs.

          Reply
  3. Stephen Embry says

    July 8, 2019 at 03:21

    Good article, but I’d like to add that SolaCure.com has some UV lights that are vastly more powerful than anything mentioned in this article, and very affordable.

    Reply
    • Steven says

      July 8, 2019 at 14:42

      I’m not familiar with those bulbs, thanks for the tip! Took a quick look at the site and the bulbs may be worth a try for sure. The website is is dire need of an update though.

      Reply
  4. Jeff says

    July 8, 2019 at 06:09

    What about a bug zapper in the tent? Would that be a strong enough uv source? Would it not also aid in pest control?

    Reply
    • Steven says

      July 8, 2019 at 14:44

      Good question. I’d say probably not, but to be honest, I have no idea how strong those things are and I also don’t know what wavelength of light they put out, so I can’t really give a good answer.

      Reply
      • Anon says

        August 25, 2019 at 08:55

        Just another form of blacklight, but with coated glass so some white light is emitted too. The lights gonna the right UV, it’s just a matter of quantity.

        Reply
        • Donal says

          March 10, 2022 at 03:32

          Should UV light be used during lights on or lights off cycle?

          Reply
  5. Rolland says

    September 13, 2019 at 20:03

    I’ve been trying to eradicate some Powdery Mildew using a UVC light,,. A couple Questions I have are ..How long should I leave the UVC on ? How far from the canopy? Lastly will a UVC light have similar effects on the plant as UVA & UVB LIGHT?

    Reply
    • Steven says

      September 14, 2019 at 00:36

      I don’t know too much about this, but I do know that UVC light is damaging to the plants. The time needed and the best distance from the plants varies greatly, so I can’t really help much there.

      Here is a pretty good article on the subject of using UVC light to filght mildew: https://auvco.blogspot.com/2018/10/can-uvc-combat-powdery-mildew-pm-in.html

      Reply
      • Ollie says

        November 10, 2021 at 00:19

        I am using full spectrum LED and hang my UV bars 10cm below them. Raise your LEDs to the point where you can only just feel the warmth when your hand is just above canopy. Then hang your UV bars 10cm lower than that. And only for the last 2 to maybe 3 weeks of flowering. Massive difference

        Reply
  6. Na says

    October 1, 2019 at 22:06

    I found an Exo terra repti-glo 10.0 compact fluorescent dessert terrarium lamp bulb in amazon for $5.33. May need a couple, they are 26w, but at that price, np. So you think a mixture of these amd normal led uva would be good? I tend to do a staged harvest so I would point them up at the top colas as I harvest going lower so as not to damage the plant below.

    Reply
    • Steven says

      October 2, 2019 at 01:08

      Yeah, I think those would work just fine.

      Reply
    • Merlyn Abbott says

      April 15, 2021 at 12:18

      Do you have a link to the bulbs for $5.33 at amazon??thank you..

      Reply
    • Smartch says

      September 18, 2021 at 11:55

      How did it go? Was it enough?

      Reply
  7. Pete says

    January 13, 2020 at 18:13

    The new Arcadia D3+ T5 12% UVB 30% UVA available in 3 sizes: 24W 22″, 39W 34″, 54W 46″. UVA and UVB are both good for cannabis UVC kills everything including your plants so don’t use it. The new T5 bulbs are very efficient and are not too powerful so can be used for a couple of three hour bursts.
    Exo Terra 10% UVB and 33% UVA. These are T8s but come in six sizes starting at 14W 15″ so are ideal for small spaces. Likewise a couple of three hour bursts.
    Agromax pure UV deliver UVA 25% and UVB 75% but are very powerful and should only be used for 15 minute bursts in the last few weeks and keep them high. Starting with one burst increasing to four bursts over the first week then possibly six bursts in the final days. The plants are as good as dead by then growthwise, everything is now going into seed production. Be careful as these can ruine your buds. These are very hard to get hold of in Europe, got mine from the UAE know one will export them from the US. They cost a lot and you can get hammered by customs adding 20% VAT + ยฃ8 handing charge in the UK.

    Reply
    • Stephen Bihn says

      October 14, 2020 at 00:07

      Just be careful to be far away and have sun glasses uv-b it will burn your eyes I was snow blind for a couple days and sun burn!

      Reply
  8. Chris says

    May 30, 2020 at 06:44

    I just purchased a uvb light by California light. My understanding is that it should be applied at short burst 15 min several times a day during it’s flowering phase. So I guess this is a 2 part question.
    1- how far away should the light be from the plants.
    2- do they make a timer that I can set to go off and on multiple times a day

    Reply
    • Steven says

      May 31, 2020 at 00:48

      California Lightworks is a US company. The big advantage of paying more for US brands is that they have good customer service that you can reach on the phone. Give them a call and ask these questions. They know a lot more about their lights than I do.

      Reply
    • Pete says

      May 31, 2020 at 01:30

      Depends on the coverage area the greater the area you want to cover the higher up it should be and the longer the light periods should be. Another thing to consider is the reflector shape you are using as they can effect the spread of light. Always use polished metal reflectors with no lacquer on as anything else will filter the UV out.
      Check out Shane’s Migro video on the Agromax UV tested
      Times yes there are plenty I recommend a mechanical one as I’ve had two digital ones that have not switched off when set to. This was ok for normal lighting as long as you notice but if this happens with UV you will have to bin the lot as they will be sunburnt to death.
      Good luck

      Reply
  9. Vernon Cyphert says

    September 5, 2020 at 19:42

    Black lights at night or with day in conjunction with other

    Reply
    • Steven says

      September 6, 2020 at 01:58

      For a few hours during the day.

      Reply
  10. sean says

    January 14, 2021 at 13:22

    I’ve just stuck x2 agromax t5’s in my little 4×4 grow (2 plants) in the last 2 weeks of flower but the 2 hours i’ve been giving them i think is too much reading the above and noting the odd curling yellow leaft etc. I’ll put them on 15 mins I think (as there is 2 in there), i’m running with 2000wat spiderfarmer. Started too strong probabaly but the girls are ok I think.

    Reply
    • Steven says

      January 15, 2021 at 02:43

      Yes, if you start to see ill effects, definitely cut back.

      Reply
  11. Freeman says

    February 9, 2021 at 02:42

    Very good site. I have learnt a lot. Thank you for doing something to help the home growers cause. You are a green hero; cape or no cape.

    Reply
  12. DrDave says

    July 27, 2021 at 11:06

    The author states that infrared light does not contribute anything to photosynthesis, but he’s dead wrong. Read and watch Dr. Bruce Bugbee, Professor of Crop Physiology at the Univeraity of Utah cite the importance of far red light on photosynthesis.

    Get u some far red diodes!

    Reply
    • Michael Gallagher says

      October 29, 2021 at 14:22

      My Kind LED fixture has Ultraviolet as well as Infrared and better plants in shorter period than HPS or MH

      Reply
  13. Brady says

    August 19, 2021 at 11:52

    Just added 2 of the SolaCure 2′ T12 UV bulbs to my 2 Cultilux 700w full spectrum LEDs in flower room one and left flower room 2 as is with just one 700w Cultilux LED and see a noticeable difference in the trichome production between the two rooms. I am running the UV bulbs about 30″ above the canopy 4 hours a day. This too me is the ultimate combo. Product quality is as good as CMH with 2/3 the heat and half the energy consumption. Also studies have shown that UVB will decrease CBD in cannabis plants but increase THC. Thanks for the write up. Got some more needed info from the read.

    Reply
  14. DonJulioGrows says

    November 26, 2021 at 18:34

    Great Thread.

    Iโ€™ve been using a Growers Choice 760-Roi with master controller. Recently added the UV-R bloom boost accessory, bust wasnโ€™t sure the best time of day to have the UV-R lights on.

    I see the question was already answered. After searching multiple threads I am pleased to have finally found the answer.

    Thanks for your services!!

    Reply

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