Can You Grow Weed With A Black Light?

Growing weed is an awesome hobby. However, you need patience, hard work and the right equipment to grow a successful plant.

One of the most important bits of equipment for a healthy cannabis plant is the light you use to grow it.

Lighting is essential to your cannabis plant and there are usually 3 different types of grow lights to choose from:  LED, HID, or fluorescent.

However, one question we get asked a lot, is  “Can you grow weed with a black light?”

This article will answer that question once and for all.

Can you grow weed with a black light?

Short answer: The short and simple answer to this question is no, you cannot grow weed with a black light. A black light simply does not have the properties which a weed plant requires to grow big, strong, and healthy. There are various reasons why a black light simply won’t do, so let’s go over the most important factors…

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1. Cannabis plants need a certain spectrum of light to grow properly

During the vegetative state, a weed plant requires a lot of blue spectrum light wavelengths to grow short and fat, thus producing a lot of leaves. When it comes time for flowering, a weed plant requires lots of red spectrum light wavelengths to grow tall and produce buds. Black lights simply don’t have enough of either of these light spectrums to be effective. They do usually emit some red and some blue spectrum light, but just not enough to be very effective.

2. Black lights tend to get hot very quickly

This is a big problem when it comes to growing weed. Cannabis plants require a very specific, regulated temperature to grow well. If the temperature is too hot, the leaves wilt, and mold and humidity become big issue. Black lights heat up the grow area too much, thus making them a bad choice for growing weed.

3. Weed plants, for the most part, like really bright light (when lighting is required)

Simply put, black lights are not very bright. This has to do with their output of lumens. Lumens are a measure of how much visible light any given bulb releases. In other words, this has to do with how bright the light is. Generally speaking, the more light there is and the brighter it shines, the better and faster your weed plants will grow. Black lights simply don’t have a high lumen count and aren’t that bright, which is another reason they just won’t do.

Simulating the ‘Moon Effect’

As you might know,  weed plants in the vegetative state need to have complete darkness for at least 12 hours per night. Some people think that it is a good idea to simulate moonlight with a black light, just like when growing weed outdoors. However, this is a big misconception. Plants in their vegetative state need their darkness.

Moonlight doesn’t affect your plant in any way because it just doesn’t reflect enough of the sun’s light to make a difference.

Even though black lights don’t have the spectrum of light your plant to make them ideal for growing, they do still emit enough light to disrupt the dark cycle your plants need during the night. Using a black light during the night to simulate moonlight is not only useless, but it can actually be quite detrimental too.

Which type of light should you use?

Led grow lights are by far the best option if you’re looking for a simple, effective setup. Although a little bit more expensive to purchase than HID/HPS grow lights,  they’ll save you money in the long run due to their durability and energy efficiency. A good LED grow light will produce the perfect spectrum of light for each stage of your plants growth.


At the end of the day, the big takeaway here is that there are many better options in terms of lights to grow weed with. When it comes to black lights, they simply don’t cut it.

James Alexander James Alexander
Hey, I'm James! The founder and head writer here at I started this site to share my passion and knowledge of all things cannabis. I used to work as a budtender in central Amsterdam, and have over 7 years of growing experience.

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15 thoughts on “Can You Grow Weed With A Black Light?”

      • What if for some reason need to enter grow tent during dark times is there a light I can use that will not damage Growing cycle????

      • Hi Clifford. I would try to do all your tending during light hours, but if that’s not possible then buy a green led headlight, which won’t stress your plants.

    • Your cannabis plant would probably start growing if you used a desk lamp, but you would get awful results. The same is true with a UV grow light. There are far better options out there!

  1. Was looking into this as I heard that UV light can increase potancy.. Can anyone confirm this and is there a low cost effective solution for this.. Using dual spectrum HPS in virtical grow..

    Thanks in advance

    • Hey Paul. Thanks for your comment. Yes, UV has been shown to increase potency. UV rays are damaging to plants, just like they are to our skin. If cannabis plants are exposed to UV wavelengths, then they go into defence mode and start producing more trichomes, which contain THC. However, using backlights in grow grow room is a bad idea. They are too hot and do not offer the right wavelengths or intensity of light you need. If you want to include UV in your grow setup, you are much better off using an LED grow light that includes a small number of UV diodes.

      • Hey James I had someone break in and steal and break all my plants is there a way I can save my plant it’s not dead but it’s not doing good been almost a week but nothing has changed

      • Sorry to hear that Bailey, must have been very traumatic. Are the stems broken? I’ve had good success using duct tape to fix broken stems. Cannabis plants are very hardy so give them some more time to heal.

  2. During veg stage you need 18 hours light 6 dark to simulate summer and 12/12 for bud. I understand the black light isn’t bright enough but all the scientific crap about growing is just that I’ve been using a $15 led workbench light from Walmart and it’s doing awesome.

  3. Very informative article!
    I’m trying to use what I have on hand… would an orange fluorescent bulb be sufficient?

  4. Hi James ,i run my aglex 3000 cob series 648w on the wall , ive added one cob 50w 385nm do you think its good or not ?


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