Can You Use Black Lights to Grow Weed?

Growing cannabis is a fun and great learning experience. When preparing to grow the “green gold” known as marijuana, many supplies are needed to gain a fruitful harvest. If you’re new to the growing game, or just plain curious, you might wonder if you can use those old black lights from Halloween to grow your cannabis instead of spending cash on new LED grow lights.

You can not use black lights to grow weed due to the insufficient wavelengths in the light spectrum emitted by black lights. It’s also unlikely the black light would be intense enough even if it had the correct spectrum makeup.

Luckily, you can read my home page where I’ve listed the best grow lights available to grow weed. There are grow lights listed in every price range.

Important supplies such as pots, soil and lights are essential items when growing weed. Many people believe they can grow cannabis by using a black light, however there are many reasons why it isn’t a great choice.

Below I have listed 3 of the major reasons about why black lights aren’t a great option for growing marijuana.

Cannabis Plants Need Proper Light Spectrum

can you use black lights to grow weed
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One of the main reasons black lights aren’t a great choice for growing weed is because they lack the correct light spectrum needed for growing cannabis.

Cannabis plants grow based off two different grow light spectrum: Blue and red (purple), or white (combination of blue, red, yellow, orange and green).

During the early phases of a plant’s life, an extended period of blue spectrum light is needed for the plant to grow.

The blue spectrum of a light encourages the plant to go through a procedure known as photosynthesis. Photosynthesis is a process in which plants turn full spectrum LED light (or natural sunlight) into nutrients to grow.

Grow lights attempt to mimic the sun with different colored wavelengths. The blue spectrum enables the plant to grow out and produce lots of leaves, both 3 tips and 5 tips leaves.

When cannabis plants transfer into their flowering phase, they need less blue spectrum light and more red spectrum. The red spectrum light promotes large buds.

While black lights emit some spectrum of blue and red light, it’s simply not enough to make the plants grow. Cannabis plants respond to brighter lights that have a photosynthetic photon flux density (PPFD). PPFD is the measure of light intensity at the surface of your plant’s leaves.

Black Lights Get Hot Fast

Weed plants don’t like climates too hot or cold, they prefer to grow in regulated temperatures. If the temperatures are too hot, the plants will mold, wilt and burn.

An imperfect temperature can lead to stunted growth or a failed crop. Not only do black lights create a lot of heat, but the intensity of light is nowhere near strong enough for growing marijuana.

What Type of Lights to Use?

Free Grow Guide to Download

The Spider Farmer SF-1000 one of the best value lights right now, and it’ll grow you one plant in a 2×2 foot space. If you have a bigger grow space you can check out the SF-4000 which covers 4×4 feet.

While black lights aren’t suitable for growing weed, LED grow lights are designed exactly for that purpose. I wrote about the best LED grow lights on my home page. LED, COB LED, and Quantum Board grow lights work the best when growing weed indoors.

Many people still use high pressure sodium or metal halide grow lights, but compared to the various LED models they’re not nearly as efficient.

The right LED lights will produce a high intensity which makes them the perfect fit for growing cannabis plants.

Are You Confusing Black Lights For Ultraviolet Light?

Black lights and ultraviolet lights are easily confused. They emit a similar color but that doesn’t mean the light is the same spectrum. Ultraviolet spectrum actually has some use for growing marijuana plants.

Combined with one of the above-mentioned lights, a higher range of THC and CBD are present when using black lights. Ultraviolet lights alone can be very harmful to a cannabis plant when used improperly. However, they help to pack on frosty trichomes during the last weeks of flowering when used with other lights.

Many LED grow lights contain ultraviolet light wavelengths mixed into the light they emit. Check out the best LED grow lights on my home page to find one that’ll work well in your garden.




My name is Nick, and I'm a home grower who spends time writing about indoor growing tools and setups. My goal is for 420ExpertGuide to be a one stop shop for indoor growers to find the information they need.