UV Light for Cannabis: Can UVA and UVB Really Boost THC?

UV light can nudge cannabis plants into making more trichomes, terpenes and possibly more THC, because the plant builds that resin partly as a natural sunscreen. UVA is the gentler, safer band for indoor grows, while UVB is stronger but can damage the plant if you overdo it. For years, terpene and THC production was chalked up mostly to genetics and breeding, but as legalization has made cannabis more available, growers have picked up more sophisticated tools that simply weren't on the table before.

Breeding techniques were once seen as the main way to raise THC and terpene content, but now they're recognized as just one of several ways to boost potency. One of the best known is adding ultraviolet radiation, or UV light, to the grow.

What is ultraviolet (UV) light?

Ultraviolet (UV) radiation, or UV light, is a form of non-ionizing radiation given off by the sun and by artificial sources. It has some upsides for people, like vitamin D production, but it also comes with health risks.

It's a band of light that sits just beyond the violet end of the spectrum, past what the human eye can see. Plenty of life on Earth relies on UV light, mainly for making vitamin D. Plants, which are what we care about here, don't really run photosynthesis on UV (that's mostly visible light's job); for them, UV works more like a signal that switches on protective responses.

Not all UV light is good, though. In fact, not all of the UV the sun gives off even makes it through Earth's atmosphere. UV light is split into three subcategories by wavelength: UVA (320 to 400 nm), UVB (290 to 320 nm) and UVC (100 to 290 nm).

What does UV light do for cannabis plants?

Today, environment, nutrition and genetic modification are understood as the levers for manipulating terpene and THC yields. Environmental factors like temperature swings can also shift THC and terpene production, but one environmental factor modern grows often ignore is how much UV light the plants get.

When plants are exposed to UV light, they make their own natural sunscreen compounds. It's a lot like how people produce melanin and tan in the sun. This response to specific wavelengths of light is called photomorphogenesis. Photomorphogenic responses to UV exposure produce a range of natural compounds as an evolutionary defense. In cannabis, research suggests these compounds may include THC, CBD and terpenes.

Cannabis plants have their own way of blocking harmful UVB rays: ramping up trichome and cannabinoid production (CBD, THC and many others) along with more terpenes. These natural sunscreens are triggered by both UVA and UVB, although only UVB damages plant DNA. UVA rays are the safer option and less likely to harm your plants while still triggering this photomorphogenic response.

How do you add UV light to an indoor grow?

UV light for indoor cannabis plants (mainly UVA) can boost terpenes, antioxidants, flavonoids, THC, CBD and vitamins without seriously harming the plants, and it carries less risk for the people working around them than UVB. That makes it the most useful part of the spectrum to add to an indoor grow.

Most of the artificial light sources commonly used to grow cannabis (HPS and LED grow lights) put out next to no UV, so indoor growers miss out on the extra trichome, cannabinoid and flavonoid production UV exposure can provide. Even greenhouse growers are leaving that potency boost on the table, because greenhouse plastic is made to block the sun's natural UV so the plastic itself doesn't break down.

So what's the fix?

You can supplement your existing lights with UV lamps, though nailing the right ratio of UV to visible light can be tricky, and standalone UV bulbs also spread UV unevenly across the canopy. That's why the ideal setup is a grow light with the right dose of UVA already built into every fixture.

And as cannabis keeps moving toward legalization on the back of wider social acceptance, expect even more innovative techniques and tech for pushing THC and CBD yields higher than ever.

Can you grow cannabis under a UV black light?

It's not a good idea for any cannabis grow, because a black light isn't strong enough and doesn't put out the right wavelengths for the job. Even if it were extremely powerful, it would probably only have a minimal effect.

When should you give your plants UV light?

During the seedling stage, it's best to switch your DIY UV light off completely. Natural light levels can be bright and welcome, but indoor lights can be very intense and seedlings are fragile, so what matters most is giving them a good, healthy start.

The vegetative stage is when you can really put this part of the spectrum to work. If your plants are outdoors, you'll notice they stay in veg all summer, when the sun is at its strongest. Give them a good dose of UV and you're telling them they don't need to stretch to compete, so they can spend their energy growing bushy and dense. Once you start, you'll see them put out more leaves, which means more photosynthesis and more energy.

By the time flowering comes around, we're usually in a pretty changeable time of year: fall. The sun is barely the star of the show anymore. That's why our take is to dial back or switch off UVB for this stage, so the plants can focus entirely on building buds.

Have you read our latest articles?