The Truth About UV Light and Your Plants

After a recent conversation with a customer about the risk of UV lighting in the grow room, we became intrigued about the full spectrum of UV light. Do plants need UV light to thrive? Is exposure to UV lights harmful? And crucially for us at Happy Hydro, can you protect your eyes from UV rays in the grow room?

What we quickly discovered in our research was that the available information sprawling across the internet is that many sources aren’t as clear-cut as we had hoped.

It took us a while to get the information we needed, from a variety of sources. Its why we’ve put together the following guide, the truth about UV lights for you and your plants.  

What is UV Light?

Ultraviolet light, or UV light, is part of the rainbow, just not the visible part. We measure light by its wavelength, a measurement known as a  nanometer (nm). The visible spectrum of light falls between 400 nm to 700 nm. Just outside the violet end of the spectrum begins the ultraviolet category, with a wavelength from 10 nm to 400 nm. While the human eye can’t pick up these waves lengths, but many other creatures, including plants, use it to survive.

When it comes to cannabis, harnessing some UV light may even improve your harvest.

There are three primary types of UV light, including UV-A, UV-B, and UV-C:


The upper end of the UV spectrum, between 400 nm to 315 nm, is also the most prevalent on earth. Of all the UV light which reaches us from the sun, 98.7 percent of it is UV-A.

Is UV-A good for plants?

Most of the evidence suggests that UV-A exposure isn't tied to DNA damage in plants. Not a surprising fact considering plants are exposed to high levels of low-intensity UV-A in their natural environment. Generally speaking, the UV-A included in most grow room lights won't cause any damage to your harvest, and at least in the case of cannabis, might improve THC and CBD content if introduced correctly.

(If you are just starting out with grow room lights, this is a great introduction to indoor grow lights)

Is UV-A harmful to humans?

In short diffused doses, there is nothing wrong with a little UV-A light. However, over time or if exposed to intense bursts of UV-A light, it may trigger the development of skin cancer. In particular, researchers link UV-A to melanoma, the deadliest type of skin cancer. Unlike other forms of UV light, UV-A isn’t filtered-out by glass windows. Which is why you have faded curtains and aging artwork. It is also the spectrum behind your wrinkles and age spots.


The second most common UV wavelength to reach the earth from the sun is UV-B. Its electromagnetic wavelengths range from 314 to 280 nm. It’s interesting because it triggers oxygen creation and the regeneration of the ozone layer. It makes up the remaining 1.3 percent of UV light reaching the earth’s surface.

Is UV-B good for plants?

In the right dose, introduction a little bit of UV-B light into your indoor grow room can be a good thing. Ultraviolet light of the B variety encourages plants to produce their own natural sunscreen. These sunscreens vary from one species to another but include trichome, terpene, and color development. The UV-B spectrum also offers a bit of natural protection against fungal infection and unwanted pests.

Is UV-B harmful to humans?

Humans have a love-hate relationship with this middle spectrum of UV light. On the one hand it's the type which triggers Vitamin-D production, but on the other hand it does damage skin, and if left unchecked can cause skin cancer. We need it, but we also need to take precautions against too much of it. Most sunscreens on the market include protection from harmful UV-B rays, as well as grow room glasses.


UV-C light accounts for a full zero percent of UV light reaching the earth’s surface. While UV-C light is now artificially made, the ozone layer completely absorbs this wavelength, which means no UV-C light penetrates through the earth's atmosphere. It ranges in wavelength from 280 to 0 nm.

Is UV-C good for plants?

This is where the truth about UV light in the grow room gets a bit confusing. Out in the great outdoors, plants are not naturally exposed to UV-C lighting, because it doesn’t permeate through the ozone layer. They don’t need it to grow, and it can be one of the most damaging wavelengths of light to expose your plants too. Many growers don't realize that UV-C does damage to the plants DNA.

But, with that said, UV-C lighting wands are often sold as a hand-held tool designed to sanitize the workroom from microorganisms. These wands borrow from UV-C technology used to sterilize water in the backcountry. Ultimately, UV-C will kill everything with too much exposure, including your plants. If you do decide to use a UV-C wand in your grow room, cover up and protect yourself and your plants from excessive exposure. Your plants certainly do not need UV-C to grow, and your grow lights don't produce it.

Is UV-C harmful to humans?

We, earthlings, take our ozone layer for granted. Few realize that without the protection of the invisible atmospheric barrier, we would face a constant barrage of danger UV-C light. UV-C light is hazardous to humans, even with limited short-term exposure. If intense enough, UV-C light can cause lesions, burns, or redness. Over the long term, it speeds up the visible signs of aging and also leads to skin cancers.

A Summary of What Light is Best For Plants?

What is the best UV spectrum for growing cannabis? When it comes to the full UV light spectrum, its best to stick with UV-A and UV-B varieties for your plants. As mentioned, there is no benefit of direct UV-C exposure to yourself or to your crop (although UV-C may help sterilize your grow room). If you are considering using the UV-C technique to disinfect your workspace, consider safer organic alternatives to pest control, including as integrated pest management (IPM) techniques.

Depending on the species of plant you intend to grow, there will also be a precise formula for just how much UV light you’ll need, and when you should introduce it. In some cases, UV light over prolonged periods has stunted growth. In other cases, it can enhance valuable characteristics of your crop. For example, for growing cannabis, you’ll only want to introduce UV light during the final few weeks of flower, to boost trichome development, but constant UV lighting for leafy greens tends to stunt their development.

But what about grow lights? Do artificial lights include the UV spectrum, and if so will these artificial lights help your plants grow? Modern metal halide and ceramic metal halide bulbs should all produce a low, but measurable UV spectrum of light, but so do many LEDs. So long as you are investing in modern grow light technology, the UV should be built right in. Importantly, reputable grow lights won't emit any dangerous UV-C wavelengths.

What Safety Precautions Do you Need to Take for UV Lights in the Grow Room?

It goes without saying, you should never look directly into the sun, and you should also never stare straight into a grow light. The sheer intensity can cause irreversible damage to your eyesight, and its the UV spectrum which growers should be especially wary of.

To protect yourself from UV-A and UV-B electromagnetic radiation, invest in a pair of LED and MH grow room safety glasses, or the HPS and MH variety. Your eyesight will thank you later. Interestingly, you may see brands advertising UV-C production as a selling point - but this is entirely unnecessary as grow lights never produce measurable levels of UV-C. In fact, even if you protect your eyes from UV-C exposure, you have forgotten about the dangers to your skin and face. 

The bottom line on UV light boils down into two recommendations:

Explore the possibilities of LED lights with UV-A and UV-B for boosting yields and valuable features of your harvest, but understand you'll need to protect your eyes and skin from prolonged exposure.

Avoid using UV-C in the grow room, especially when emitted from a grow light. Exposure to UV-C can damage your skin, eyes, and more, even after only short bursts. Your plants do not need any UV-C to flourish.

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