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 ultraviolet light harmful? And crucially for us at Happy Hydro, can you protect your eyes from UV rays in the grow room?

Modern grow lights provide a full spectrum of light for your plants, including UV wavelengths that are harmful to human eyes. If you’re interested in protecting yourself from harmful UV-A and UV-B electromagnetic radiation, LED and MH grow room safety glasses—or the HPS and MH variety—are an excellent investment.

During our research on the subject of UV light, we quickly discovered that many sources aren’t as clear-cut as we had hoped. To make the research easier on you, we put together the following guide regarding the truth about UV lights—and how they affect both you and your plants.

If you prefer visual content, we go deep into this topic in our YouTube video here.

Ultraviolet light, or UV light, is a form of electromagnetic radiation. Light is measured and referred to by its wavelength in nanometers (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 wavelengths, many other creatures, including plants, use it to survive. When it comes to cannabis (and many vegetables), harnessing some UV light may 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 the most prevalent spectrum of UV light on earth. UV-A makes up 98.7 percent of all UV light that reaches us from the sun.

Most evidence suggests that UV-A exposure isn't tied to DNA damage in plants. This is not a surprising fact, considering that 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 in the case of cannabis, they might even improve THC and CBD production.

If you are just starting out with grow room lighting, check out our introduction to indoor grow lights.

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 ultraviolet light, it may trigger the development of skin cancer, according to the Skin Cancer Foundation. 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. This is what leads to faded curtains and aging artwork. It is also the light spectrum responsible for 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. UV-B 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.

In the right dose, introducing a small amount of UV-B ultraviolet light to your indoor plants can be a good thing. This light spectrum encourages plants to produce their own natural sunscreen. These “sunscreens” vary from one species to another—in the case of cannabis, the development of trichomes, terpenes, and colors will be affected. The UV-B spectrum also offers a bit of natural protection against fungal infections and unwanted pests.

Humans have a love-hate relationship with this middle spectrum of UV light. On one hand, UVB light triggers Vitamin D production. On the other hand, it can damage the skin, and will eventually cause skin cancer if left unchecked. Us humans require UV-B, but we also need to take precautions against too much of it. Most available sunscreens include protection against harmful UV-B rays—grow room glasses serve the same purpose for your eyes.

UV-C is not present in the UV light that reaches the earth’s surface. While UV-C light is now artificially made, the ozone layer completely absorbs this wavelength, meaning that no UV-C light penetrates through the Earth's atmosphere. It ranges in wavelength from 280 to 0 nm.

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 plant's 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.

Us 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 dangerous UV-C ultraviolet light. This spectrum of 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.

What is the best UV spectrum for growing cannabis? When it comes to the full UV light spectrum, it's 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 thinking about using the UV-C technique to disinfect your workspace, consider safer organic alternatives to pest control, including 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, as well as when you should introduce it. In some cases, UV light over prolonged periods can stunt growth. In other cases, it can enhance valuable characteristics of your crop. For example, when growing cannabis, you’ll only want to introduce UV light during the final few weeks of flower to boost trichome development. 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. Most LED lights also provide these UV benefits. 

So long as you are investing in modern grow light technology, the UV spectrum should be built right in. Most importantly, choosing reputable grow lights will ensure that no dangerous UV-C wavelengths are emitted.

Take a look below at some of the best uv lights for plants.

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 it's the UV spectrum that 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 should never produce measurable levels of UV-C. In fact, even if you protect your eyes from UV-C exposure, your skin and face are still susceptible to damage.

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

1. Explore the possibilities of LED lights with UV-A and UV-B for boosting yields and valuable features of your harvest—but always keep in mind that you'll need to protect your eyes and skin from prolonged exposure.
   
   
2. 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 require any UV-C to flourish.