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The Effect of Light Leaks on a Sealed Growrooms

Light leaks in your sealed growroom can be disastrous for some kinds of plants. They can also be a sign that more than just light is getting in, including pests and other environmental variables.

When planning and setting up indoor gardens, growers spend time and money in the creation of environments where plant species thrive. For sealed growrooms, this process represents a careful balancing act between temperature, humidity, light, and CO2. However, once an indoor grow is operational, these synthetic environments create challenges for growers that are non-existent in the natural world. The reaction of indoor plants to light leaks during dark periods (nighttime) presents one of these unusual phenomena.

Certain species of plants are subject to photoperiodism, in which the varying duration of light cycles between day and night cause plants to enter new phases of growth. To illustrate, uninterrupted 12-hour periods of darkness (nighttime) causes some plant species to start flowering. This process occurs when a hormone called photochrome reacts to sunlight intensity and durations, directing plants towards the different phases of growth.

If a sealed growroom is not 100 percent dark during the nighttime period, photoperiodism cycles can be interrupted, causing photochrome imbalances as related to specific plant processes. These hormonal imbalances can have negative and sometimes detrimental effects on an indoor harvest.

For those looking to avoid issues with light leaks in their sealed growrooms, consider the following points.

Inconsistencies and Stress

Indoor gardeners should always strive for consistency in their growroom environments. Most crops perform best in stable environments, and bountiful harvests are the result of constant environmental balance—including stable lighting intervals—during both vegetative growth and flowering. A common misconception amongst indoor growers is that light leaks during the vegetative growth phase won’t disrupt crop growth. However, any irregularities in lighting patterns can stress plants out. Along this line of thought, all environmental stressors inhibit essential plant functions, such as nutrient uptake, and retard growth.

Light leaks can also prove troublesome regarding photochrome levels in plants, as unexpected or irregular doses of light can alter stable hormonal conversion processes. During flowering phases, excess light during dark periods can push photochrome activity to the point of converting a plant back into vegetative growth.


One of the most widely known negative side effects of growroom light seepage has to do with the transformation of female plants into hermaphrodites. For those looking to grow seed-free flowering plant varietals, hermaphrodites can prove devastating for a crop. This is because male flowers on a single plant can pollinate an entire growroom and greatly devalue a harvest.

Expert horticulturists agree that certain plant species turn hermaphrodite as a result of environmental stressors, and light leaks are notorious for being associated with this phenomenon. However, it should be noted that the occasional beam of light on a garden from a headlamp won’t cause plants to “herm.” While indoor growers should strive to avoid any disruptions in regular light cycles, it takes rather consistent light exposure to force a plant into hermaphrodite growth. These sorts of leaks come from constant sources, such as under doorways and walls, that occur on a daily basis.

Light Leaks Mean Other Leaks

Indoor growers can be assured that if their sealed growroom is leaking light, it has issues with other leaks. Seasoned cultivators go to great lengths to ensure that their sealed gardens are functioning at their best when it comes to atmosphere, temperature, and sterility. All these contingencies are compromised with an improperly sealed growroom.

If leakage issues arise, growers, sacrifice the total environmental control that is so essential in sealed room growing. With this issue comes potential problems with maintaining ideal, static levels of temperature, humidity, and CO2. Moreover, as sealed rooms are wholly dependent upon CO2 injection technology, the regular loss of CO2 to leaks is financially burdensome and operationally threatening.

If light leaks can penetrate the confines of a sealed garden, so can airborne pathogens. As such, it is virtually impossible to fully sterilize an indoor grow if bugs and spores (of powdery mildew and botrytis) can continuously access the grow space via leaks.

Locating Light Leaks

Many indoor growers don’t know that their rooms have light leaks until it is too late, and the problem expresses itself by way of hermaphrodites and seeded flowers. As a result, it’s a good idea for cultivators to regularly check their growrooms to make sure they are 100 percent dark during the nighttime period.

To inspect an indoor garden for light leaks, it’s best to enter the grow with a green light when the primary lights are off. Once situated in the garden, turn off the green light and sit still for a while to let your eyes adjust to the blackness. At this point, it should be easy to canvass the walls and ceilings of the room and discern any potential points of light leak trouble. Also, this inspection process should be done during various parts of the day, as different angles of sunlight outside can cause light leaks during isolated time frames.

Gardeners should also be advised that control panels on grow equipment, such as atmospheric controllers and AC units, often give off light. This light is usually red or green and is residual from the digital readout. As such, it is recommended that growers cover up these light sources with electrical tape or some sort of removable opaque material.

Compared to any other form of controlled environment agriculture, sealed room growing provides the most mastery over environmental factors. However, these growrooms present novel challenges of their own, as seen with the issues surrounding light leaks. For the conscientious gardener, regular inspections of one’s garden should alleviate any light leak problems. All things considered, this knowledge will provide a better platform for troubleshooting on the macro-level moving forward.

Using Ultraviolet Light to Stop Gray Mold, Powdery Mildew & Other Marijuana Plagues

This is an important update to an article we posted a while ago, about using ultraviolet light to fight gray mold, powdery mildew and similar marijuana enemies. The photo you see just above this text shows you two marijuana leaves–one treated with ultraviolet light, and the other not treated. The untreated leaf is infested with powdery mildew, but an ultraviolet light device made by a company called CleanLight blocked powdery mildew on the other leaf!

Ultraviolet light can kill molds and fungi, which are some of the biggest causes of crop failures in marijuana grow rooms. Obviously, you want to know how to do that same mildew-killing ultraviolet treatment in your marijuana garden. In the original version of this article, we discussed an ultraviolet device called the Reme Halo. The Halo is a tiny device that has to be installed by an HVAC professional inline in your grow op building’s air handler. It uses a low-watt ultraviolet light and an ionized hydroperoxide generator to scrub air. The manufacturer claims it removes mold, fungi, and odor.

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The Halo manufacturer doesn’t say a word about the marijuana growing industry in their marketing: they only talk about using the Halo in hospitals, food prep areas, and similar facilities.

When I engaged an HVAC professional to install the Reme Halo and asked him about molds and mildews, he warned that those organisms thrive in most buildings, especially in ductwork. Replacing ductwork is expensive and difficult, and pretty soon, the new ductwork has mold and fungi spores in it again anyway. Carpets are full of spores too, he said, and so are dogs, couches, clothing and other absorbent things.

“If you want to cut down on molds and mildews,” he said, “get rid of your carpets.”

Several of us on the Growing Marijuana Perfectly team had the Reme Halo installed in our grow op buildings, and monitored the results for several grow seasons. We discovered the Halo is great for getting rid of cannabis odor; it’s probably the best inline air handler odor-reducing unit you can use. You can smell the hydrogen peroxide-cleansed air coming out of your air conditioning vents. But you can’t count on the Halo to totally block molds and fungi, nor will it eliminate them if those organisms have already invaded your grow room.

Ok, now for fantastic news! We discovered a different and much better ultraviolet technology made by a highly-esteemed Dutch agriculture-focused company called CleanLight that’s far more reliable and scientifically proven to prevent and eradicate gray mold and powdery mildew in marijuana grow rooms.

CleanLight ultraviolet crop protection units are successfully protecting crops in all kinds of agricultural settings worldwide, including licensed legal North American marijuana production facilities such as Aphria, Pure Sunfarms, Cronos, Zenabis and Aurora. CleanLight units are also protecting marijuana crops at licensed legal marijuana grow ops in Colombia, including cultivation companies such as Blueberries, Colombian Organics, and Clever Leaves.

Craft and connoisseur home growers use smaller hand-held units such as the CleanLight Pro, shown in the picture below. It’s fun, safe and effective to wage war against powdery mildew and gray mold by bathing your marijuana plants in ultraviolet light instead of in fungicides, Neem oil, other foliar sprays, and systemics. Using CleanLight is much healthier for you, your plants and bud consumers than those other methods.

Before we talk more about CleanLight’s impressive new technology, let’s discuss some useful background information…

I’ve been growing marijuana indoors for many years and because of security concerns and worries about pest and pathogen vectors, my home is a fortress. Clandestine locked grow rooms with intensely bright light pouring out from under doors, smelling of skunks, diesel, pine, and other cannabis odors are things you can’t easily explain to visitors.

I keep my windows tightly closed and filter all my incoming air, due to airborne pathogens and pests. I have to be sure not to wear the same clothes into my grow room that I had on outdoors while doing yard maintenance, because pests, molds and mildews can transfer into my grow room via my clothes.

Growing marijuana indoors puts much more humidity into your indoor space than normal and molds and mildews love humidity. Yes, I have professional dehumidification and air conditioning, but live in a humid region. Gray mold spores, powdery mildew, leaf septoria and other harmful organisms are everywhere around here. You could seal up a grow op like a NASA spaceship and still not block all those spores.

And you probably already know that gray mold and powdery mildew can wreck your plants in a few days if not sooner. Most tactics marijuana growers use against gray mold and powdery mildew, such as lighting a sulfur burner or foliar spraying, are costly and have a negative effect on the quality, safety and market value of your buds.

So-called organic or “natural” materials such as Neem oil, yeast, natural extracts and other interventions never were 100% effective against gray mold and powdery mildew. They’re messy to use, can pollute your buds, clog leaf stomata, and harm plant health.

When buds smell and taste like sulfur, Neem or fungicide, nobody wants them! There’s another risk: in regulated legal cannabis states, buds contaminated with most types of systemic or foliar natural or chemical fungicides are dangerous for consumers, and are rejected after testing, meaning dispensaries can’t buy them from you.

Even if you make it all the way to harvest without being victimized by gray mold (botrytis cinerea) or powdery mildew, these pathogenic destroyers can invade your buds during drying, curing and storage.

Mold and fungi can survive for years in nearly sterile environments, even in harsh conditions. Once they get into your grow space, air conditioning system and throughout your building, they’re almost impossible to totally eliminate using traditional methods such as fungicides, bleach, and sprays.

When professional licensed commercial cannabis growers told us about CleanLight’s effective ultraviolet product line designed specifically for horticultural use, we contacted CleanLight and explained to their exceptionally helpful rep David Symanzig that we’d used a different ultraviolet device and it didn’t work well.

David immediately sent documents from third-party testing of some of the several different types of tech that CleanLight makes. The company manufactures two sizes of hand-held ultraviolet generators, a floor-standing UV air cleaner with ultra-precision filtration, an tubular unit that scrubs exhaust and internal air flow, and larger, industrial-sized units for use on the world’s biggest indoor commercial horticulture facilities and field crops. The company also manufactures water-cleansing ultraviolet units.

I’m impressed by CleanLight test results, and by photographic and video evidence that clearly show how CleanLight treatment blocks powdery mildew, (see the main article photo for an example).

The company’s credibility is bolstered by photos and videos of CleanLight products used by big-money industrial agriculture operations, including outdoors. I figure that when multimillion-dollar professional farms and food processing and storage facilities are spending big money on CleanLight technology, the company’s gear must be effective.

CleanLight has a laudatory backstory. For example, the Reme Halo manufacturer only makes products for hospitals and other uses, but doesn’t pay any attention to horticulture. CleanLight focuses only on agriculture and horticulture, and has been done so for nearly 15 years. More than 3,000 farms, greenhouses, glasshouses and grow ops of all types and sizes in 26 countries use CleanLight products to kill molds, viruses, and bacteria on crops, equipment, and in irrigation water and the air.

CleanLight has massive industrial installations in some of the biggest commercial agriculture facilities worldwide. Their UV-C fixtures are often mounted on movable carts, tractors, or overhead hydraulics so they can bathe acres of plants with pathogen-killing UV-C. Take a look at this photo and you’ll see what I mean…

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Treating your plants with ultraviolet light is easy and effective. According to Cameron Scott, a researcher at Canada’s Simon Fraser University, the most common diseases that arise within cannabis operations are powdery mildew and botrytis, but there are also harmful soil-borne fungi that infect plants, such as Fusarium.

Cameron started testing CleanLight UV technology and reported the following results: “We conducted powdery mildew trials with cannabis plants treated with Clean Light, or left untreated to compare against. At the end of the trial, which was for 28 days with daily exposure to UV-C, we compared the results. We conducted disease assessments weekly, we measured the disease progress curves, and we analyzed the findings. We found out that plants treated with Clean Light had much lower powdery mildew development compared to those not receiving the treatment, by up to 50%.”

Cameron also noted that growers have a variety of CleanLight choices. There are handheld units such as the ones he used in his research, a hand-cart system, and overhead automated booms equipped with Clean Light modules.

“The booms are particularly suitable for large facilities, as it is easier to cover big spaces. The hand-held unit is ideal for smaller operations,” he says.

Testimonials about CleanLight from professional growers include the following:

  • “We have been using CleanLight since 2015 on our leek and strawberry plants. We see a great effect on powdery mildew, which resulted in reducing the use of chemicals for this disease by 100%.”
  • “The CleanLight is the greatest innovation in the last 20 years. Our customers are extremely strict with regard to residues. The CleanLight makes it possible for us to reach extremely low residue levels.”
  • “I must say that we have never had any problems with plagues since we use CleanLight. The crops are really beautiful. Also, CleanLight does not have a negative effect on the functioning of biological protectors. By treating our crop daily with CleanLight, we see no mildew, we have lower insect pressures and a healthier- looking crop. Good results with the CleanLight hand trolley convinced us to invest in an automated system.”
  • “CleanLight, while labor intensive, did reduce our dependency on chemical fungicides by 66%.”

In my view, the ideal CleanLight intervention for small-scale home, craft and connoisseur cannabis growers are the handheld CleanLight Pro or CleanLight Hobby units. These units dose clones, seedlings, seeds, buds, growing plants and your entire grow op and building with pathogen-killing ultraviolet light. I suggest you also get the CleanLight Air, which functions as a pathogen-killing floor-standing air cleaner.

The handheld units are fun to use. You can deliver UV-C to any surface–plants, walls, hydroponics reservoirs, grow room equipment, ceilings. Of the two, the CleanLight Pro is the one I recommend most because it has more coverage area and penetration.

Initial testing shows when you UV-C treat your grow op and building BEFORE you ever start an indoor garden, you’re killing off any mold and fungi spores present in the space. You routinely repeat the process as your plants grow, at harvest time, and throughout drying, curing, and packaging. There’s solid evidence that CleanLight gear eliminates or effectively controls predatory molds and fungi even after an outbreak occurs.

Along with the handheld units, there’s the CleanLight Air filtration unit. This is the world’s most effective floor-standing anti-biological pathogen stand-alone air cleaner, but it doesn’t reduce odor much. Using the CleanLight Air all the time, and the handheld units periodically, eliminates harmful molds and mildews from your marijuana grow op, and can control or completely vanquish these pathogenic organisms if they’ve already infested your grow op.

If you have a larger facility or for any other reason want more ultraviolet sterilization capacity, go for the CleanLight Pro Air 800. This ceiling-mounted unit has its own fan, and can be used as for exhaust filtration and to cleanse internal circulating air. It can protect your crops from molds and mildews in a space as large as 28,000 cubic feet!

Some people purchase weak UV bulbs and attempt to make their own amateur copies of CleanLight gear. These DIY units don’t work well, aren’t scientifically tested and engineered, and cause fires and electrocution.

Based on clear evidence we’ve seen so far, CleanLight tech is the safest, easiest to use way to protect your grow op from the gray mold, leaf septoria and powdery mildew plagues. Contact CleanLight today and explain your growing situation to them to find out how their products can help you.

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?

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.

What is UV Light?

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.

Is UV-A good for plants?

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.

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(If you are just starting out with grow room lighting, check out our 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. 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.

Is UV-B good for plants?

In the right dose, introducing a small amount of UV-B light into your indoor grow room 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.

Is UV-B harmful to humans?

Humans have a love-hate relationship with this middle spectrum of UV light. On one hand, UV-B 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.

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 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.

Is UV-C harmful to humans?

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 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 Light is Best For Plants?

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.

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 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:

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.

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.