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.

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…


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.