Gray mold, known also as Botrytis cinerea, bud rot, and bud mold, can quickly trash your bloom phase hopes. Gray mold also attacks harvested buds. And when you put botrytis-infected buds in the freezer, the mold goes dormant, only to spring back into action when buds thaw.

Gray mold doesn’t just destroy buds, it destroys your lungs. Medical evidence shows gray mold can survive combustion, vaporization, or the making of concentrates and edibles. It’s bad for your lungs and can cause other body toxicity issues.

Gray mold loves temperatures between 70-78°F and relative humidity above 70%. This temperature and humidity range is too often found in indoor grow rooms or outdoor marijuana growing locations, and it invites gray mold.

Indoor marijuana growers attempt to block gray mold spores from getting into grow rooms by using carbon filters, micro-filters, and sealed grow ops. But mold spores are microscopically small, traveling on wind and via vector contact.

If you live in a region where gray mold flourishes outdoors, it’s virtually impossible to create a grow op so sealed and filtered that all spores are blocked.

As soon as new air enters the grow op, spores come in with it. One key way to prevent catastrophic gray mold bud destruction is to keep spore density levels as low as possible while ensuring grow room conditions are as inhospitable to gray mold as possible.

Gray mold has a very hard time getting established on your plants when relative humidity is 60% or below. But this humidity level is costly to achieve and may interfere with vapor pressure deficit management.

One big problem for marijuana growers is it’s impossible to accurately measure relative humidity where it counts—inside the bud.

In agricultural settings where growers are producing fruits, for example, ambient humidity only needs be kept lower than 75% to block gray mold.

But the deep interior of a fat marijuana bud may be so moist as to be almost visibly wet. A grow room with 75% ambient humidity could easily have fat buds with near-100% humidity in the bud interior, perfect for grey mold.

That’s why some marijuana growers keep their grow rooms below 60% humidity, if not lower, down to 51% humidity. By overdrying grow room ambient air, moisture in the bud interior evaporates, creating a less-favorable environment for humidity-loving botrytis.

Growers have to be careful though, because low relative humidity may interfere with ideal vapor pressure deficit, and can also cause plants to suck down more nutrients water, leading to overfeeding and root zone toxicity.

In some grow op situations, a combination of exhausting moist air, and using a dehumidifier, is a hybrid strategy that works to keep vapor pressure deficit in ideal range while also ensuring low humidity.

You need an extremely accurate hygrometer to measure humidity. Unfortunately, most hygrometers sold for under $100 are inaccurate. When you’re extremely serious about accurate humidity measurements, get a sling psychrometer or use a professional humidity monitor and data logger.

Several companies make gear and apps for wireless monitoring and transmission of real-time grow room climate data. In some cases, high-tech hygrometer data systems allow you to set alerts so you get text or email warnings if humidity and other grow room factors are going out of range.

Monitoring for Gray Mold

Gary mold most often starts out of sight, hidden away on the inside main stem of fat buds. The photo accompany this article shows a fat Auto Glueberry OG bud that looked just fine until I detected a rotten smell and pulled the bud apart to reveal grey mold growing inside.

By the time you see the mold’s white cloud of ugliness, it’s too late to save that bud. Constant scrutiny of your marijuana plants, especially during peak bloom and late bloom phases is required. There are two types of scrutiny: visual, and olfactory.

Look closely at your plants from top to bottom. Botrytis often gets its start on leaves and stems just above the root zone, because root zone moisture creates a high humidity area extending from the root zone a foot or more up into the plant, and because gray mold often lives in soil and other root zone media.

Using magnification, look closely at and into your buds. If you see bud areas that are grayish, brown, reddish, or curling, dead or detaching from the main bud, gray mold is likely festering at that site from inside the bud.

Some growers chose to only excise the moldy bud portion and leave the rest of the bud on the plant. Big mistake!

The entire bud, or better yet the whole branch or stalk it’s on or the entire plant should be removed from the grow op immediately and quarantined or destroyed.

And you can bet that mold spores have spread to adjoining plants, and equipment, filters, air conditioning ducts and other structures that are extremely hard to disinfect.

You can often smell gray mold before you see it. Some people use the word “dank” to denote cannabis, but a dank smell means buds are too moist and possibly molding.

A small percentage of cannabis strains have terpenoids that smell like rotten fruit. But that’s different than the rotten smell of mold. If fat buds smell rotten, it’s because Botrytis is living inside them.

As you’re drying and curing your buds, be on the lookout for gray mold. If it gets on your buds, even freezing them won’t purge the mold.

Gray Mold Prevention & Intervention

Along with keeping grow op humidity low, there are other methods for making your plants and grow op less hospitable for Botrytis. One tactic is to use less water volume and less frequent watering than normal, especially when fat buds are present.

Water as infrequently and with as low a volume of water as possible, but don’t cause plant wilt. Water early in the day so moisture has time to evaporate out of the root zone and transpire out of leaves before lights-off.

Lights-off cycle is often a time of lower temperatures and stagnant air. If you water roots or you foliar spray just before grow lights go off, you’re asking for gray mold trouble.

Foliar spraying buds starting in peak bloom, even thin Sativa buds, isn’t recommended. You really don’t want to add sprayed moisture to buds.

When you’re cloning, misting cuttings and using a humidity dome creates favorable zone for gray mold. You might not see the mold until bloom phase though. Its spores are smart enough to lie dormant from clone phase onwards, springing into action when they have moist buds to attack.

You already know that bringing clones in from outside your grow room is a major vector for pests and diseases. It’s best to make your own clones.

Overwatering causes many problems in your cannabis grow room, including overhydration of stalks, stems and leaves that create humid bud conditions favorable for gray mold.

Growers are advised to reduce watering to lowest safe levels starting in peak bloom, especially when dense, fat Indica buds are present.

Reducing nitrogen fertilizer percentages is another tactic to deter gray mold. Most hydroponics base nutrients designed for bloom phase contain less nitrogen relative to phosphorus and potassium compared to grow phase nutrients.

But even that amount of nitrogen can be too much for bloom phase and may create conditions favorable for gray mold.

Further, some bloom phase supplements, especially organic supplements, contain nitrogen. Your bloom phase plants need nitrogen, but not very much.

Adding any nitrogen beyond base nutrients is often too much, so carefully examine your bloom phase supplements and if they contain nitrogen or nitrates, use them sparingly if at all.

Any standing water on plants or in the grow room creates micro-zones of high humidity favorable for gray mold. Water on leaves is especially harmful. Also harmful are any debris in the grow room, especially old leaves, pet hairs, and other detritus. The cleaner the grow room, the less likely pathogens can infest it.

Botrytis is especially likely to flourish where there are wounds on your cannabis plants. So if you’re doing pruning, leaf removal, lollipopping or other work that leaves wounds on your plant, beware, because those wounds present open tissue that attracts pathogenic diseases and insect pests.

Another grow room problem comes from lack of air circulation, filtration and exchange. Use grow room oscillating fans set at different heights to push air from the floor all the way to the ceiling and back again.

Use cloth pots instead of plastic or other hardened pots, because they lower humidity in and around the root zone. You do well to have the highest grade of filter material on air intakes, and to have a free-standing HEPA filtration unit and carbon filtration units operating to scrub grow room air.

Never let pets inside the grow room, especially if they’ve been outdoors, and strictly limit who comes in the grow room and for how long. If I’ve been outdoors during humid months of the year when gray mold is hanging around, I change clothes before I enter my grow room.

Fungicides for Stopping Gray Mold

Chemical controls don’t work well for marijuana growers because gray mold evolves so quickly that fungicide effects are diminished or eliminated.

Fungicides are toxic chemicals that harm cannabis plants, growers and consumers. In states where you grow weed for sale and buds are lab tested for contamination, fungicides and pesticides cause those buds being blocked from distribution.

And tainted buds produce harmful cannabis concentrates. 

There are so many state-specific restrictions and specs on which if any fungicides can be used on cannabis that your only choice is to consult regulators in your location to find out which if  any fungicides are allowed in your marijuana grow op.

Many marijuana growers use Eagle 20 fungicide against Botrytis and powdery mildew, but it stays in their buds longer than the manufacturer says it does, and cannabis industry regulators often order quarantine or destruction of buds contaminated with it.

Chemical fungicides create safety problems for those applying them and for people consuming buds treated with them. Many fungicides are systemics, not just surface application materials, and can show up in buds at harvest.

Marijuana Strains Resistant to Botrytis?

Many marijuana growers believe specific terpenoids (and perhaps cannabinoids) naturally repel or even kill Botrytis. Conversely, some strains appear to have terpenoid and cannabinoid profiles  favorable to gray mold.

My experience with Botrytis cinerea suggests that Sativa-rich strains in the Haze, Diesel and Chem families with scents and tastes such as lemon, pine, diesel, gassy, pepper and turpentine might contain anti-Botrytis compounds.

On the other hand, some strains attract Botrytis. My grow room data shows that these strains have tastes and scents such as mango, fruity, berries, and skunk. These tastes and scents are correlated with thick Indica buds that have humid interior microclimate Botrytis loves.

Using UV-C Light Against Bud Rot

Ultraviolet light kills gray mold or controls its spread so an infestation is less disastrous. Ultraviolet light fixtures include handheld, free-standing units, and units placed in-line in air conditioning and ventilation systems.

Just 1-3 minutes of UV-C applied to plants every 2-3 days during bloom phase, followed by at least four hours of lights-off, is the optimum way to use UV-C to kill gray mold in live plants.  Studies show that more UV-C exposure than that damages plants by interfering with photosynthesis.

Gray mold treated with UV-C and subjected to four hours of darkness is unable to reproduce and re-infect. But gray mold treated with UV-C and then placed under light is later still able to reproduce and re-infect.

Apply UV-C light to harvested buds, starting at harvest time and all the way through drying and curing. Duration of application can be constant during the drying and curing process, because there’s no photosynthesis to interfere with.

In fact, UV-C degrades chlorophyll (which is why you should use it sparingly around live plants), and the less chlorophyll in buds, the better the buds taste and smell.

I use free-standing CleanLight UV-C units in my drying chamber and I also use it to treat my plants during bloom phase. Always remember to use UV just before lights go off.

Using Dehumidifiers in Marijuana Growing

There are two basic types of grow room dehumidifiers. One is the common free-standing unit that costs $150-300. These generic units aren’t meant for indoor agriculture. They’re meant for making sure basements don’t get too damp, and similar uses.

There have been repeated recalls of such units, with the world’s major manufacturer (Gree) having to pay huge fines because their dehumidifiers caused fires and other damage.

Even if these dehumidifiers weren’t generically defective and dangerous, they’re so inefficient, noisy, unreliable, and unpleasant to use that they’re not worth running in your grow room.

The cannabis grow op industry has embraced the Quest brand of dehumidifiers. Quest makes industrial-strength units superior to regular dehumidifiers in manufacturing standards, efficiency, operation, operating costs, reliability and warranty.

Quest units are quieter, put out less heat, and are way more energy-efficient than regular dehumidifiers. Made in the USA and tested to perform flawlessly even in commercial grows with thousands of plants, these units are rock solid and outperform any other dehumidifier brand I’ve used.

To summarize what we’ve learned about gray mold:

  • Gray mold loves temperatures between 70-78°F and grow room humidity above 60%.
  • If you grow thick Indica buds, humidity inside the bud is near 100%, so you have to dial your grow room to 51% humidity to counteract that moisture.
  • Fungicides poison you and your buds and are only partially effective, if at all.
  • Overfertilization in bloom phase, especially nitrogen, fuels gray mold. So does overwatering.
  • Some cannabis strains are naturally resistant to gray mold; others attract gray mold.
  • If you see growing plants with gray mold, get them out of the grow room immediately and treat them with UV-C, or trash them. Then treat the entire grow op and grow house with UV-C.
  • An indoor grow room must have a quality dehumidifier. The best we’ve found is the Quest dehumidifier brand.
  • Ultraviolet light as administered by the CleanLight floor unit and handheld unit have been proven to give near 100% prevention or elimination of gray mold on growing plants, and protect harvested buds from gray mold.
  • Daily, intense scrutiny of your bloom phase plants using your eyes and nose is essential for early detection of bud rot.
  • Treating your grow room like a fortress, inspecting clones, and otherwise being super vigilant against gray mold vectors saves you a lot of trouble.

Now you know the real deal when it comes to bud rot. Most marijuana growers have had their hearts broken by gray mold at least once. I hope it never happens to you!