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Letters to Rev – Humidity and Mold

Letters to Rev – Humidity and Mold

Humidity and Mold - Powdery Mildew

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Howdy Skunkers! Well holy hell man, this Fall is warm and wet. That atmospheric river is a killer for indoor and outdoor growers alike. Warm and wet is especially problematic for indoor growers, humidity and mold wise, if their gardens don’t have proper humidity controls. The real spawning ground for mold is wet and cool. Like in the early morning hours just after lights on or sun up. The reason warm and wet is so dangerous indoors is because normally some additional heat can keep your humidity in check overnight. But, if your ambient temps are like 74 or 76 at night, heating them up beyond that can have ramifications like deficiencies due to nutrient absorption failures. Things like sulfur are harder for the plant to get from the soil with extremely warm nights. Damned if ya do, etc.

My Flowering Room Mini Dehumidifier
My Flowering Room Mini Dehumidifier

Having a dehumidifier is really fairly essential for me indoors, and I run one at night, every spring/autumn night, in my flowering room, except during summer months. You want a dehumidifier that you can select the humidity level. I run mine at 45% humidity so if the humidity is higher than that, my unit is running. They are a tad spendy to run but not extremely so. I use a smaller dehumidifier meant for a single room in my flowering room, which is all you need. 

Rev’s Tip: New Houseplants Anywhere Near Your Grow are Ill Advised

Before we roll, I want to alert you guys to houseplants, yup. If your cannabis garden is in your household, you need to avoid bringing in new houseplants. The problem is that these plants are almost exclusively raised synthetically using chemical combat measures to fight pests, like mildew. This results in super-strains of powdery mildew (PM) that I have found to be fairly common on all new houseplants. So, be thee aware my friends. Super strain PM is uber invasive and resilient, able to withstand virtually any combat measures. Same goes for outsourced clones, fairly often.

Alright then earthlings, let’s rock and/or roll with today’s “Letters” to Rev…

Humidity and Mold, Question #1: Sprouts Falling Over?

FROM: Kurt

“Rev! Longtime fan. I just preordered your new Druid’s Edition TLO, I can’t wait. About once or maybe twice a year I have a big problem with my sprouts having their little stems rotted through by mold. I always run good humidity in that space and there’s plenty of air extraction in that room as well. But it just keeps happening. Do you think this is somehow strain specific? I just can’t get to the root cause. Any thoughts? Thank you, man.”

Rev’s Answer to Q1
You Always Want Some Airflow at Ground Level When Raising Sprouts
You Always Want Some Airflow at Ground Level When Raising Sprouts

Hey Kurt. Yeah, as you likely know this is called Damping Off. It is essentially mold eating right through the stem of the sprouts and causing them to tip over and die. Think about it, and I bet these occurrences happen in either springtime, or fall. It’s always just a bit extra wet indoors at these times. Damping Off is always spawned by humidity. Specifically, it is caused due to the wet sprout’s soil affecting the area just above it. The stem. The level of humidity at the ground level is in far excess to the ambient humidity of the room. You dig?

What you need to focus on my friend, is air circulation. Air circulation down at the ground level of the sprouts. I have solved this issue for peeps many times in the past and that is almost always 99% the reason. You don’t need to aim fans at your sprouts, just make sure your airflow causes them to “dance” slightly in the airflow. The mold(s) that cause Damping Off are always in the air so there’s no avoiding it if you make the conditions right for it to spawn. So, as long as your ambient humidity is good, this airflow enhancement will fix ya right up.

Humidity and Mold, Question #2: Basement Grow Mold?

FROM: Paul

“Hello. I’m having big problems with mold, Powdery Mildew during veg and Bud Rot during flowering. I grow in a basement. After the first run I installed like 8 house fans to keep all the air circulation excellent in there. Now it’s all happening again. I don’t know if my amps can handle a dehumidifier. I do have a 6-inch extraction fan as well. Can you help me out Rev?”

Rev’s Answer to Q2

Greetings Paul. First of all, having a 6” extraction fan is all well and good. What you need to do is calculate the cubic footage of your basement (L x W x H) and then find out the CFM (cubic feet per minute) value of your fan. You need to be exchanging all the air, around once every few or several minutes. The amount of humidity that larger plants (over 1 foot) can put into the air in a room is nothing short of amazing. Not to mention flowering plants!

Here’s some ideas I have for ya Paul…

  • Switch to tents in your basement and vent the tents. You can get away with a lower CFM fan value here. Just calculate the cubic footage of the tents. Draw in basement air to the tents, all good.
  • Maybe your amps could handle a smaller sized dehumidifier, like my mini in the photo above, depending on the size of your basement.
  • Upgrade your extraction fan to accommodate your grow room size in cubic feet. Maybe 8 inches.
  • If you can heat up your basement, you can just keep it a bit warmer at night than it would be normally. So, if your overnight low is, say 56, then just set a thermostat to keep it 60. This will help a lot. This also is likely quite spendy, depending.
Humidity and Mold, Question #3: Mold Resistant Strains?

FROM: Fubar Fillion

“I was wondering if you know of a way, I can tell which plants will have better mold resistance than others? Sometimes everything works out great, other grows I get hammered with mold during flowering even though I have good venting and air movement. Are there certain strains I should be looking for here? Thank you for all you do Rev.”

Generally Speaking, Super Dense Buds are the Most Vulnerable to Mold Due to Lack of Circulation Within
Generally Speaking, Super Dense Buds are the Most Vulnerable to Mold Due to Lack of Circulation Within
Rev’s Answer to Q3

High there Fubar, LoL. More or less mold resistance during flowering, in my experience/opinion boils down to floral morphology. Some strains/plants just have a better “build” on their buds allowing adequate ventilation of those buds.

Plants that have grown in places like Afghanistan for generations have no need to develop extra floral (pro circulation) morphology because humidity is never a thing in Afghanistan. Many Hashplant types of indica fall into this category. Many sativas and some sativa dominants on the other hand, can handle much greater humidity—hell, even rainfall without succumbing to mold. Check out the Black Poison over at Kingdom Organic Seeds. She’s a very decent mold warrior, even with some rainfall.

Humidity and Mold, the Good News
Grower’s Ally Crop Defender 3
Grower’s Ally Crop Defender 3

Wait, what? There’s good news? Heh heh, well actually there is. The good news is that if you let your humidity get out of hand in your grow, the PM that will show up is likely the normal kind of PM, and not the super-strain kind. As long as you get your humidity under control you can make it to harvest with far less damage using a product like the one in the photo to the right. Now, an older product like Serenade is better at kicking PM ass, however, the damage to your terpenes is noticeable indeed. Flavors and smells will be fucked up a bit if Serenade is used within 3 weeks of harvest.

This product is essentially a bunch of organic oils. I have tested this product myself and while it isn’t as powerful at killing PM as Serenade, it is MUCH BETTER at not damaging terpenes. I have used this product within a week of harvest on one plant and it was fine at harvest. Not perfect, but way-way better than if I had let the PM run its course. Crop Defender 3 really serves more to halt the progression of PM than to eradicate it utterly. You use it about once per week, lightly, on affected areas.

If you get super-strain PM from outsourced clones etc., you are basically boned pretty bad and even using a powerful remedy like Serenade won’t help that much.

See Also


The Rev

Having a dehumidifier is really the bomber solution to any problems with mold indoors. However, they do cost a little coin and they are a tad spendy to run. I got mine for around $150.00 and it works great for my 10×10 flowering room. I set it for 45% at night and 50% during the day. That way it works fine but also isn’t constantly running. Larger ones are about $200.00. Don’t bother with those little tabletop dehumidifiers in a grow room, they just don’t have the oomph to get the job done. Remember, a room full of flowering plants will raise the ambient humidity in a grow room by AT LEAST 20%, and that’s a lot!

I keep my overnight humidity below 64% and I almost always get some PM if it goes above 68%. During lights on I normally set my dehumidifier to 50% so it only needs to turn on fairly often during uber humid conditions.

Humidity and Mold

I hope you all found this humidity and mold article helpful. Want a little more Rev babbling? Check this article out: Cloning Cannabis Plants – The Rev. Also—YaY—my latest TLO (True Living Organics) book is almost here! You can preorder it right now, here: TLO Druid’s Edition Preorder Link. Yup. L8r G8rs…

  • REv ?
November Crossword Time!

This one is medium hard I would say and deals with today’s article regarding humidity and mold. It’s just a quicky so give ‘er a go, yeah?

Crossword Key


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