What percentage of RH(humidity) should ones grow room(s) be at to avoid mold?

Crusader Rabbit · Jul 18, 2013

In some ways this is like most of the site's cloning threads. People chime in with stories of success and speak as if their methods are the holy grail. Similar to asking old people the secret to longevity. You might get an answer that the secret is to smoke a cigar every morning. Some of us are just fortunate. Not speaking of anyone here, but I think the real answers come from those who truly experienced failure and found a way to correct the problem.

Vegan · Jul 18, 2013

Clayton Bigsby said:
its temps you need to worry about. Ive grown in 90%+ humidity rooms with temps around 80 and have never seen mold once in 15 years.

DHF said:
Well CB.....Tell us about your environmental control in place that allows you to grow with 80 degree temps and 90% RH during lights on and off......

I`m just sayin......I live in the Hell of the deep dirty south with RH as you talk about , but without SERIOUS attention to airflow and filtration into and out of each of my fliprooms , airborn pathogen possibilities were inherent from constant perfect conditions for em to form , grow , and proliferate.......so.....that said......

Gotta respectfully disagree about temps bein the only factor to be watched out for concerning airborn pathogen formation in ANY grow area , cuz NOTHIN could be further from the truth IME , but I digress......

Please come back with your thinking behind this theory.....

Peace....DHF...........

actually what I think you are saying is that CB is full of shit . which is exactally what I was thinking when I read that crap, :peacock:

gmanwho · Jul 19, 2013

I think one big factor is the size an INTENSITY of your grow. Every situation different. Keep temps arorund 80, 50% rh an i've had good success. Air movement 24/7 an not directly blowing on leaves.

I've gotten mold at 45%rh on some mammoth colas!! then ran a 55% with now issues. In fact i think that to low of an rh late flower is no good. The plant will transpire more from within the bud then normal, Creating a worse scenario then higher room rh!!

good luck.. & b-safe

M.R.GT · Jul 19, 2013

78 degrees lights on 68 lights off,fans 24/7 humidity below 50% works for me.
I kinda thought this was target room standards from back to OG days.

I use a converted 2 ton air handler and 2 6" passive intakes for air exchange. 2 16" oscillator fans for movement. portable A/C and dehumidifier in flower room for temp and humidity . all of that and sometime its not enough .

I also had mold once in a lui Ortega pheno above 50%

Sats will take heat and high humidity much better than an indica.

lui swt next to the ort never had an issue . Ort has been my only mold prob and it wasn't much..

any further south and I would be in the gulf.

Good luck

DeBudman · Sep 8, 2013

I'm tailing off my small outdoor grow.
I've had previous bad experiences with the Tobacco Moth attacks on my biggest colas and had to spend hours plucking out worms with tweezers...

This year I decided to be a bit proactive and carry my plants into my garage at night. All was going well until the last few weeks. I noticed a couple cola leaves turning brown and found a few chunks of rot. This is exactly what happens during a bud worm attack. My first thought was that somehow they attacked again, but found no evidence. I clipped the bud about 1 inch below the damage and found no other issues.

This got me thinking about relative humidity and the effects on my plants while tranfering them into my garage at night. I noticed the last few days have been getting almost 100 degrees outside and my garage doesn't get much airflow. But as you know the hotter the temperature is the lower the relative humidity. I figured I would be out of danger. My relative humidity was reading about 20 something at about 90 degrees temp.. One mistake I believe is I haven't been using any fan.
I suppose I can add a 14-16 inch oscillating fan out there.

Any advice would be appreciated..
I would bring the plants in at night, but that isn't an option cause I don't have an available room that I could leave them for 12 hours with no light.

Thanks..

BlindDate · Sep 9, 2013

It is a dilemma for sure. High humidity is great for the plants throughout veg and halfway into flower. At some point you benefit by lowering the RH if you can so as to avoid mold. One easy solution is to spray the buds every few days with some baking soda solution. It raises the PH on the surface of the bud and mold can't grow on the higher PH. It is also harmless and tasteless to smoke.

mowood3479 · Sep 9, 2013

Botryitis thrives in lower temp, higher humidity rooms..
So as dhf said.. Keep temps within ten deg.. Lights on/off and keep dehuey set at or below 55%..
But that being said.. Even with good temps, rh, and air circ.. U can get some rot in very dense buds if u have a room with a spore infestation in it or near where air is being pulled in..
So get ur environment in check, then set off a fogger, clean with an amonia product, let it dry, then clean with a bleach product n let it dry, then bring plants back into room..

I fought rot all last winter/spring.. Ruined some of the biggest nicest colas ive ever done..
So anyway, cb did have a point.. Rot will not thrive(read total crop
Failure in temps above 84.. So if u cant lower rh try bumping night temps somehow..
But lowering rh is preferable n controlling both rh n temp is best..
But i know everyone doent have a sante fe dehue
My two cents, learned through research and applied with trial and a few errors..

Cheesegez · May 21, 2016

you just needed a dihumidifier and decent airflow sealed rooms will go to 99%RH very quickly when lights are out

TreehouseJ · May 22, 2016

My buddy has grown the best nugs ever in sealed tents with high humidity (as high as 85%) with 100% air exchanges every hour and in 10 years hasn't ever struggled with mold. He is probably a shaman though. Results may vary.

Cheesegez · May 22, 2016

TreehouseJ said:
My buddy has grown the best nugs ever in sealed tents with high humidity (as high as 85%) with 100% air exchanges every hour and in 10 years hasn't ever struggled with mold. He is probably a shaman though. Results may vary.

that's not a sealed room if air exchange is involved ... id rather not waste all the CO2...

peace
CG

TreehouseJ · May 24, 2016

Cheesegez said:
that's not a sealed room if air exchange is involved ... id rather not waste all the CO2...

peace
CG

This also bothered me. I should note everything my grow mentor does bothers me, and I don't do anything he tells me to until I have beaten my head into a bloody stump trying to one-up him somehow. Last run I had all my parameters in the zone (80-85f night, 70-75f day, 50-60 percent humidity), but no air exchanges, and I lost about a half ounce to mold. This time I will do fresh air exchanges every 6 hours or somethin along those lines. Bottled Co2 is expensive, but a couple air exchanges can cut down on the heat produced by your dehu and take some strain off your AC, offsetting the cost of wasted co2. I use propane, so co2 comes out pennies on the dollar. 1 lb lasts me a week in full bloom under 1200w. If my propane expenditures quadrupled I would be spending ~$40 a month instead of ~$10. This f'n turd uses the stupid yeast and sugar buckets from the grow shop with all those exchanges and still pulls a gram per watt of the dankest nugs for 10 years and running. Everything this guy does infuriates me, so I totally understand if any of this infuriates you guys. If I read something like that on the internet I would think someone was pulling my leg.

OG Tree Grower · May 24, 2016

A fresh air exchange will not prevent you from getting mold in a sealed room, if anything your risking bringing spores in. But if you keep your humidity in check , plants properly trimmed so air can flow through them, have air movement through the whole room so there is no dead spots, evenly space large colas so there not touching each other ect then you won't lose much at all to mold. It's all about airflow and keeping your night time temps within 10 deg of daytime temps.

That being said I do exchange air in my sealed rooms, just not to prevent mold

Billy Liar · May 24, 2016

If you run a controlled environment with a higher temp for day than night, a good tip is to raise the temp to your daytime level an hour before the lights come on to avoid the build up of condensation on the plants.

I strive for a max temp difference of 5°c night-day.

Peace
BL

Douglas.Curtis · Jul 28, 2016

These humidity and temperature recommendations are upside down for quality cannabis. I wanted to point out that mold doesn't grow well at all with 72F Canopy temps and 20-25% humidity.

What you do get with this environment? Significantly higher quality cannabis, through higher trichome count and maximum terpene/cannabinoid expression and retention. Massively lowered chance of having any mold issues at all.

When combined with HEPA filters on the intakes, you can grow some seriously high quality cannabis.

MedicinalLifer · Jan 27, 2017

best gro room I ever had is one I have now, small 3x6 table 8 plants in 1 gal coco, 400w hid over one side 600 hps on other , indiacs under hps side cause they will finish faster, box fan at doorway, and tller stand fan just above box height push air in to the side, screen material on backside of fan blowing on bulbs so no fuzz or fibers get blown onto budds, hot air rushes out above fans, plus have 4 inch air exhaust constantly on sucking out intake is 3 inches or so from celing, plan to carbon filter it for night time smell, room is a windowless den, landlord has access door on other side but its sealed shut with brackets and poly over the door, at night dehu is on set at 40% and not one gnat or any type of insects, no mould, newer house, so lots air movement in day, exhaust sucks smell into my livingroom/kitchen, so I cook evenings and smell at lights off covered by garlic etc, exh fan is merely a 60 cuft/m bathroom fan with home made shroud to direct air thru a 3.5 inch connector...is top of big plastic cat litter container from costco, green in color, lol...cost all of 30 to rig it up with a dimmer switch to slow it if needed...my 2 cents

Tynehead Tom · Jan 27, 2017

I've been told that stuff like powdery mildew can't exist under 35% rh but the plants don't seem to like it that low unfortunately.

moses wellfleet · Jan 27, 2017

Cheesegez said:
you just needed a dihumidifier and decent airflow sealed rooms will go to 99%RH very quickly when lights are out

This post is on the money if you ask me. A sealed room is most vulnerable to condensation at lights off. Temps drop as soon as lamps shutdown humid air quickly reaches dew point and your buds are bathed in moisture. You need to have your dehuey running 2 hours before lights off to try dry the room out BEFORE the lights go off. Activating the dehuey at lights off is too late, it won't have time to be effective!

So to answer the OP, yes it can be the same during lights on and off, but the RH spike directly after lights off is very difficult to control. Obviously in a air exchange room you would run your fans heavily to exhaust all that humid air! I have been sealed for years now so was mainly answering from that perspective!

Cheesegez · Jan 27, 2017

Bottled Co2 expensive ? I get 35kg for around 50 quid English ....

If your having Mold problems still look into Bipolar Ionization this kills all airborne pathogens mold and odour

I know of people in Oregon using this on a commercial scale :tiphat:

Douglas.Curtis · Jan 27, 2017

Tynehead Tom said:
I've been told that stuff like powdery mildew can't exist under 35% rh but the plants don't seem to like it that low unfortunately.

Partially correct.

Cannabis does not like low RH + High temps. I've spent over a decade flowering cannabis at around 15-20% RH with LOW temps and cannabis does just peachy. :tiphat:

High temp, low humidity is low transpiration.
Low temp, high humidity is low transpiration.
Both of these environments will significantly slow the growth of cannabis plants.

Traceofvenom · Oct 28, 2018

moses wellfleet said:
Activating the dehuey at lights off is too late, it won't have time to be effective!

I am not being argumentative here but this statement is false. At least in my particular set up it is. other factors like where you live, time of year and size of your space are points you have to consider, I live in southern midwest and I have my dehumidifier set to kick on right when the lights go out. Usually, my RH is around 55% to 60% right before the lights kick off then literally, in about 30-45 minutes my RH has dropped to the low 40% range. I have it set on a timer to run 30 minutes on 60 minutes off and repeat. This keeps the RH in check in a range of 40% to 43% till the lights kick on then I don't run the dehumidifier during lights on and the RH never goes above 55%. Trying to get the most out of the equipment I have from an energy/efficiency stand point. During the late stages of flower when the fruits get fatter I will run the dehumidifier to keep the RH 45% and below till harvest. Been doing it this way grow after grow with no issues.

Peace

What percentage of RH(humidity) should ones grow room(s) be at to avoid mold?

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