Humidity Issues! Help

Hello forum lovers.

I am deep into my grow (12 weeks) and my humidity is at 87%
I had to pick off some mould on one bud. The temp is cold here in Australia 17 degrees (62 Fahrenheit).

In a grow tent and i have only ever used a small fan. I don’t have an exhaust fan.

Any tips on reducing the humidity please???


Are you in flower currently?

Air exchange can help manage humidity. Suggest both input and exhaust fans. It should lover humidity assuming the humidity in the air outside the tent is < 87%.

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Yes I am, tipping I am only 2 weeks away from harvest

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Unfortunately, that’s probably not going to happen, buddy.

If you can see fungal hyphae on the outer part of a cola, your plants are (IMHO) unusable. Do you have a picture?

You need to regulate your environment if you hope to prevent this from happening again. Cycle fresh air in, stale air out, and control temperature and humidity.


No I picked it off. It was only a small amount of furry looking mold.

That’s not really the way mold works though. You cut off exposed hyphae, but the problem is probably systemic at that point. Sometimes grows just don’t work out. Like I said, you have to regulate your environment to avoid this in the future.

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To help reduce moisture, try using a large bag of charcoal. It’s a natural moisture gatherer. You’ll have to change it every 1-3 days but could help.

I would use a heater, dehumidifier, air conditioner, ventilation, or some combination of those 4 things to get the humidity down into an acceptable range.


So I have been fighting rh late in flower too albeit not quite as high as yours (mine was max 65% during lights out) I tried many things feverishly lol…bought damp rid products and placed them inside the tent and in or by all the openings…had virtually unmeasurable results…bought an “Eva-Dry” unit and had unmeasurable results…I don’t know where you’re set up but I am in my basement which was great for humidity in veg but in my battle I ended up A- placing baking soda in a large tray inside my troublesome tent and I bought the “pass through” style boxes of baking soda and put them in all the unused sleeves of the tent and even outside of any of the bottom rectangle vents with Velcro covers. I also cut a vent into my central air duct that runs right by my tents so it’s constantly cooling the air (partly because I haven’t run 30’ of duct to get the tent exhaust away from the tents)as well as having a tiny 4"x6" space heater running during lights out …I have my central air set on no lower than 72 at any point of the day and i seem to be on top of the humidity battle finally.

My ac infinity screen now never reads above 60%…in my 4x4 tent it’s effortless because I have a 600w mh bulb and a 1000w KingLED so the heat naturally battles the rh…I have a hlg 600rspec in my troublesome 5x5 which doesn’t create nearly the heat.,…all that being said I feel it’s extremes lol but necessary …my electric app says I pay $7.13/day now…but it also says only 9% higher than last year… probably cuz I weld at home regularly…useful or useless I guess that’s my 2¢

Gidday mate I in vic had huge issues last winter dehumidifier for 8ft tent $500 bar oil heater can get pretty cheap. These two things along with good air in. Things sitting pretty good. How big your tent?

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He’s referring to vpd.

It’s a science, explained in these charts with use of an infared thermometer to read leaf temperature. Then using the tent temperature minus the leaf temperature to find your vpd.

Ofc without actually reading the leaf temperature many of these charts seen shared as guidelines. When they are and aren’t… As a set of 0-5° they are. If you only have one it’s useless to what it’s trying to convey.

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Basically lights out is the worst time to have high humidity.

Bloody hell
Gotta delete my post.

Hi mate, 120 x 120 x 200cm size grow tent. Got a dehumidifier in there today. Hoping when I go check in a couple of hours walls are dry and humidity has dropped.

I opened the tent up and had the fan running today. Dropped to 70%

70 fine in veg but got to drop it in flower especially if have issues keeping it warm Or will learn all about the dreaded mould. Good thing dehumidifier. . Good luck mate.

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The dehumidifier is making a difference in terms of the walls are no longer wet.
Can’t get the humidity to drop past 70%

Is it because it is too cold in Melbourne? Need more heat in tent?

Yes. Good chance. And maybe lack of airflow

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During the day (light off period) in the dark garage I open side of tent with fan on to get fresh air in