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Humidity too high

Bloom SA

Member
Hi guys
Plants have really done well during veg cycle in my greenhouse. The problem is humidity at night time now that they are in flower. Where i am, summer humidity is up to about 90% at night time with temperature between 18-20 degrees celcius. Not sure what that is in farenheit but it is minimum temperature for good growth. I bought a dehumidifier and mounted onto my humidity controller. Day time humidity inside drops to about 50% with max temperature being around 33 celcius. So day time with ventilation running keeps humidity nice and low. Night time ventilation doesnt help as outside humidity is way too high so i close up the vents and let dehumidifier do its thing. The problem is the dehumidifier only drops it to about 80% rh. Greenhouse is 3mx3mx2m or 10ftx10ftx 7ft. I sealed the whole thing up with about 5 tubes of silicon. It has 8 plants inside as well as 3 tomato vines. Are the plants dumping too much moisture at night? They only get 1.5lt each every second day and early on in the day and have not shown any heat or dehydration stress so im not pumping water into the greenhouse excessively and water only ever goes directly onto rootzone of plants, I dont spray water everywhere.
What can i do to dehumidify further bearing in mind i just blew about 300 dollars on a dehumidifier thats supposed to dehumidify 45sq metres. It pulls out about 5 lt of water per night. Have i crammed too many plants into this space? What is a roundabout figure of what a plant transpires in water vapour at night time? The dehumidifier is an olympia spendid aquaria thermo 16 condensor dehumidifier with a built in 1000w heating element. I normally run it on "laundry mode" which is max power non stop. Where have i gone wrong here? What can i do to drop to max 50% rh at night
 

Itsmychoice

Well-known member
ICMag Donor
My advice

My advice

Raise the temperature with a pellet or wood stove. If that's not an option use an electric heater or get a better dehumidifier. Defoliating all the small lower and inside stuff will also make a big difference.
 

Douglas.Curtis

Autistic Diplomat in Training
When I cannot control humidity, I make sure air is flowing through every part of the plant. Whatever else you do, be sure to trim leaves and thin under branches until air flows very well. Leaves touching constantly will cause a problem area.

Wish I had more info
 

2011rex87

Member
Hmm this a tough situation due to the fact that you are sealed up inside with the RH outside being so high...

50%RH is high for flower cycle once buds develop. 50% is okay for the first week or two. Week 1-2 50%, week 3-4 40%, week 5-6 30%, and try to drop the RH a low as possible for the final week or two as the buds will try to protect themselves from the low RH by forming MORE RESIN glands AKA more Trichomes. Most growers bottom out around 25% or 20% RH.

How will you accomplish this? HMMMM If you want to save your cannabis crop perhaps you will need to sacrifice the tomatoes. I say that because you want to remove as much wet media (aka soil in pots) as possible to reduce the overall amount of moisture inside.

Try to only water when the sun/lights first come on. This way the plants have all day to drink the water and the pots should be dry before the lights go out or before the sun goes down. Not too dry but dry enough so that your plants will be ready to water first thing when the lights come back on. Remember they won't be drinking with the lights off specially if the RH is 90% they will not drink at all like that. So make sure those pots are dry just before lights out. This way you do not have wet media in your green house in the dark when the RH outside is already so high.

As the previous poster said, artificial heat can dry out your environment. An electric space heater that uses a hot coil with a fan blowing over it could work to drop the RH a few points depending on how powerful the space heater is. That said, you will want to monitor the temperature because you would never want to overheat your grow space for the sake of dropping the humidity. However, plants can generally tolerate heat well when everything else is on point - so you could get away with temps as high as 85f but at the same time you want your daytime temps to be slightly higher than your night time temps.

No real easy way out if you do not want to spend money on more dehumidifying power. Some pictures could help us advice you on your HVAC situation.

Generally when the RH is too high inside the grow space you would exhaust all that air out but ONLY IF the incoming air itself is not too humid. Is there any way you can intake air from a dry source? If your grow space is close to a house or a garage where the RH is lower, that could be a source of dry air. Something to think about..
 

2011rex87

Member
Also one more note - make sure your dehumidifier is draining the water outside of the grow space... It is a common mistake growers make to simply drain the dehumidifier into a 5 gallon bucket or tub inside the grow space. Drain to the outside.
 

Lester Beans

Frequent Flyer
Veteran
Yes raise the heat. Wood fire would be great as it's dry heat but any heat will help. Also as suggested above, make sure you have a lot of air moving around in there.
 

petert

Member
Defoliate as much as you can tolerate, more air movement, room heater, and make sure you attach a hose and run the drain hose outside the tent.
I’m in a 10x10x8 tent with 16 plants inside. I empty my dehumidifier twice a day. More than a gallon each time. And my RH is nowhere near yours! Is your tank filling and sitting for any period of time?
 

Spaventa

...
Veteran
Aspirin. 1 dissolved /litre. Apart from being salicylic acid that is good for all sorts of other things in plants, its an antitranspirent. It works on the stomata opening.
 

Bloom SA

Member
Aspirin. 1 dissolved /litre. Apart from being salicylic acid that is good for all sorts of other things in plants, its an antitranspirent. It works on the stomata opening.

Gotta love ICMag, you folks are very helpful. Curious AF on the aspirin. When does one apply it and how often? Won't stomata being closed be more harmful than good once the sun rises again?
 

Spaventa

...
Veteran
Gotta love ICMag, you folks are very helpful. Curious AF on the aspirin. When does one apply it and how often? Won't stomata being closed be more harmful than good once the sun rises again?

Antitranpirants are widely to increase crop yields, quality, resistance to pests, disease etc etc They use them on fruit and veg to increase tumescence/swell - nothing harmful, they will just transpire less and compensate by drinking less - that the other thing you should see - all that water that ends up in your dehumidifier.. more of it will stay in your res.
Water it in dissolved at 1 pill per litre and repeat as needed, maybe each week but let your plants and RH/water consumption guide you.
 

farmerlion

Microbial Repositories
Mentor
Veteran
Bloom SA. one simple this that costs you nothing. Is to water in the evening. Your moisture will soak and spread more thoroughly. Your plants will take what they need. I have also used multiple heavy wire plant markers. I pull them out before watering so the water drains down better, them replace markers. I use the markers as a cheat sheet for specific plant nutritional requirements. Personally I'm not a big fan of defoliating a plant until a few days before harvest. But I do remove dead and dying leaves thoroughly, daily. Defoliating isn't wrong by any means. If you over do it you screw yourself hard. Peace
 

Bloom SA

Member
Bloom SA. one simple this that costs you nothing. Is to water in the evening. Your moisture will soak and spread more thoroughly. Your plants will take what they need. I have also used multiple heavy wire plant markers. I pull them out before watering so the water drains down better, them replace markers. I use the markers as a cheat sheet for specific plant nutritional requirements. Personally I'm not a big fan of defoliating a plant until a few days before harvest. But I do remove dead and dying leaves thoroughly, daily. Defoliating isn't wrong by any means. If you over do it you screw yourself hard. Peace

very interesting point of view this. Everything ive read so far says to only water early on in the day due to RH issues at night
 

redlaser

Active member
Veteran
What are your outside temps at night? I’ve been dealing with the same issues, last fall I just either let the exhaust run all night or exhaust for 10-15minutes per hour on a timer. I’ve talked with others that only vent 2-3 times nightly, some call it burping the baby.

I was running into December of last year, air temps hitting 30-33F (0 celcius). Soil temps still in mid 60’sF. I have propane heat, but quit using it because of the humidity. Everything was actually going pretty well before winter rains came. Up until November I could keep humidity at night at 60-65%, and temps 73-78 degrees, which is where I want it to be. Once the first inch of rain fell after no rain for six months my undersized dehumidifier was overwhelmed.

I plan on adding a wood stove of some sort, debating pellet versus regular heat since firewood is abundant. Should help with humidity as well. I’ve seen electric heated houses but unless you get cheap electricity, kind of expensive.
 

farmerlion

Microbial Repositories
Mentor
Veteran
I'm very far north, RH hasn't been an issue for me. All locations present their own problems. Best of luck. Peace
 

2011rex87

Member
Keep in mind if you are supplementing CO2 into the grow space you are not going to want to exhaust that out before your plants have a chance to benefit. That can complicate your humidity/environmental issues. Are you supplementing CO2 at all? Sounds like you are not but it's good to ask...
 

hamstring

Well-known member
Veteran
Hoping to build my own greenhouse next year. Like the original poster I have concerns about high humidity too. I live in the midwest and come October the rains start. It would seem even with fans you are bringing in humid air?

I was looking online at greenhouse heating to control humidity. I have zero experience but wanted to ask others about the some of what I found.

First most of the articles you find center around heating rather than to control humidity.

Wood Stoves- Easy to use and maintain but only have a 4hr burn time before they need to be reloaded.

Pellet Stoves-Don't need to be refueled but heard the augers can rust easily and stop working. Especially in the summer when not being used and higher humidity conditions.

One thing many recommended was drip irrigation over hand or spray to cut down on excessive moisture.

Which begs another question for MJ cultivation are containers better than raised beds in a green house to control humidity during flowering? Was hoping to go with raised beds and go completely organic, earth worm, living soil etc but sounds like could have a lot excess moisture in the ground to maintain this soil culture.

Sounds like a damed if you do damed if dont situation.
 

Bloom SA

Member
Keep in mind if you are supplementing CO2 into the grow space you are not going to want to exhaust that out before your plants have a chance to benefit. That can complicate your humidity/environmental issues. Are you supplementing CO2 at all? Sounds like you are not but it's good to ask...

No CO2 supplimentation. I've made a supersoil with horse shit, bonemeal, chicken shit, molasses, dolomite lime, river sand and a huge army of red worms. I was hoping the soil alone with the decaying Organic matter, red worms and mycorizal fungi would bump up co2 levels but i havent tested the air. I don't run any ventilation at night, only the dehumidifier and oscillating fan so theoritically at first light there should be plenty of co2 from what the plants let out into the greenhouse at night
 

2011rex87

Member
Which begs another question for MJ cultivation are containers better than raised beds in a green house to control humidity during flowering?

I would think you want to minimize the surface area for the moisture to evaporate off into the air so maybe pots would be better for you than a raised bed but you will lose the benefit of all the extra cu/ft of soil.

Flowering in high humidity is not necessarily a death sentence. I want to put that out there incase you end up having to try it. It does increase the risk of mold but it's not a promise that you will get moldy buds just because. You can take other measures to try to mitigate mold like avoiding the use of wood in the grow space and filtering all your incoming air, cleaning your grow space, etc. I have seen great plants grown in high humidity, it's just not always ideal. So don't panic and just keep on and maybe you'll have a great success in the end regardless.
 

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