[Bloom SA]

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]

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]

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

[Bloom SA]

cheers guys. There sure is a lack of info/options to read about

[2011rex87]

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]

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]

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.

[Wendull C.]

Get a bigger dehumidifier.

[petert]

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]

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.