Vapor Pressure Deficit & What It Means To You!

[Gazoo]

Great grow environment article for the aficionado!
But I have a unique humidity problem in my 4x4 grow tent that has really stumped me how to fix. My temp naturally stays a perfect 70-75 degrees with just the circ. fans and lights..... Unfortunately my humidity also stays at a constant 70+ and the only way to bring it down is by adding heat... which I don't want to do. It is commonly recommended here to bring the external room humidity down...unfortunately this is not an option because the tent is in a very large basement. I need a way to decrease humidity INSIDE the small tent... so full size dehumidifier placed inside the tent is not an option. Also, it is my understanding the smaller compressorless (Peltier) dehumidifiers do not adequately remove enough RH in a grow tent? True?... Thanks in advance for any help!
Slipp

Interesting problem, Just a quick IDEA, I have not looked into it but it just occurred to me
that WHITE RICE is a Desiccant, it absorbs moisture. So thinking if you placed a large tray
of UN-COOKED white rice in the tent it may Help.

Again I have not researched this so have no idea how often you may need to replace the
Rice but its fairly inexpensive so may be a PARTIAL solution

 

[bobrown14]

@slipp,

The problems with high-ish RH is mold.

To answer your question inside your tent - yes upping the temp will bring the humidity down - but not a lot. Say maybe down to 65RH.

The real issue is again mold.

You should be able to tell if you have mold (and mold spores) in your basement.

If its damp and musty it's probably an issue for you. Likely reason for damp basements is your rain water isn't getting far enough away from the foundation or the water table is high where you live. Water table you cant fix.

Where I live in the summer - we get high RH and high temps. We have mold outside in the form of powdery mildew its everywhere.

Cure for me is to foiler spray on an IPM (integrated pest management) and include Southern Ag - Garden Friendly Fungicide. It can be sprayed up to the day before harvest. It's a bacteria that kills mold and the spores.

It works and I only needed to treat my plants say 3-4 times in early flower. It took care of any potential issues. I had 1 plant that had PM on it and it was gone when I chopped the plant. The PM was on the LAF so I tossed LAF it in the compost bin.

This stuff I pointed to to OK to use on cannabis and certified organic in Oregon and also mild enough to not require any special clothing or protection. Its used on a lot of fruit & veggie crops as well as cannabis.

Active ingredient can be found in many products so if you in another country its likely available with a different product label.

 

[TurboBucket]

Great grow environment article for the aficionado!
But I have a unique humidity problem in my 4x4 grow tent that has really stumped me how to fix. My temp naturally stays a perfect 70-75 degrees with just the circ. fans and lights..... Unfortunately my humidity also stays at a constant 70+ and the only way to bring it down is by adding heat... which I don't want to do. It is commonly recommended here to bring the external room humidity down...unfortunately this is not an option because the tent is in a very large basement. I need a way to decrease humidity INSIDE the small tent... so full size dehumidifier placed inside the tent is not an option. Also, it is my understanding the smaller compressorless (Peltier) dehumidifiers do not adequately remove enough RH in a grow tent? True?... Thanks in advance for any help!
Slipp

Is there a reason you won't condition the entire space? Putting the dehuey in the room with the tent would be sufficient to keep you out of the danger zone.

 

[InTheShed]

Southern Ag - Garden Friendly Fungicide. It can be sprayed up to the day before harvest. It's a bacteria that kills mold and the spores.

I bought that at your suggestion and sprayed all three of my flowering plants as a precaution. So far I only got mold on one small bud, and I've left that bud on and resprayed that to see if it spreads. So far it hasn't!

mild enough to not require any special clothing or protection.

Har! I wore sweats, socks, rubber gloves, glasses, and a half-face cartridge respirator!

 

[bobrown14]

WOW - prolly protect you from the mold spores!

How do you get any trichomes on you with that outfit?? lol

I like to work in the flower room in my skivvies and a T bare foot. lol

 

[InTheShed]

Har! That was just to spray this stuff. I love getting trichomes on me

 

[slipp]

Thanks for the suggestions! Tried the rice inside tent... RH stayed same @ 70+. Will use the fungicide if I notice any Powdery Mildew or Mold.... But I would prefer to eliminate the root cause of my problem ie: The high humidity in the tent. I definitely have moisture issues in my basement because the musty smell is rather strong. Treating the entire basement with a larger dehumidifier (I have a 50 pint unit) has proven futile because it is a large space(1600 sq ft). So... I'm back to my original question: Has anyone had luck with the smaller Dehumidifiers (Peltier or other) inside the tent?
Thanks again in advance for any help!
Slipp

 

[Archiweedies]

Great grow environment article for the aficionado!
But I have a unique humidity problem in my 4x4 grow tent that has really stumped me how to fix. My temp naturally stays a perfect 70-75 degrees with just the circ. fans and lights..... Unfortunately my humidity also stays at a constant 70+ and the only way to bring it down is by adding heat... which I don't want to do. It is commonly recommended here to bring the external room humidity down...unfortunately this is not an option because the tent is in a very large basement. I need a way to decrease humidity INSIDE the small tent... so full size dehumidifier placed inside the tent is not an option. Also, it is my understanding the smaller compressorless (Peltier) dehumidifiers do not adequately remove enough RH in a grow tent? True?... Thanks in advance for any help!
Slipp

Thank you for reading my article Slipp!
I think I can help you out a little...

Thanks for the suggestions! Tried the rice inside tent... RH stayed same @ 70+. Will use the fungicide if I notice any Powdery Mildew or Mold.... But I would prefer to eliminate the root cause of my problem ie: The high humidity in the tent. I definitely have moisture issues in my basement because the musty smell is rather strong. Treating the entire basement with a larger dehumidifier (I have a 50 pint unit) has proven futile because it is a large space(1600 sq ft). So... I'm back to my original question: Has anyone had luck with the smaller Dehumidifiers (Peltier or other) inside the tent?
Thanks again in advance for any help!
Slipp

Alright man, in case you missed it, I grow in SoCal in an uninsulated garage. I have a 4x8 and the right side of my tent is devoted to my environmental control apparatus. I have a dual hose AC and a 30 pint Dehuey INSIDE my tent.

You can do it if you can make the room in a tent. I bought a 6x6 tent but the supplier sent a 4x8 instead. This was a blessing because the extra space is awesome. Basically, I grow on one side of the tent and the other side is just gear.

I want to caution you because, as you likely are aware, the dehuey is going to produce heat as a byproduct and that heat has to be either vented out really well or you may need to cool your room as well.

I know that’s probably not what you want to hear, it wasn’t what I wanted to do either but it’s what I had to go through for my unique set of circumstances. Cheers and I hope that helps man. Please feel free to keep any questions coming.

 

[Gazoo]

I also have to improvise when I run the De-Humidifier, It has to be vented out side or it really
heats up the space.

Also have had issues with it shutting down do to The Coils Icing up , This will
commonly happen if I run it Continuously. Seems some models are worse than others with this
issue

 

[TurboBucket]

Thanks for the suggestions! Tried the rice inside tent... RH stayed same @ 70+. Will use the fungicide if I notice any Powdery Mildew or Mold.... But I would prefer to eliminate the root cause of my problem ie: The high humidity in the tent. I definitely have moisture issues in my basement because the musty smell is rather strong. Treating the entire basement with a larger dehumidifier (I have a 50 pint unit) has proven futile because it is a large space(1600 sq ft). So... I'm back to my original question: Has anyone had luck with the smaller Dehumidifiers (Peltier or other) inside the tent?
Thanks again in advance for any help!
Slipp

For the sake of the indoor air quality I would be upgrading my home dehumidification.

If you keep the air in the rest of the house more dry you went need the dehumidifier for your tents specifically and your home would be more comfortable. Lowering the Rh in your home will also reduce the load on your air conditioning system. I have two 50 pint units in my house. One in the basement living room and one on a night timer in the room my tents are in. When we can afford it I'll install a bigger single unit into the duct work and get rid of the living room unit.

 

[Archiweedies]

That’s a great point Turbo. As luck would have it, the humidity stays pretty much 45-50%RH in my tent without any input on my part. Thats with lights on, we all know the RH shoots up during the dark cycle and that hurts. That’s good enough for veg and flower if we aren’t necessarily beating ourselves up to try and keep VPD locked in.
@slipp I looked at your posts but I couldn’t tell if you are experiencing high RH at lights on as well. If you aren’t, you might consider just running autoflowers for a while. That way you can keep the lights on as long as you want really. I’ve run autos at 24/7. Just a thought. Cheers

 

[Gazoo]

Just finished an interesting article comparing LEAF SURFACE Temperature using
HPS, MH, CFL and LED Grow Lights . Granted it was written by a LED light maker

It actually confirms what I have noticed. Using MH/HPS and T5s I should/need to keep
the AMBIENT TEMP at 77F for best LEAF TEMP readings

Noticed if the AMBIENT Temp in my tent goes above 79 F I start to see LEAF TEMP STRESS
in the form of + readings compared to Ambient.
EXAMPLE
at 81/82 F ambient, leaf temp is 84+
at 80 F ambient its + 1 or 81 F deg leaf temp
at 79 F ambient its THE SAME, Leaf temp = ambient

I suspect if I can get it to 77 F Ambient I would be at the -2C range that is charted on
one of my VPD charts (I have to adjust now for the zero difference to use that chart)

CONCLUSION after reading this article it Looks like if using LED you can run a higher Ambient

I can now confirm KEEPING my ambient Lights on temp between 75 to 77 F
Will give me the needed MINUS leaf TEMP needed to keep my VPD range and
allow me to keep the R.H. on target

 

[bobrown14]

Dont the plants have the ability to create a micro climate that suites there needs right at EACH leaf?

I'm thinking this is why we see curling, turning toward the breeze (fans) and turning away or turning towards the light (to change temps and also make food). The leaves have many functions not just using light energy and transpiration.

I think they have the ability to change the environment around them quite a bit more than we give them credit for.

 

[Pennywise]

It shows that plants are aware of their surroundings and will manipulate temps, light and RH to their advantage if we let them.

 

[Gazoo]

Dont the plants have the ability to create a micro climate that suites there needs right at EACH leaf?

I'm thinking this is why we see curling, turning toward the breeze (fans) and turning away or turning towards the light (to change temps and also make food). The leaves have many functions not just using light energy and transpiration.

I think they have the ability to change the environment around them quite a bit more than we give them credit for.

I am sure they can. Checking leaf temp at canopy I will get different temps at different spots, even
on the same Blade. I just want to make it as easy on them as I can

Not sure about the science and proof but have always understood if the leaf temp is higher than
the ambient that is a sign of heat stress ? have only been able to accomplish the minus temps recently by lowering
my Ambient to the mid 70s. Seems to be working well

Edit Oh I also keep the RH at 60 to 70% rh, early veg

 

[bobrown14]

Yeah I agree keep the proper temps and RH under control and we dont have to worry about it.

For 2 months a year (gonna be the last time I hope this year) I have crazy high temps and RH all over the place. All I really do is give the plants unlimited water supply and fans on high moving a lot of air in and out so they can transpire a lot.

I get very large plants this way, thats sort of a plus. Some plants can handle the high heat and some can not.

IF I ever grow in heat like this again I'll run equatorial Sativas and do a short VEG time.

 

[Gazoo]

In my down stairs environment i use SIX Humidifiers to keep it in check.
Found each COOL MIST humitifer will lower the ambient by 1 to 2 deg F.
The T5s burn off the Humidity very Fast, but I have been able to control it with
3 room units combined with the 3 smaller cool-mist units and I can keep it 75 to 78F and 63 to 69 RH.

616W of Light I need 210W of humidifiers + 125W of fans (intake, Hood cooling Exhaust, 6 Inch fans)

Cant wait to open up my 4 x 8. Should be able to control things much easier
with a closed tent

 

[Baron Von Blurple]

Read this and is having a light bulb moment.

 

[TurboBucket]

Mine is almost the opposite. I need to run a 50 pint deheuey in the tent room and an additional 70 pint in the living room to keep overall humidity through the house at ≈ 50%

 

[Archiweedies]

Read this and is having a light bulb moment.

I’m glad you’ve found it useful brother!

Mine is almost the opposite. I need to run a 50 pint deheuey in the tent room and an additional 70 pint in the living room to keep overall humidity through the house at ≈ 50%

Yeah this down to your regions climate a lot of times. It’s hot and dry where I live and therefore I had to go big on my humidifier. On the flip side, I get by with a tiny 30 pt. Dehuey and it rarely turns on.
Cheers guys!