Sealed room VPD question.

I have been growing in a couple 5x5 tents and they have worked fine, but I usually can tell some sort of stress on my plants by the end. I am wanting to get better control of the environmental conditions to try and reduce plant stress that I am guessing is mostly from a bad VPD. I have several questions but the first is this. In a sealed room is the goal to keep the temperature at the same point during lights off and lights on to have a consistent Temp to work against with humidity level to get to the VPD level you want? With lights out dropping temps and thereby raising the relative humidity it would seem that might get complicated even with a controller to get this dialed in. You would definitely need a controller that could handle multiple set points to handle day/night cycles. So it would seem that having a fixed temp day and night would be much easier to keep humidity where you want it but are you losing anything by not having the temperature swing?

What you said makes sense to me. Less temperature fluctuation would mean less variance in humidity levels during lights out. I guess it depends how much control a person wants. I have a quest dehumidifier/ trolmaster controller combintion, also a heat pump and mini split. Bit of an outlay to begin with but really nice for keeping things spot on and vcd optimum.

MojoRizing said:

but are you losing anything by not having the temperature swing?

Click to expand...

No. If anything you will get better growth with warmer nights. I like to drop the day temps during ripening, with light intensity and CO2 levels. This gives denser buds and better quality, limits fox tailing and even saves energy while giving more yield.

delacruz said:

What you said makes sense to me. Less temperature fluctuation would mean less variance in humidity levels during lights out. I guess it depends how much control a person wants. I have a quest dehumidifier/ trolmaster controller combintion, also a heat pump and mini split. Bit of an outlay to begin with but really nice for keeping things spot on and vcd optimum.

Click to expand...

Thanks for the comment that is almost the exact setup I am wanting to do. Quest Dehumidifier and Trolmaster controller. What heatpump/mini split did you gto with. I would like one that I know works with the trolmaster so thanks for any input.

Also how do you like the controller overall?

Renfro said:

No. If anything you will get better growth with warmer nights. I like to drop the day temps during ripening, with light intensity and CO2 levels. This gives denser buds and better quality, limits fox tailing and even saves energy while giving more yield.

Click to expand...

Thanks for chiming in. I see the knowledge that you share around here and its been amazingly helpful on several issues for me when planning this whole thing out.

Renfro said:

No. If anything you will get better growth with warmer nights. I like to drop the day temps during ripening, with light intensity and CO2 levels. This gives denser buds and better quality, limits fox tailing and even saves energy while giving more yield.

Click to expand...

What temps do you recommend shooting for consider LEDs and CO2 usage.

Well I am running HPS, canopy at 81 - 83F and CO2 at 1000 - 1200 PPM. I just start about 3 weeks before chop and incrementally bring the CO2, light intensity, temperature all down and by the last 7 days or so I am at like 500 - 600 PPM CO2, fully dimmed HPS, temps around 72 - 75 canopy. With your LED you may wanna add a few degrees.

Renfro said:

fully dimmed HPS

Click to expand...

Interesting. I've gone up in a wattage the last few weeks of growth before. How dimmed are you talking if you dont mind me asking?

Renfro said:

Well I am running HPS, canopy at 81 - 83F and CO2 at 1000 - 1200 PPM. I just start about 3 weeks before chop and incrementally bring the CO2, light intensity, temperature all down and by the last 7 days or so I am at like 500 - 600 PPM CO2, fully dimmed HPS, temps around 72 - 75 canopy. With your LED you may wanna add a few degrees.

Click to expand...

So this is one of the other questions I have about VPD. When you say your canopy temps are 81-83 is that the air temp at your canopy level or is that leaf temp? If I am using an IR thermometer to measure the leaf temp do I even need to consider the air temp in a VPD equation or just use the leaf temp to cross reference in those online charts (which are mostly all different which is very helpful).

First off I don't care about VPD and my flowering RH% indicates that. IMO VPD is a recipe for PM and bud rot when high RH% used in flower. Cold temps slow growth to a crawl. So VPD isn't gonna float in my grows.

My temps are air temps, the leaf surface is lower, around 79 - 80F.

natureboygrower said:

Interesting. I've gone up in a wattage the last few weeks of growth before. How dimmed are you talking if you dont mind me asking?

Click to expand...

From 1000 - 1150, incrementally all the way down to 600. This was against my every instinct to throw more light at them to get more flower. When I finally did it I never looked back. The bud density was so much better, terpene content (THCA is a diterpene) was increased, weight was increased, fox tailing on strains prone to such behavior was minimized or eliminated. Basically we are trying to mimic nature in the fall without the super chilly temps and high RH%.

I've been trying for a perfect vpd for 2 runs now and still think it's smart to drop temps and RH in late flower.

SwiSHa85 said:

I've been trying for a perfect vpd for 2 runs now and still think it's smart to drop temps and RH in late flower.

Click to expand...

Anyone that gets away with high RH% in late flower has to be running a mold / mildew resistant strain or just the luckiest person on earth lol.

Renfro said:

First off I don't care about VPD and my flowering RH% indicates that. IMO VPD is a recipe for PM and bud rot when high RH% used in flower. Cold temps slow growth to a crawl. So VPD isn't gonna float in my grows.

My temps are air temps, the leaf surface is lower, around 79 - 80F.


From 1000 - 1150, incrementally all the way down to 600. This was against my every instinct to throw more light at them to get more flower. When I finally did it I never looked back. The bud density was so much better, terpene content (THCA is a diterpene) was increased, weight was increased, fox tailing on strains prone to such behavior was minimized or eliminated. Basically we are trying to mimic nature in the fall without the super chilly temps and high RH%.

Click to expand...

Yep, i follow vpd until midway through flower, ever since the bud rot struck couple crops ago, no more vpd late in flower, i start dropping rh the last few weeks for sure. Better success and no bud rot since.

Renfro said:

Anyone that gets away with high RH% in late flower has to be running a mold / mildew resistant strain or just the luckiest person on earth lol.

Click to expand...

Yea first run with everything on point I had 1 single cola on a Blue Dream go bad. Caught it the day it started and dropped the whole room 4 degrees and 10% and finished out strong. Blue Dream is known to mold but air exchange and everything was perfect so had to be RH.

Renfro said:

First off I don't care about VPD and my flowering RH% indicates that. IMO VPD is a recipe for PM and bud rot when high RH% used in flower. Cold temps slow growth to a crawl. So VPD isn't gonna float in my grows.

My temps are air temps, the leaf surface is lower, around 79 - 80F.


From 1000 - 1150, incrementally all the way down to 600. This was against my every instinct to throw more light at them to get more flower. When I finally did it I never looked back. The bud density was so much better, terpene content (THCA is a diterpene) was increased, weight was increased, fox tailing on strains prone to such behavior was minimized or eliminated. Basically we are trying to mimic nature in the fall without the super chilly temps and high RH%.

Click to expand...

I could theoretically replicate that by raising my lights, correct? I’ve also experimented with an 11/13 schedule in the past that I really liked.

InThEwOoDs said:

I could theoretically replicate that by raising my lights, correct? I’ve also experimented with an 11/13 schedule in the past that I really liked.

Click to expand...

Maybe 'ish. I notice when I dim my lights my whole room is noticeably less bright. When you raise lights you just spread the light around more and do experience a decrease in canopy intensity in the middle but may actually increase it in the perimeters, and reflection losses come into play, and you aren't changing the heat input.

Renfro said:

Well I am running HPS, canopy at 81 - 83F and CO2 at 1000 - 1200 PPM. I just start about 3 weeks before chop and incrementally bring the CO2, light intensity, temperature all down and by the last 7 days or so I am at like 500 - 600 PPM CO2, fully dimmed HPS, temps around 72 - 75 canopy. With your LED you may wanna add a few degrees.

Click to expand...

interesting. I’m going to try this out this run.