Humidifier/Echaust question

I try for best the best vpd the conditions allow sometimes its ideal other times its well out.
Screenshot_20240818_222541_Engbird.jpg
the uk isn't great for growing its cold and rarely below 90% rh, surprisingly rot isn't a big problem I think the humidity tale has been blown way out of proportion, humidity is conductive to mould no doubts but live plants aren't, with good air flow 90% healthy plants will take mould spores in their stride.
Folk in low humidity areas of the world still have bud rot issues if the plants aren't healthy, I'm not trying to promote high humidity lol I'm saying chill 60/70% isn't too concerning to healthy plants.
 
If you plug in leaf temps +2-3 degree you pretty much get what ive been recommended by growbro/growshop dudes for HPS growing for years.
Without that radiant heat the vpd recs look very different and scary to someone used to hps grows, it looks very mold inducing. But with good air flow this doesnt seem to happen. I can understand someone in a tent being scared of those high rh but it really isnt much more difficult than moving air above and below cannopy, and lolipopping anything to deep down, something you should anyway really. If you cant keep a match lit in any part of your grow space, you eont get mold unless you introduce it by carelessness intracting from a moldy place or if youve gotten mold spores in you led heatsinks that you cant clean.
VPD doesnt grow great weed, it increases transpiration. You can run vpd at the lower range of transpiration if you compensate by raising nute levels; less volume of a stronger juice. In any case its more of a guideline, showing where to take your environment when you have transpiration related issues. See slouchy plants; check which way to go with vpd.
 
Well, somehow I didn’t notice all the replies to my thread. Thank you
Star Dog said:
I try for best the best vpd the conditions allow sometimes its ideal other times its well out.
View attachment 5437238
the uk isn't great for growing its cold and rarely below 90% rh, surprisingly rot isn't a big problem I think the humidity tale has been blown way out of proportion, humidity is conductive to mould no doubts but live plants aren't, with good air flow 90% healthy plants will take mould spores in their stride.
Folk in low humidity areas of the world still have bud rot issues if the plants aren't healthy, I'm not trying to promote high humidity lol I'm saying chill 60/70% isn't too concerning to healthy plants.
Click to expand...
I’ve read several people state that they grow in 70s humidity through flower and with good airflow they never have bud rot. Either way I decided to raise the airflow and vpd as I watered them all really good yesterday and gave them some excellent worm castings with live worms inside from uncle Jim’s worm farm. Scratched in some espoma garden tones about 2 teaspoons a plant (1tsp for the little ones in 2 gallon). The other big plants are in 3 gallons. One thing I’m noticing is I believe the small veg tent is in need of some fresh air, so I’m gonna go and put a small exhaust fan on it. May end up buying another drum fan to move the air around in this room.
 

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Rocket Soul said:
Yo always need exhaust or humidity will slowly rise to 100% or close to it. I guess if you open every once in a while the tent but really some exhaust is necessary.
You can try extracting from the bottom and intracting from the top; this will keep the tent more hot and moist
Click to expand...
Yes I’m noticing with my small grow tent that the plants are drooping and don’t look as lively because the airflow is not as good in there. I lm sure the stagnant air is doing them good
 
Rocket Soul said:
If you plug in leaf temps +2-3 degree you pretty much get what ive been recommended by growbro/growshop dudes for HPS growing for years.
Without that radiant heat the vpd recs look very different and scary to someone used to hps grows, it looks very mold inducing. But with good air flow this doesnt seem to happen. I can understand someone in a tent being scared of those high rh but it really isnt much more difficult than moving air above and below cannopy, and lolipopping anything to deep down, something you should anyway really. If you cant keep a match lit in any part of your grow space, you eont get mold unless you introduce it by carelessness intracting from a moldy place or if youve gotten mold spores in you led heatsinks that you cant clean.
VPD doesnt grow great weed, it increases transpiration. You can run vpd at the lower range of transpiration if you compensate by raising nute levels; less volume of a stronger juice. In any case its more of a guideline, showing where to take your environment when you have transpiration related issues. See slouchy plants; check which way to go with vpd.
Click to expand...
I really am scared of mold so I took heed to what others warned me and did raise the vpd for my plants that are still early in flower this month. It’s sitting around 68rh and 84f in my 2x4. Plants must be around 20 days in
 
Avering said:
Well, somehow I didn’t notice all the replies to my thread. Thank you

I’ve read several people state that they grow in 70s humidity through flower and with good airflow they never have bud rot. Either way I decided to raise the airflow and vpd as I watered them all really good yesterday and gave them some excellent worm castings with live worms inside from uncle Jim’s worm farm. Scratched in some espoma garden tones about 2 teaspoons a plant (1tsp for the little ones in 2 gallon). The other big plants are in 3 gallons. One thing I’m noticing is I believe the small veg tent is in need of some fresh air, so I’m gonna go and put a small exhaust fan on it. May end up buying another drum fan to move the air around in this room.
Click to expand...
The key issue for bud rot is temperature. When temps fall <75, that's favorable for botrytis.

Another way to reduce the chances for bud rot is to use a silica supplement. The addition of silica improves the strength of cells and will tend to stave off mold and mildew.
 
Rocket Soul said:
If you plug in leaf temps +2-3 degree you pretty much get what ive been recommended by growbro/growshop dudes for HPS growing for years.
Without that radiant heat the vpd recs look very different and scary to someone used to hps grows, it looks very mold inducing. But with good air flow this doesnt seem to happen. I can understand someone in a tent being scared of those high rh but it really isnt much more difficult than moving air above and below cannopy, and lolipopping anything to deep down, something you should anyway really. If you cant keep a match lit in any part of your grow space, you eont get mold unless you introduce it by carelessness intracting from a moldy place or if youve gotten mold spores in you led heatsinks that you cant clean.
VPD doesnt grow great weed, it increases transpiration. You can run vpd at the lower range of transpiration if you compensate by raising nute levels; less volume of a stronger juice. In any case its more of a guideline, showing where to take your environment when you have transpiration related issues. See slouchy plants; check which way to go with vpd.
Click to expand...
Amen!

It's amazing to see the amount of mishegas about VPD. It's not just here on RIU. I frequent another cannabis site and people get completely twisted around the axle there, as well.

VPD is not the Holy Grail. It's just a way of combining temperature and RH to help plants grow well. If VPD is 1 or so and 1.2 or higher in flower, you're going to do fine. As you point out, as temperature increases and/or RH drops, plants have to transpire more. If temperature is really high and/or RH is really low, transpiration can get so high that lots of nutes get taken up and nutrient issues will show up.

Everyone running around with their hair on fire will not that I wrote "temperature increases and/or RH drops". Well, "VPD" is just an easier way to put and sure as heck is easier to use with a controller device.

When growers have nutrient issues, check that they've mixed the nutes correctly (I f'd up 28 gallons of nutes yesterday but caught it before I swapped the res, so yeh, dumbshit happens) and if they're watering correctly. If both of those are being done right, check the temp and RH and it's amazing how often those two parts of the grow environment are out of whack. A little hot and dry or a little cold and wet aren't a big issue. But when growers are running EC 2.5 with 45% RH and 85°, things aren't going to work out well.

VPD doesn't have anything to do with that condition. It's too hot and dry or, in some cases, cold and wet, and the nute strength needs to be adjusted.

And the idea that it's only something that works in a laboratory - that's true only if the laws of physics don't apply to cannabis plants.
 
Avering said:
I really am scared of mold so I took heed to what others warned me and did raise the vpd for my plants that are still early in flower this month. It’s sitting around 68rh and 84f in my 2x4. Plants must be around 20 days in
Click to expand...

This is from Mitch Westmoreland's super videos on cannabis cultivation. He's a PhD student studying under Bugbee and he released a video about a year ago (title in the screenshot) in which he discusses four major topics re. cannabis, on of which is temperature.

The graphic below is from his recent video but he also did a video in 2021 that discusses cannabanoid levels and temps in flower. Bottom line - if the temperature at the tops of flowers is in the 80's, your cannabinoid levels will plummet.

I've generally grow with ambient temps in the low 80's in flower but I've had to re-think things after this research.

The 78° mark is the highest it should be. Some cultivars should be kept cooler.

Check out the video. It's excellent.
1730492688907.png
 
Delps8 said:
Amen!

It's amazing to see the amount of mishegas about VPD. It's not just here on RIU. I frequent another cannabis site and people get completely twisted around the axle there, as well.

VPD is not the Holy Grail. It's just a way of combining temperature and RH to help plants grow well. If VPD is 1 or so and 1.2 or higher in flower, you're going to do fine. As you point out, as temperature increases and/or RH drops, plants have to transpire more. If temperature is really high and/or RH is really low, transpiration can get so high that lots of nutes get taken up and nutrient issues will show up.

Everyone running around with their hair on fire will not that I wrote "temperature increases and/or RH drops". Well, "VPD" is just an easier way to put and sure as heck is easier to use with a controller device.

When growers have nutrient issues, check that they've mixed the nutes correctly (I f'd up 28 gallons of nutes yesterday but caught it before I swapped the res, so yeh, dumbshit happens) and if they're watering correctly. If both of those are being done right, check the temp and RH and it's amazing how often those two parts of the grow environment are out of whack. A little hot and dry or a little cold and wet aren't a big issue. But when growers are running EC 2.5 with 45% RH and 85°, things aren't going to work out well.

VPD doesn't have anything to do with that condition. It's too hot and dry or, in some cases, cold and wet, and the nute strength needs to be adjusted.

And the idea that it's only something that works in a laboratory - that's true only if the laws of physics don't apply to cannabis plants.
Click to expand...
Its also notable that VPD is only one factor that will affect transpiration, CO2 levels, spectrum, light intensity and air pressure are a few more. So in essence you shouldnt be targeting a specific VPD value, you should be targeting a plant posture indicative high transpiration together with high nutrient load without defs or toxicity. Its only cause with leds and low leaf temps this factor becomes outsized in importance, but in the end it should always be the plant that gives you your cues. I think of vpd more as an indicator in case of transpiration related issues (saggy and shitty plant posture) occur; it shows the direction where to take your climate but likely not the exact end destination. And part of the extra heat regime is down to metabolism needing it, which strictly speaking isnt the same as vpd and can likely be mediated with some heat (though not constant!) around the rootzone.
Though in general i would go the other way around with leds and defs: first establish: is this plant transpiring properly and if not why? Fix transpiration and then dial in nutes: not really great if you fox your nutes but plant is still doing shitty, only to get toxicity when the plant is back into full transpiration.
More of weaker juice, or less of stronger juice; its generally better to go the more of wealer juice but then you need to see at least how much drinking your environment and plant in general can do to start with. If you can get it there then try colder and drier in the end; it will help with crop quality, but if you cant get there (our case) then you have to strike a balance.
 
Last edited:
Avering said:
I really am scared of mold so I took heed to what others warned me and did raise the vpd for my plants that are still early in flower this month. It’s sitting around 68rh and 84f in my 2x4. Plants must be around 20 days in
Click to expand...
Main way of fighting bud rot is by air movement all the way thru your grow space, it will help more than climate adjustment. If in a tent and no space for oscillating fans under cannopy try this: horizontal fans at pot levels with air flow direct towards a vertical surface: pots or walls, making sure there is plenty of "diffused" air movement but not leaves trashing and getting wind burnt. Its not climate above cannopy causing molds its the intra cannopy environment that is the main culprit. Ymmv :)
 
Rocket Soul said:
Main way of fighting bud rot is by air movement all the way thru your grow space, it will help more than climate adjustment. If in a tent and no space for oscillating fans under cannopy try this: horizontal fans at pot levels with air flow direct towards a vertical surface: pots or walls, making sure there is plenty of "diffused" air movement but not leaves trashing and getting wind burnt. Its not climate above cannopy causing molds its the intra cannopy environment that is the main culprit. Ymmv :)
Click to expand...
I was thinking of buying some small oscillating fans for the bottom of the tent, there still is some space. Do you have any you would suggest?
 
Rocket Soul said:
Its also notable that VPD is only one factor that will affect transpiration, CO2 levels, spectrum, light intensity and air pressure are a few more. So in essence you shouldnt be targeting a specific VPD value, you should be targeting a plant posture indicative high transpiration together with high nutrient load without defs or toxicity. Its only cause with leds and low leaf temps this factor becomes outsized in importance, but in the end it should always be the plant that gives you your cues. I think of vpd more as an indicator in case of transpiration related issues (saggy and shitty plant posture) occur; it shows the direction where to take your climate but likely not the exact end destination. And part of the extra heat regime is down to metabolism needing it, which strictly speaking isnt the same as vpd and can likely be mediated with some heat (though not constant!) around the rootzone.
Though in general i would go the other way around with leds and defs: first establish: is this plant transpiring properly and if not why? Fix transpiration and then dial in nutes: not really great if you fox your nutes but plant is still doing shitty, only to get toxicity when the plant is back into full transpiration.
More of weaker juice, or less of stronger juice; its generally better to go the more of wealer juice but then you need to see at least how much drinking your environment and plant in general can do to start with. If you can get it there then try colder and drier in the end; it will help with crop quality, but if you cant get there (our case) then you have to strike a balance.
Click to expand...
Agree.

VPD is one number for two of these. And, unless you really muck up, temp and RH (VPD) aren't high on the list. I've never grown anything else but it does seem that cannabis is very hardy.

10 Parameters of Growth.png
 
Avering said:
I was thinking of buying some small oscillating fans for the bottom of the tent, there still is some space. Do you have any you would suggest?
Click to expand...
Its probably a bad idea to go for oscillating; id rec small standing fans directed towards a vertical surface; oscillating is likely to fall and direct airflow towards bottom cannopy and trash the leaves and wind burn your crop. Diffuse air movement seems to work but i never done tent grows, only outdoor and open space where we had enough height to have +18" of clean stem before cannopy and leaves start. But from what others have tried it seems like the airflow broken by walls/pots seem to work fine. Ymmv, im actually also after another solution like this and awaiting word from another gear head i know making investigations in alichina.
 
Delps8 said:
Agree.

VPD is one number for two of these. And, unless you really muck up, temp and RH (VPD) aren't high on the list. I've never grown anything else but it does seem that cannabis is very hardy.

View attachment 5437285
Click to expand...
I agree. My main point is that in order to get the final reading on all these factors conpounded you still have to be able to read the plant like a proper gardener; this is why most autosystems are doomed to fail unless you make some evaluation and adjustment as a grower: if the plant is not happy it doesnt matter if you have all you factors supposedly dialed in, the plant knows better when its happy than any tables or online recommendations:) a grower still has to grow it, almost like a symbiot. Automation and controllers can help you but youll still always need to look and understand the plant.
 
Avering said:
I was thinking of buying some small oscillating fans for the bottom of the tent, there still is some space. Do you have any you would suggest?
Click to expand...

Fan on left is an AC Infinity (osc), white fan is a no name brand, AC Infinity next to it (on the bar), and I found the little fan in the lower right on line at Walmart. It's a USB fan with two speeds and it comes in pretty handy. The hose coming into the tent on the right is for an AC Infinity heater.
1730499021284.jpeg
 
Rocket Soul said:
I agree. My main point is that in order to get the final reading on all these factors conpounded you still have to be able to read the plant like a proper gardener; this is why most autosystems are doomed to fail unless you make some evaluation and adjustment as a grower: if the plant is not happy it doesnt matter if you have all you factors supposedly dialed in, the plant knows better when its happy than any tables or online recommendations:) a grower still has to grow it, almost like a symbiot. Automation and controllers can help you but youll still always need to look and understand the plant.
Click to expand...
We're singing from the same hymnal.

"A fool with a tool is still a fool."
 
Avering said:
I was thinking of buying some small oscillating fans for the bottom of the tent, there still is some space. Do you have any you would suggest?
Click to expand...
Might also consider a tower fan. Small footprint to air movement ratio, and most of them have an oscillating function.
 
weedstoner420 said:
Might also consider a tower fan. Small footprint to air movement ratio, and most of them have an oscillating function.
Click to expand...
Theres two ways to make good air movement in a tent or room,.oscillating: good for clear spaces like on top of a flat cannopy. You can even make it hit the light a bit but you want your leaves to only tremor and move ever os slightly, no trashing!
The other way is stationary fans in several spots making air flow in directions or vortex or even just straight into the wall; you basicly have to design a system which makes sure that the air flows, not jets thru the cannopy. As soon as leaves look trashing in the wind rather than just tremor in the breeze youll have troubles with wind burn. Tower fans and oscillating fans down low only really works if you remove enough leaves thru lolipopping that the fan can sweep over the pots (theyll break the flow if fan is too low) nut below the the first leaves. This generally means a good 16 inches and in a tent thats orime vertical real estate. Maybe a small 360 oscillating right at pot height. But other than that your likely better off by using a few mini fans directed towards pots and walls which break the main airflow up into "diffuse" airflow that gently pushes air from below upwards from every direction of below, instead of a "air jet" of an oscillating or tower fan.
 
Rocket Soul said:
Theres two ways to make good air movement in a tent or room,.oscillating: good for clear spaces like on top of a flat cannopy. You can even make it hit the light a bit but you want your leaves to only tremor and move ever os slightly, no trashing!
The other way is stationary fans in several spots making air flow in directions or vortex or even just straight into the wall; you basicly have to design a system which makes sure that the air flows, not jets thru the cannopy. As soon as leaves look trashing in the wind rather than just tremor in the breeze youll have troubles with wind burn. Tower fans and oscillating fans down low only really works if you remove enough leaves thru lolipopping that the fan can sweep over the pots (theyll break the flow if fan is too low) nut below the the first leaves. This generally means a good 16 inches and in a tent thats orime vertical real estate. Maybe a small 360 oscillating right at pot height. But other than that your likely better off by using a few mini fans directed towards pots and walls which break the main airflow up into "diffuse" airflow that gently pushes air from below upwards from every direction of below, instead of a "air jet" of an oscillating or tower fan.
Click to expand...
This is a fair point. I have mine pointed in the space between plants, but any branches that lean into that space do get wind burned over the course of the grow...
 
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