VPD useless for multiple plants in same environment?

[Death The Cultivator]

Honestly the big thing is I haven't had a consistent temp and humidity. In 10 days, I will have been growing for I believe 5 months exactly to the day. I'm very new and it started out a bit rough. I tried germinating 5 seeds and all of them except one germinated. I put them in the coco and only two ever popped up. I was running these two without following VPD the first couple weeks. Then I read something about it. Well roughly one month after that, after practicing how to germinate seeds, I tried again with a 3 pack of cream caramel autos. I ended up germinating all 3, and by my error I killed two of them. So this left me with two plants entering flower, and one seedling. At this time, I maintained a VPD between 1.2-1.6, however I was noticing it seemed like my seedling was having a hard time growing. I erred on the side of caution though and decided not to risk moldy buds. Recently I haven't had a set temp and humidity, because I've been seeing what works best for each plant. I think I more than likely don't have any hotspots, mostly because I have so much air circulation in the tent. I have an inline fan bringing in fresh air, two small fans that push air around the outside edges of the tent, one fan that pushes air around the canopy and one fan pushing air underneath the canopy. This is in a 3x3. The different heights thing is definitely plausible. Though I will say the biggest difference I have between plants is 6 inches so the height difference isn't huge.

I'd still take the average but it doesn't really around like you've ever filled the tent up. Not having maintained consistent VPD has been your biggest issue as cultivars don't different that much from each other to necessitate the worry for each individual one. Focus on consistent VPD, I work in a commercial facility with 30+ cultivars that all receive the same VPD based on growth stage with no issues.

Inconsistencies in temperature can lead to uptake issues, pest development, and molds. What are your temps and humidity like outside the tent? What type of light are you using and on what cycle? Do you know your day and night differentials?

You may need to get control over your exterior room in which the grow tent is and manage the temp and humidity from there.

 

[narmohare]

I'd still take the average but it doesn't really around like you've ever filled the tent up. Not having maintained consistent VPD has been your biggest issue as cultivars don't different that much from each other to necessitate the worry for each individual one. Focus on consistent VPD, I work in a commercial facility with 30+ cultivars that all receive the same VPD based on growth stage with no issues.

Inconsistencies in temperature can lead to uptake issues, pest development, and molds. What are your temps and humidity like outside the tent? What type of light are you using and on what cycle? Do you know your day and night differentials?

You may need to get control over your exterior room in which the grow tent is and manage the temp and humidity from there.

Yea so I've been trying to manipulate the temp and humidity of the tents rather than the lung room because I have two different tents at two different temperatures. I have a drying tent which I try to keep at 60/60 and my grow tent I keep at 72f/45rh. I noticed in your comment you said "all receive the same VPD based on growth stage". For someone like me who has a perpetual grow(and plants in several different stages) what would the balance be?

 

[ExNavyInSTL]

I keep this VPD Tool "Pinned" in my browser. It's a tool from the folks at Pulse. You may find out the subtle differences in leaf temps may still all be good to go.

Before checking it out, make sure to note it is a 2-part tool.

In Part 1, you put in your tent temp, RH%, and leaf temp. It gives you VPD for the room and leaf. Then you go to Part 2 (tabs are at bottom of the page - See Red Arrows).

In Part 2, you have 3 fields at the top of the page. The last one to fill in is simple math. For instance, if the leaf is 74ºF and the room is 75ºF you would put -1. If the leaf was 74 and the room 73, then you would put 1 in the field. Once you enter this you will see the graph shift to match your conditions. Make sure you fill in the blank for Veg or Flower (See Blue Circle and Arrow).

Click HERE: for Pulse Google Sheet VPD Tool

I've included screenshots below of Both Pages:

 

[Death The Cultivator]

Yea so I've been trying to manipulate the temp and humidity of the tents rather than the lung room because I have two different tents at two different temperatures. I have a drying tent which I try to keep at 60/60 and my grow tent I keep at 72f/45rh. I noticed in your comment you said "all receive the same VPD based on growth stage". For someone like me who has a perpetual grow(and plants in several different stages) what would the balance be?

I think a comfortable balance would be 76f/55% from seed to harvest but I would use humidity domes from seed to early veg. In propagation we run 70f/50% but in veg and flower it is 76-78f/53ish%.

It's better to manipulate the lung room otherwise you're fighting you're lung room due to intake.

 

[Delps8]

At this time, I maintained a VPD between 1.2-1.6, however I was noticing it seemed like my seedling was having a hard time growing.

Recommended VPD for seedlings is 0.8 (hot and wet). They had to do a lot of work to deal with VPD of 1.2 kilopascals (much warmer and much less RH).

 

[Delps8]

RH is a crapshoot. I use the PulseGrow + an Accurate + the Acurite Hygrometer. All of those are only certified to be ±5% RH. I've seen them give me the same readings and, a few minutes later, they're different - some high, some low. And that's in a 2' x 4' tent. I can't imagine the fluctuations you folks deal with working a large, open space.