Help! I'm super confused by the vpd chart

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adam2706

Active Member
Hi everyone.

I am currently running my tent at about 80 degrees and am in week 5 of flower. Many things online say that I should be reducing my RH down to between 45% - 50% to prevent mold and other problems.

But when I look at a VPD chart, that tells me I should be between 72% - 76% RH at 80 degrees. That seems wrong to me and that it would invite some major problems with rot or PM.

Is there a point in flower that I should stop looking at the VPD chart? Or is that crazy high RH something I should shoot for?

TIA
 
Kassiopeija

Kassiopeija

Well-Known Member
VPD tells you how to optimize your environment for optimal photosynthesis/stomatal conductance. Preventing mold is just a different pair of shoes. I've seen grows at 90% rH where there was 0 mold. And some at 30% rH where all tops developed greymold. Botrytis initially dwells on dead tissue, and if you have fat colas in between the buds you will always have a high rH, no matter the outside rH.
 
A

adam2706

Active Member
Kassiopeija said:
VPD tells you how to optimize your environment for optimal photosynthesis/stomatal conductance. Preventing mold is just a different pair of shoes. I've seen grows at 90% rH where there was 0 mold. And some at 30% rH where all tops developed greymold. Botrytis initially dwells on dead tissue, and if you have fat colas in between the buds you will always have a high rH, no matter the outside rH.
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Got it. So the VPD chart probably isn't worth following in late flower. I'm currently sitting between the 45-50% range. Should I keep it there for now and dial it back again to about 40% starting in week 7or 8? That is my normal plan, but this is my first time trying to use the VPD chart as a guide.
 
Hook Daddy

Hook Daddy

Well-Known Member
In late flower I try to stay 45% RH or slightly lower, and right at 80 F. No point inviting mold. As most have already said, keep your RH on the lower side during flower, then as you get more experienced you can try raising it.
 
R

rmax

Well-Known Member
Tolerance Break said:
Get a few grows under your belt, and then worry about vpd. Just my 2 cents.
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I've got a tent going w/60%RH and 77°F. The VPD tool indicates 80%RH would get the tent into optimum zone. Should I jack-up humidity to match the VPD tool or wing it?
 
Tolerance Break

Tolerance Break

Well-Known Member
rmax said:
I've got a tent going w/60%RH and 77°F. The VPD tool indicates 80%RH would get the tent into optimum zone. Should I jack-up humidity to match the VPD tool or wing it?
Click to expand...
How many weeks in? If buds have developed I'd run it at 60% for now and drop below 55% in the last few weeks. Mold/mildew when you're almost at the finish line is the worst. If they're young, and you have good airflow, I say go for 80%.

Don't worry about optimizing. It's a weed, it grows in a very wide array of conditions. The more you narrow those conditions down, the more difficult it becomes, and doing so at the very start can be overwhelming.

Rule of thumb, if the plants look happy, don't make any big changes. Incremental changes, one or two variables at a time allows you to know what does and doesn't work.
 
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