Vapor Pressure Deficit!

[Todd gunter]

Have any of you guys actually paid attention to VPD and actually stayed in the green area for an entire grow? The amount of humidity required is insane!! How would you not get PM while in flower sticking to those RH requirements. If anyone has how much better were your results?

 

[PE636]

The chart I've seen floating around appears to be just a general purpose chart not specific to cannabis and therefore with no concern for anything like bud rot, PM etc. VPD would be pretty important in a large scale operation growing pretty much anything for profit. I've never ran a grow as humid as the chart, I usually battle low humidity. But I do notice faster growth and happier plants with higher RH when they are young, the humidity seems to matter more then.

 

[Todd gunter]

I think it’s spot on but the risk of mildew is what scares me and everyone, but if your plants are healthy I think you’d be good up until the 5th or 6th week and then dial it back considerably. I have seen recently in my own grow what it does to the growth rate and it’s pretty convincing. Just be aware of the risk in the later stages of flower.

 

[Weaselcracker]

Could you post the chart you’re talking about?

 

[ChefDGreen]

You can keep decent rH in late flower as long as other considerations are taken. Running an IPM (integrated pest management), keeping good air movement, and proper nutrition can all be factors in plant health and pest infestation.

the point im making;
If you grow healthy plants, you don’t need to worry about higher humidity for proper VPD in late flower.

 

[Todd gunter]

Could you post the chart you’re talking about?

 

[Modest Grower]

Yeah, I'm outside of that chart in flower. I'm around 77*F and 50% humidity. I was thinking I should gradually lower it to 40% rh now until harvest

 

[Bush Doctor 77]

Is that chart for cannabis or iceberg lettuce, or both, or neither? My greenhouse in the summer runs above 90F and below 35%RH on a daily basis, and I grow trees! Unless you can measure the actual conditions in the microclime on the leaf surface the chart is not helpful. Also, wind plays an important part, and is not considered with the chart. Plants control their own environment with their stomata, to a great extent.

 

[Weaselcracker]

I’ve kicked at this can half-heartedly before. I think a lot of growers work with their climate as best they can- others may have the climate control to tweak this aspect. My outside air hovers between 90 and 100% humidity for most of the year. As long as the RH in my flowering room is below 75 everyone is very happy and mold free and I don’t worry about RH beyond that.

There’s a fairly large thread on this somewhere. I think it’s called ‘VPD and what it means to you’. For myself the answer is ‘not much’. For others there’s obviously a different answer.

Edit - here Vapor Pressure Deficit & What It Means To You!

 

[Todd gunter]

I’ve kicked at this can half-heartedly before. I think a lot of growers work with their climate as best they can- others may have the climate control to tweak this aspect. My outside air hovers between 90 and 100% for most of the year. As long as the RH in my flowering room is below 75 everyone is very happy and mold free and I don’t worry about RH beyond that.

There’s a fairly large thread on this somewhere. I think it’s called ‘VPD and what it means to you’. For myself the answer is ‘not much’. For others there’s obviously a different answer.

Edit - here Vapor Pressure Deficit & What It Means To You!

Yeah, I'm outside of that chart in flower. I'm around 77*F and 50% humidity. I was thinking I should gradually lower it to 40% rh now until harvest

Look it up and read it, I am pretty sure all of you guys would change your minds. I know it’s scary to think about that much rh but it’s just science. I’m going to run it and the last two weeks I’ll drop my rh considerably.

Right on I’m going to give it a real good go and see what happens, so far they are kicking ass!!

 

[Weaselcracker]

I grew with very high RH for years before I got the dehumidifier. I find when growing with RH in the 80s I get mould in my bigger buds and that’s not worth it.

Usually I can’t consistently run much higher than 75 RH without some of the buds getting hit. Realistically I also can only run the RH so low, considering my climate.

The mould factor really depends on the strain, and lots of constant airflow helps. Pure sativa strains are better at mould resistance. Heavy dense large colas mostly rot every time.

Not to disagree with you Todd. If you’ve got the climate control, then great.
For me it’s just not worth the struggle, and in the end I wonder what the difference would really be. If my plants seem great, and my bud seems great... I just smoke it. Cheers!

Carn2

• Weaselcracker
• Feb 18, 2020

Carn 2

• Weaselcracker
• Feb 11, 2020

 

[Calvados]

I use this one. After a while you understand what range you need and how to get there.

A9BD79B8-0FAA-4B71-AF1F-5293872323FA.gif

• Calvados
• Jan 7, 2020

Vapor Pressure Deficit Chart

 

[Todd gunter]

I grew with very high RH for years before I got the dehumidifier. I find when growing with RH in the 80s I get mould in my bigger buds and that’s not worth it.

Usually I can’t consistently run much higher than 75 RH without some of the buds getting hit. Though mould factor really depends on the strain, and lots of constant airflow helps. Pure sativa strains are better at mould resistance. Heavy dense large colas mostly rot every time.

Not to disagree with you Todd. If you’ve got the climate control, then great.
For me it’s just not worth the struggle, and in the end I wonder what the difference would really be. If my plants seem great, and my bud seems great... I just smoke it. Cheers!

Carn2

• Weaselcracker
• Feb 18, 2020

Carn 2

• Weaselcracker
• Feb 11, 2020

 

[Todd gunter]

Yeah I definitely need to be careful as soon as they start to fatten up. Thanks for the input and I’ll definitely post my finished results

 

[Bush Doctor 77]

A very large fan(I run an 18" wall fan on my 4'X4' SCROG)can help negate the less than optimum conditions. That helps the plant self regulate. Remember this is a weed that thrives in places like the mountains of Afghanistan with very hot dry conditions, and tropics with hot moist.

 

[Weaselcracker]

Exactly. In most of Southeast Asia humidity is 100% and 40° plus for much of the year. You’re sweating like a pig all the time. Head a little north - western India and Afghanistan and it’s bone-dry desert country- frying temps in the day and at night you’re huddled in blankets freezing your ass off. Great bud grows throughout those places though.

 

[safeman]

Jungle Boy's (breeders) grow using this an swear by it. If you grow room is not air-tight - would not consider it

 

[Calvados]

I use the one I posted as a guide. I grow inside, but I don’t have AC. So I don’t have complete control over my environment, but I do have enough to keep it in the right zone. I don’t understand the one that Todd Gunter posted. Mine’s very different. Within those margins there is an inverse relationship between higher/lower humidity and higher/lower temperature. It seems to make sense with what little I know about the RH range in the lifecycle.