VPD vs. humidity?

[soulshine]

VPD vs. humidity?

Confused! Have been reading a lot about keeping the VPD during propagation and early vegetation around .48 to .8 and the temp between 75 and 80 which according to the VPD chart will have the humidity around 80%. Mars Hydro just put something up about keeping humidity between 45 and 60% for optimum growing conditions, which if I did that my VPD would be up around 1.6 with the same temperature.

Which is best to follow, lower humidity or lower humidity?

 

[bluter]

it's all a range. your rh will match to a particular temp for any given vpd value.

vpd is just a tool. you want to create the best conditions you can indoor. in most instances it means you are growing within the vpd chart whether you followed it or not.

don't let the chart stress you.

 

[soulshine]

it's all a range. your rh will match to a particular temp for any given vpd value.

vpd is just a tool. you want to create the best conditions you can indoor. in most instances it means you are growing within the vpd chart whether you followed it or not.

don't let the chart stress you.

So is it better to have lower humidity or lower VPD?

 

[Nunyabiz]

If Mars Hydro is referring to veg stage then they are simply wrong. 45% bit too low

But your VPD will change from seedling to veg to beginning flower to final two weeks of flower.

I usual try for around 78⁰ and 75+% humidity for seedlings.
82 to 84⁰ and 70% humidity for veg .
80 to 82⁰ and 70% humidity at flip then each week I lower humidity by 5% until harvest.
I want to be in the 30s to lower 40s in humidity and a bit over temp like 78⁰ last week or so.

 

[Phytoplankton]

IMHO, the VPD thing is over-rated. Don't drive yourself batty trying to get it perfect. Think about plants grown outdoors, the VPD swings widely nearly every day, the plants do fine.

 

[Delps8]

So is it better to have lower humidity or lower VPD?

Neither. - they're different things.

This site has good info on VPD.

Re. the temps + RH you've cited are correct. Speaking in general terms, cannabis grows well in temperatures between 68 and 85 and RH from 50-70 (rough numbers).

As you've observed, if you check the VPD values for that pair of ranges, it's pretty wide and that's correct, too - cannabis will grow well in range of temperatures and humidities but controlling VPD is one way to optimize your grow.

 

[Regrowth]

Imagine trying to get the VPD right outdoors in the fall (at least here in Canada)...
Would need to hit unrealistically low or even negative rh values? lol

 

[Porky82]

VPD vs. humidity?

Confused! Have been reading a lot about keeping the VPD during propagation and early vegetation around .48 to .8 and the temp between 75 and 80 which according to the VPD chart will have the humidity around 80%. Mars Hydro just put something up about keeping humidity between 45 and 60% for optimum growing conditions, which if I did that my VPD would be up around 1.6 with the same temperature.

Which is best to follow, lower humidity or lower humidity?

I wouldn't take to much notice of what Mars says!!

 

[Porky82]

IMHO, the VPD thing is over-rated. Don't drive yourself batty trying to get it perfect. Think about plants grown outdoors, the VPD swings widely nearly every day, the plants do fine.

Couldn't agree more!
Personally I think it's utter rubbish!

 

[Porky82]

Neither. - they're different things.

This site has good info on VPD.

Re. the temps + RH you've cited are correct. Speaking in general terms, cannabis grows well in temperatures between 68 and 85 and RH from 50-70 (rough numbers).

As you've observed, if you check the VPD values for that pair of ranges, it's pretty wide and that's correct, too - cannabis will grow well in range of temperatures and humidities but controlling VPD is one way to optimize your grow.

You do realise that link is an advertisement for pulse monitors disguised as an information article!

 

[Keffka]

So VPD is important indoors because of the entire setup. The plants are being exposed to far greater concentrations of very directed light for far longer indoors. Not to mention that if you’re indoors you’re in a container which changes literally everything. You can’t compare and outdoor grow, and especially an outdoor grow in the ground, to a container grow indoors. The plant evolved under the sun with unlimited space and ground to work with, not in a 3 gallon container in someone’s closet.

With that being said, VPD is a perfectionist measurement. You can get away with low or high VPD easily as long as you’re offsetting the nutrient requirements. Dry air under LED vastly increases plant transpiration which in turn vastly increases K and so on and so on. If you’re growing synthetically you probably won’t even notice this, but if you’re growing organic you definitely will notice this and it’s a quick way to have to start top dressing


TLDR: VPD is situationally important but it’s far more important to understand basic gardening principles

 

[bluter]

So is it better to have lower humidity or lower VPD?

it's best to shoot for the best conditions you can, and let the plant grow through it.

i have extremely low humidity where i live, it would take more expense and gear to get into an optimum vpd for me, so i don't bother. i have learned to grow with my conditions. it's not hard.

don't stress about a stupid chart. if your given conditions aren't perfect we can coach you through it.

 

[Sueet]

It is more important for your conditions to be stable, even if they don't quite fall inside the lines of the vpd chart ..... If you are close, just keep looking til you find a different chart where your numbers fit in (I've seen several versions)

 

[Keffka]

it's best to shoot for the best conditions you can, and let the plant grow through it.

i have extremely low humidity where i live, it would take more expense and gear to get into an optimum vpd for me, so i don't bother. i have learned to grow with my conditions. it's not hard.

don't stress about a stupid chart. if your given conditions aren't perfect we can coach you through it.

It is more important for your conditions to be stable, even if they don't quite fall inside the lines of the vpd chart ..... If you are close, just keep looking til you find a different chart where your numbers fit in (I've seen several versions)

Both of these answers right here. Cannabis growing is legitimately all about balance. Nothing else matters as much as the balance. A stable environment is much less stress on the plant. However, if you can’t get a stable environment, then in the name of balance you’re gonna need to crank up nutrients, pot sizes, etc. whatever it takes to create a balanced grow.

This is a plant that can grow to the size of a tree outdoors, or stay half a foot in a closet. It’s pretty versatile and doesn’t need us to be exact.

 

[Delps8]

It is more important for your conditions to be stable, even if they don't quite fall inside the lines of the vpd chart ..... If you are close, just keep looking til you find a different chart where your numbers fit in (I've seen several versions)

"keep looking…" - that's horse before the cart.

VPD charts for have different values because optimal VPD values are different for different stages of growth. VPD makes it easy to understand how to change the grow environment to improve the outcome of the grow.

Real life example - my 2' x 4' tent is in an unheated garage in Southern California. For my grow this Spring, I used AC Infinity equipment to keep VPD at 0.8, 1, and 1.2 for seedling, veg, and flower. It's not easy for me to control temperature so I control RH to keep VPD ≈ optimal.

Prior to this year, I had an Inkbird RH controller and I had to check RH multiple times per day to keep VPD in the optimal range. It was a PITA to do that and, when the AC Infinity finally released their humidifier, I retired the Pulse and used the Controller 69. Thanks to the mostly mild, very predictable (cool) temps that we've had for the past couple of years, the C69 was able to keep VPD in optimal range almost 100% of the time. The C69 is, arguably, the most valuable piece of tech in my tent.

Keffka has posted about keeping a grow in "balance". I agree with his premise completely. This text from a web page for what I believe to be the hands down best value in lighting for a 2' x 4' grow tent* - "Recent studies show that PPFD levels as high as 1,500 umols/m2/s will deliver a proportional increase in yields so long as other critical inputs are kept in balance. These include CO2, water, and nutrients."

A grow that has VPD (which is just a proxy for different combinations of temp + RH values) in the optimal range just means that the temperature and RH will allow cannabis to thrive. That still leaves (pardon the pun) plenty of other opportunities for growers to screw up their grow areas to address to optimize their grow. To that end, I've posted a screenshot from one of Bugbee's videos:

VPD is a good tool to keep RH and temp in range but the other issues need to be dress right dress to maximize the quantity and quality of the crop.



*spetrum-wise it's a $280 knock off of my $640 Growcraft flower light and it's got the best PPFD map I've seen on any grow light

 

[Sueet]

"keep looking…" - that's horse before the cart.

VPD charts for have different values because optimal VPD values are different for different stages of growth. VPD makes it easy to understand how to change the grow environment to improve the outcome of the grow.

Thanks for the lesson, I didn't understand the different charts were for different situations. Makes perfect sense, I was being cheeky, sorry.

 

[bluter]

"keep looking…" - that's horse before the cart.

VPD charts for have different values because optimal VPD values are different for different stages of growth. VPD makes it easy to understand how to change the grow environment to improve the outcome of the grow.

it's just an informational tool. it's not worth dying by. as pointed out a lot will depend on which chart you hang your hat on.

piles of those stupid charts don't even agree with each other. definitely not something that would fall into the trusted category.

 

[Porky82]

Jeez I wonder how the 100s of millions of plants were grown inside the world over the last 35 years without these magical VPD readings?

 

[CaptainLucky]

If Mars Hydro is referring to veg stage then they are simply wrong. 45% bit too low

But your VPD will change from seedling to veg to beginning flower to final two weeks of flower.

I usual try for around 78⁰ and 75+% humidity for seedlings.
82 to 84⁰ and 70% humidity for veg .
80 to 82⁰ and 70% humidity at flip then each week I lower humidity by 5% until harvest.
I want to be in the 30s to lower 40s in humidity and a bit over temp like 78⁰ last week or so.

I keep my tent @ no higher then 50% the entire grow without any issues. I just set the inkbird to the setting and forget about it. But I’m sure there’s better ways of doing it. CL

 

[Nunyabiz]

I keep my tent @ no higher then 50% the entire grow without any issues. I just set the inkbird to the setting and forget about it. But I’m sure there’s better ways of doing it. CL

Oh yeah they'll grow.
But in a totally organic grow if you have everything dialed in including the VPD you can get really explosive growth.