Easy way to think of Vpd is how thirsty the air is. Too thirsty and your plants cut if off from sucking so much water, don’t want the air thirsty much at all for a seedling or clone that doesn’t have any or much of a root system yet, don’t want it too high at any time because stomata will close to prevent rapid water loss
I can be guilty of this. But sometimes, stuff I think will be complicated is deceptively easy to understand. I’ve noticed that most smokers (myself included) like to overcomplicate stuff.
Building on that you see how much water is moving through your plants and conversely, how much nutriments they are picking up and you can strongly influence that process by the VPD.
Add the light intensity as another environmental variable you can manipulate and you see you have some powerful ‘tools’ to work with.
Once you have an understanding of the interactions you see how to ‘tweak’ the processes to improve the grow.
This is great, thanks. Yes. I get the feeling that VPD is kind of the accelerator pedal for the plant’s evaporation via stomatas. The right VPD means the right exchange rate of water in the plant, whether liquid or vapor. And yeah, your view of the environmental variables – they are like levers you can pull to steer your truck. Temp, humidity, light, air flow, etc. I like understanding each lever independently, and then in relation to all of the others. It’s just how I learn. Thanks for helping me.
I look at vpd as the way temp and humidity relate. Meaning usually indoors heat is an unavoidable due to hot growlights. So you usually have a steady temp in the high 70s or low 80s that exhaust fans can only get so low. Vpd is how you know what humidity the plants want at whatever temp you are forced to grow at.
Don’t for get you need to adjust your feed accordingly.
High transpiration rates combined with a strong feed can lead to plant burn.
And the contrary, weak feed in time of very low transpire rates can lead to hungry plants.
I have never seen that written anywhere but I am confident it is true besides it is really common sense if you think about it.
BTW I pay no attention to VPD but I do pay attention to transpiration rates.
Not your feed per sey…One needs to adjust one’s feed in low-humidity situations as well as high-humidity situations. So this has nothing to do with your feed in particular, just an expression.
Well yes and no, I don’t use a chart or track temps and humidity.
I kinda just know if they are sucking up a lot of water quickly, not to feed them a strong solution.
And if they are not drying for 4-5 days I know to give them just a touch more food.
I guess you can be the judge if it is the same or not.
The barometrics values influencing the VPD is just about the atmospheric pressure surrounding the space (let’s say at the house scale). And it change with weather, high when it’s clear and sunny and low when it’s cloudy and rainy. With all the intermediates states between. For the same temp by example, it influence drastically how dense is the air, how your MPAC performance is running but also how the RH is dialed.
Let me find a VPD table quickly (i’m very not the guy interested by this).
I took a random one on google, maybe this one is initially bad idk. As you can see it’s quite simplistic with two axes : temps and RH.
Lets consider a sweet spot, the 68°f/55rh@1.06. It’s measured in pascal in bonus …
Now consider the VPD pressure during a cloudy day and a sunny day, so with high and low barometric pressure … it totally fuck up the whole table by a factor that change almost everyday if you live in a temperate sector.
I’m sure there is somewhere a smart ass that have set controllers etc to fine tune it adapted to the localized barometric pressure, but we reach degrees of accuracy that i’m not sure that are very relevant with cannabis and most of illegal indoor crops level of technology ^^
I hope it’s more clear. I don’t say it’s a bad thing, it’s not because i don’t give a fck about it that it can’t have an utility somewhere. But i doubt, at a certain level of ratio of [tech used : micro point of VPD pressure], that it have more importance that just the static pressure of the room where the crop is by example and/or the rate of renewal of the fresh air.
Thanks for taking the time to write this. It’s terrific and I’m gonna keep it around forever.
I appreciate the “not overthinking it.” That’s what I naturally do. So with growing weed, my initial mindset was deliberately, “It’s just a weed. How hard can it be?” So I watered it, kept it lit and comfortable, and it produced ok.
Then I changed my mindset – again, deliberately – to, "Okay, it’s a weed, but I’m trying to optimize for this specific Thing (big, healthy flowers and good chemistry in the plant).
So with each grow, I try to explore something a little new, like pulling on a single lever and getting the feel for it. Humidity one time, lighting every time, etc. It all builds and interrelates, I get it. But again, I gotta keep it simple for myself, so this is my approach.
My eternal question, for example is: “how much will VPD matter to my goal of getting better weed”? How much better, away from baseline, is my grow with a well-managed VPD?
I’ll tell you one thing that may shock you: I don’t pH my water at all. I think it’s might. It’s tap water, filtered through a Brita and with ascorbic acid added to control chloramines. But that’s it. I don’t do PPM readings yet or pH readings at all.
Hell you hooked me the right way ^^ Work out your instincts, look like you’re a good casino player or something like this lol
1st : I grow PH free since over 20 years indoor (not when in pure hydro of course), and it’s on purpose. And i always choose my houses/lab location for their quality of water first ^^
2nd : I decided that the plants have to work for me and not the reverse, so i use genetics leverages to fit my minimalist vision of what should be a grow with a competitive weed.
3th : At first i wanted to have at home the same quality of weed than my neighbor. Then the one of the best weed dealer of the town. Then the one of Dam coffee shops. Then the one of cups … i totally got you on this matter ^^
The journey is just more difficult than universal tricks, knowing yourself and the environment you’re performing the most is imho the stronger leverage before fine tuning it.
Great replies, @Fuel . Solidly explained how there are always more factors to try and “control,” and that mitigation and awareness are most of the battle.
@shag , I think you broke my brain. This is something I’ve been trying to work out in RDWC, but haven’t quite been able to figure that simply. I think I’m starting to get it with such a succinct explanation.
Damn good details in a refresher on vpd, y’all.
Stay up
Coffin_Dodger
It’s just a guideline and you need to measure it with ir at the leaf surface anyway which on a mature healthy plant will be lower than ambient under sun or lights.
Media temperature matters too especially in veg if you want fast veg.
