How do you use vpd with drying material? It's not just Rh and temp...Very well explained @Rocket Soul, just to add to this, the VPD is also crucial when drying and curing. We're just developing a product and this is one of the main metrics for us.
So even after the plant is dead it's a good idea to keep an eye on the VPD.
Hopefully we'll be able to go to market soon
Very well explained !I love itNo, but the concept of vpd is not "ideal cannabis environment" its about the environments tendency to make the plant transpire. Which is really important to the plant, especially with leds which dont heat the leaves in the same way as the sun or HID which means that you have to tweak the environment a lot more than with "hot" light sources in order to get it to drink properly, which in turn determines how much nutes the plant draws.
It is temps and RH as a combined measure, to see how much water in absolute terms could be absorbed by the air before it reaches full saturation. RH is relative humidity, how many % relative to total saturation. But the amount of water that can be dissolved in the air increases with higher temps. So say (just using example values here) that we have a situation of 10C /75% rh and just for this example lets say you could dissolve 10ml in a liter of air before it reaches 100% RH and reaching the dew point; where water from the air creates due in your grow space.
Now say we got another situation of 30C / 75% rh and (again just example values to demonstrate) here the air could dissolve another 40ml per liter of air before reaching 100% rh.
In both examples we have 75% rh but in the high temps situation you could dissolve 4x more water; thus making the air "more thirsty" and forcing the plant to transpire more heavily in higher vpd environments. The charts you see online gives you an indication where youd like to be for different stages of growth: if you have too low transpiration then the plant gets saggy and wont take up all the nutes it needs. If you have to high transpiration there are troubles aswell, the stomata can close due to draining too much water out of the plant and again youll see the plant looking unhappy.
Transpiration is a complicated issue cause its not only influenced by this, its also influenced by light intensity, spectrum (blue opens the stomata while green closes it) co2 levels (the higher the co2 the more the stomata closes which inhibit transpiration) and i think even the airpressure influence it.
So its a good idea to think of vpd as a tool rather than a goal: if you can see your plant looking low and saggy then look at your vpd for your grow space. Adjust accordingly and see how the plant reacts. For a fully happy looking plant (like the ones in your pics) there is little need to adjust it.
Another issue would be when you see nute deficiencies: if your vpd is on point then you can deduce that the deficiency isnt transpiration related but down to either low nute levels or ph being slightly off. But if your vpd is out of wack (and consequently you see saggy sloppy leaves) then your best bet is try push it towards the ideal. With leds youd also want to keep an eye on temps if you are running really intense light since the temps will also affect the plants metabolism; cold plants cant take full light intensity even if you have your vpd in order.
I hope this helps, and also encourage you to not to worry too much about it until you actually see problems in your grow. Maybe experiment a little but do not think that vpd supercedes what your plant is telling you with its own behaviour. A tool, not a goal.
There are also sometimes reasons to abandon the whole concept: for example towards the end of the grow, if you care more about quality than yield, you may wanna run slightly colder and less rh than vpd would indicate just to not lose terps and thc, as heat and high rh tends to diminish them both.
I dont know if theres a better way of explaining this and i might be wrong, anyone with a better definition or explanation is very welcome to correct me
I easy to understand the science behind
