I ran across this subject in my research and looks like it might be a good a way to help gauge temperature and humidity set points. Anybody using it and if so does it help keep things in check?
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I try to keep my environment set to what VPD calls for at each stage of growth. It’s not absolutely necessary for a successful grow, but it is beneficial for healthy growth.
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I was just wondering the same, some charts call for different temps and humidity regarding the lights. I found this one about LED/VPD.
It gets pretty hard to keep humidity so high.
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I used this one throughout my grow. I managed low 20s/ hi 40s-lo 50’s. Getting my RH any higher would have required a humidifier to hold - and I didn’t want to do that. In hindsight, I will aim for some humidity control. 
Thanks for sharing these. I’m on day 18 of my first grow and I’ve been going crazy wondering if I’m doing things right. I’m currently sitting at 71 degrees with 65% Rh. I’m happy that I’m hitting in the good range
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Great stuff here you all… I downloaded all of these charts and will look over them… I added a slider to my dashboard that reads VDP in real time and was playing around with it this evening. Only thing i am not liking is how high i have to set the humidity to keep it under 1.2. I am concerned about mold growth, but being this is my first grow I don’t have any idea when things get where mold ‘can’ be an issue.
So the whole point of the VPD is to adjust the optimum humidity to temperature in order to allow the stomata of the plant to exchange gasses with the environment easily. Obviously transpiration is the mechanism the plant uses to transport water carrying nutrients up into the plant. When the VPD is not correct the transpiration rate drops and thus water and nutrients stop flowing. The green zone in the chart represents the correct ratio of humidity to the ambient temperature.
If you intend to mimic nature, you can raise the humidity in the plants juvenile phase, dry the the plant out as it matures. Notice as you dry the air the temperature must be adjusted to remain in the green zone, and vise versa. When a plant in nature gets too hot, the stomata close to retain moisture, this is seen as wilting. The wilting action is because of reduced hydrostatic pressure due to lack of evaporation, this causes the leaves to change there angle to the sun and this reduces the intensity and heat on the leaves surfaces.
It is a guidline. In reality, nature in North America swings from super humid in the spring to super humid in the fall. Niether of which are ideal for plants but does kick off the changing of the leaves and raking!
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It is a tough thing to maintain in the optimal zone but when you do, the growth and amount of water they consume is pretty drastic.
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Thank you for the great summary of how VPD works. I am starting to visualize how this works a little better. I have been doing a lot of reading and learning and I am a little surprised I just now came upon this subject. Lots of guides talk about temperatures and humidity levels but don’t mention this at all. VDP ties the two together in a meaningful way. Just thinking out loud ‘relative’ humidity probably isn’t the best measurement to use because it is directly dependent of temperature hence the relative part. I might start charting wet bulb temperature along with %RH. I have a tendency to beat on things until i feel i have a good grasp on them. But if i understand VDP correctly it is the vapor (water) ‘pressure’ of the ambient air. And as you say this has a large impact on transpiration. Might look at a psychrometric chart through the eyes of my plants and see it that tells me anything. For me this hobby is one of those cases where the more i learn the more i realize i have even more to learn. Tricky stuff!
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There are a lot of different charts for VPD
My goal is
Seedling/clone humidity around 1-5 lower than temps (day or night.)
Veg humidity around 10 Lower than temps.
Flower humidity around 20 lower than temps.
So if day temps are 80 during veg I’ll try to keep humidity around 70% and if night temps are 70 I’ll try to keep humidity around 60%
If flower it would be day 60% night 50% etc etc.
This way I’m about in the middle of the kPa number for the stage of growth.
Seedling .6-.8 kPa
Veg .8-1.0
Flower 1.2
They go higher or lower than above but it makes it easy to know where you want to be without actually looking at the chart. It’s a range I look for and not going for that perfect number just get it close enough as getting it exact would cost me too much money.
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Much of humidity and temperature control comes down to rate and frequency of ventilation. Play around with that to optimize your environment.
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If you can keep your temps in check, use a vaporizer and add steam to your room. Hard to do late in flower though. That’s how I control my humidity. It can be quite dry where I’m at from time to time.
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Skydiver- nice common sense method that is super easy to use… i love it. The VDP is super easy to calculate with my control setup, just a couple lines of computer code. I am in the process of tweaking it out which is what brought me to VDP.
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Hi Landmaster. Now that winter is getting here i think it will be easier to control my environment. I have a small electric heater, for now a ultrasonic humidifier (from what i understand i have to watch for mineral buildup) and several small fans pushing and pulling air from here and there. Everything is controlled from one of those programmable logic chips. Temperature control seems easy and i can keep it within .75 degrees of my set point. Humidity a little looser and + or - a couple percent. This morning it is running at 72 degrees, 61% RH and a VDP of 1.0
Tap water can contain a fair amount of minerals like calcium that will be dispersed into the air and settle on things like your pre filter of the in line fan and lights and walls of tent. I switched to using RO water and you can get a inexpensive one for $60 or so I’ve seen others use Available on Amazon to not have those issues.
I now use RO water for the ladies rather then dealing with the changing city water and with RO you have a clean slate to work with so you control the inputs without the unknown of changes in city water. Not necessary but works for me.
Good deal on coding your HVAC for your grow! VPD control helps optimize their growth.
Nice! I was referring to csfutbol comment about it being hard to keep humidity levels that high. I run into that issue even with my humidifier in mid summer where I live. I add extra steam to my room if necessary to increase my humidity levels above 70% in summer which can be quite difficult at times. Temps can get high quickly if your not carefully though. Oh and reverse osmosis water is the best of you can use it as skydivers states as well. I try and use it or distilled water at all times.
Yes Landmaster, I could imagine summer being really tough to keep the temperatures and humidity levels correct. My tent is small (see my original post) and it is located in small room that is about 6’ x 6’ x 7’ tall. My heater and humidifier has to be located exterior of the grow tent and i cycle fans on and off with the heater or humidifier to circulate air into the tent. This works pretty well for adding to the tent but there are a few times when the temperature or humidity gets above my setpoints which puts my fans in exhaust mode. I exhaust air directly to the outside of the house with a fan and turn on a fan to replace it with air from an adjacent room. So far odor has not been an issue but very soon I expect it will. I am working on my carbon air scrubber now and the deeper we get into winter the less i will need this exhaust only mode. But I will need to rethink things and incorporate true AC and dehumidification when spring gets here. Fortunately I have several months to think about some cool ways to do that and come up with the money to buy the stuff
Lol, I’m in the same boat. I’l be adding in an ac to my room in the spring as well. I’m in a 12’ X 15’ room and plan on filling it all the way up on my next grow. I’m actually kinda thinking I might end up having to high of a humidity problem during the latter stages of flowering. I just use regular box fans for ventilation right now as I’m not in a tent, and it works well. I do filtrate the air I’m bringing in the room though. Let me know what your plans end up being for your spring setup!