Vapor Pressure Deficit?

I bought a Govee portable wifi/bluetooth transmitter and in addition to temp and humidity it is also displaying VPD which is says is Vapor Pressure Deficit?

I have never heard of that term.

Googled it and it says it is related to cannabis leaves ability to transpirate etc.

Anyone measure and use VPD or is it useless or redundant?

Thanks. Here’s a display from my Govee.


Oh boy, this question will bring on the replies. Here is a chart that I follow very loosely. It may help understand the temperature/humidity relationship to the transpiration rates or VPD.
I don’t have the equipment to control temperature or humidity precisely, and I just use it to insure I am not very far off.


I follow it loosely as well… once you put a humidifier or dehumidifier in the tent, the temperature will rise. Then it’s a constant balancing act. Same when you put a heater or AC in the tent. One thing affects the other.

Warm air can possess more water vapor (moisture) than cold air, so with the same amount of absolute/specific humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer.


:point_up_2: I gave up chasing VPD and as long as temps are 68-80 and RH 45-55ish me and the plants are happy :love_you_gesture:


Also, growing outdoors you get what you get… my outdoor grows are really good.


I also quit chasing vpd, and went with a fixed temp and humidity …UNTIL I got the AC pro 69 controller. Now the humidifier comes on to fullfil the vpd setting … that is; according to whatever the temp is.
It’s a given that a vegging plant will thrive in the perfect atmosphere… so I’m giving the settings a try, and letting the controller hunt . It’s Doing an excellent job too.
If i didn’t have this controller, I would not even try.


As long as it isn’t like the Sahara or a tropical rainforest, i leave vpd alone.


If you can control everything perfectly, then yeah, it’s great, but very hard to keep in the perfect range according to the chart.

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I actually bought the controller for WIFI access, and unaware of the VPD feature. So far, I’m pleasantly surprised AC incorporated this into their software… No doubt Someone in that company is a growmie!

This certainly gives us a chance to test the charts, and see


Ive been running clones for a while now ( just harvested grow 21) and i can tell the difference when im in the vpd zone vs when im not and the difference is substantial. Of course dli is dialed in as well.


Absolutely, VPD chasing is fruitless, IMO
My 45%RH lights on is 60-65% (or higher) lights off.
And lower temps bring higher moisture %.
Knowing my leaf temp is -3degreesF from ambient is important and lets me know the ladies are not being stressed.

Summer 2022 PNW was the best (my first outdoors). The Oct rains held off for a record 15 days.
So we got an extra 15 days without monsoon starting. Years of living here, I know the calendar for rain.
Now learning temps and sunlight. The Mason Bees had trouble with the cool spring. No apples canned this year. TBT, we ate only a dozen apples, not a prize year.


@DEEPDIVERDAVE, Did your leaf temperature change with the 350R?

My leaf temps change with the level of lighting vs ventilation. Multiple fans and filters for the tent and lung room.The biggest quantifiable change was the tent temp lowered (leaf temps follow). when I removed a Vipar Spectra R600 (leaving the V1200), added the HLG350R and started acclimating for 50% more light. The HLG 350R (350watts) produces double the LUX of V1200 (565watts). PPFD numbers better, also.
At MAX lighting the HLG350R, Viper V1200, and two more cheapies 100watts blast the tent with bigger numbers than before (PPFD/LUX). The flower tent 1000 watt draw is the same because I added a T-8 w/ filter (removed the V600 with 300 watt draw in the tent. REVEGED ladies like life, now and are starting to excite me.
The V600 is diong just fine in my 2x2 clone tent.

I do not chase VPD either. If humidity is under 40 ill turn on humidifier at times if have a lot of sprouts in tent. Otherwise i sit at 40-60% depending on how much they got watered. Never do I stress it.

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