Chasing VPD

Spring is upon us. In my area that means drastic temperature swings from day to day. Last week we had day time temps ranging from 75-90 degrees Fahrenheit. Next weeks forecast says day time temps will be 60-70 all week. This makes controlling my grow room environment a little tricky. I have a hard time cooling my grow room. I can get temps down to about 82-84 degrees Fahrenheit when outside temps are high but that’s about as cool as I can get it until adding an A/C unit. Is it ok to adjust humidity to optimize VPD on hotter days and then lower on cooler days when I can get the temp down into the 77-80 degree range? Basically can I have 84°f with 68-70%RH one day and 77°f with 60-62%RH the next day with both days equaling a VPD of about 1? What are the pros and cons of this method? I’m on day 22 of veg.

Just my 2cents but following vpd chart will screw u up. U can only follow it if your environment is optimal or u will get bud rot or mold late bloom. They suggest your rh be super high. In veg it's OK tho. Best solution for me to get temp and rh down was opening my tents up and leaving my basement door cracked so more fresh air and my ac from main room could enter. Get control before u flip em.

Unless your having prolonged exposure to temps above 85f or below 55f I wouldn't worry to much. We all want optimal conditions at all times but it's difficult and expensive to maintain such an environment. Mid 50s-mid 80s for a few hrs shouldn't stress a otherwise healthy plant

One gd way I use to get best air flow out of fans is to use a humidifier. Run it to see exactly how your air movement is then adjusted your fan. It showed me how one fan placed correctly can be better then 2 or 3. Multiple fans unless one for top of canopy and one for bottom can actually cause air pockets of heat when they hit each other.

Also your plant will be fine in temps of 85f-90f (not gd but it will survive np) the problem will be the trichs and flavors/taste u burning off once reaching those temps.

Hi this is how I see VPD with out making it more complicated.

I adjust temperature based on my RH%.

for example: my flower room (aero/nft) fluctuate between 60-70% RH. my safe zone temperature based on the chart is 80-82f.
Here are my numbers from the chart:
80.6f - 82.4f (mini split a/c, CO2 700ppm)

My safe zone RH% with air circulation

RH% VPD (kPa)
85% - 0.54 Fungal Pathogens
80% - 0.71 MAX (big chance of fungal)
75% - 0.89 x Safe Zone Max
70% - 1.07
65% - 1.25
60% - 1.43
55% - 1.61 x Safe Zone Min
50% - 1.78 Min
45% - 1.96 High Stress Zone

VPD Chart


Happy Growing

This is how I control vapor pressure deficit (VPD) and dial in my rooms. I adjust Temperature based on my room Relative Humidity (RH%) to maximize plant transpiration.


My flower room (aero/nft) relative humidity it’s around 60-70%. my safe zone temperature based on the chart is 80-82f.
Here are my numbers from the chart:
80.6f - 82.4f (mini split a/c, CO2 700ppm)

My Relative Humidity with air circulation:

RH% VPD (kPa)
85% - 0.54 Fungal Pathogens
80% - 0.71 (big chance of fungal)
75% - 0.89 RH% or VPD in range
70% - 1.07
65% - 1.25
60% - 1.43
55% - 1.61 RH% or VPD in range
50% - 1.78 (big chance of stress)
45% - 1.96 High Stress Zone

Ps I fix my chart for better understanding

Happy growing

Ann the way you are doing it is the way to go. I try to stay around vpd of 1 as long thru flowering as I can before raising vpd but never higher than 1.5. The plant's susceptibility to botrytis determines how soon into flowering I start raising vpd. If the cutting is resistant I wait till 6 weeks.

i let my flower room swing between 55%-75% or 1.60 - .90 VPD, the perfect spot is between 55%-70%. I’m trying to make it easier so other growers could understand how to play with plant transpiration rates.

I use a growtronix system to monitor everything.



Happy growing

Its worth to mention that vpd is not the only thing that controls transpiration: blue and uv light increases transpiration and high co2 will close the stomata and lower transpiration.

Mont@n@ said:

This is how I control vapor pressure deficit (VPD) and dial in my rooms. I adjust Temperature based on my room Relative Humidity (RH%) to maximize plant transpiration.
View attachment 5292198View attachment 5292199

My flower room (aero/nft) relative humidity it’s around 60-70%. my safe zone temperature based on the chart is 80-82f.
Here are my numbers from the chart:
80.6f - 82.4f (mini split a/c, CO2 700ppm)

My Relative Humidity with air circulation:

RH% VPD (kPa)
85% - 0.54 Fungal Pathogens
80% - 0.71 (big chance of fungal)
75% - 0.89 RH% or VPD in range
70% - 1.07
65% - 1.25
60% - 1.43
55% - 1.61 RH% or VPD in range
50% - 1.78 (big chance of stress)
45% - 1.96 High Stress Zone

Ps I fix my chart for better understanding

Happy growing

Click to expand...

Rubisco affect.