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VPD: Ideal temperature/relative humidity

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    VPD: Ideal temperature/relative humidity

    So you thought your lights were too hot ... maybe you just need a bigger humidifier

    Here is a great post by @Kcar:

    Originally posted by Kcar View Post
    Here is a simpler explanation

    Humidity and Vapour Pressure Deficit (VPD)

    For years Relative Humidity (RH) has been used as a measure of how much water vapour is present in the air and is probably still the preferred method used by experienced growers. In a greenhouse, the amount of water vapour present has a direct effect on a plants ability to transpire and hence grow.

    Another measure called vapour pressure deficit (VPD) is also used to indicate humidity and is felt to be more directly related to a plants wellbeing. VPD combines the effects of both humidity AND temperature into one value and so gives a good indication of plant wellbeing without the need for the grower to do any mental arithmetic. VPD values run in the opposite way to RH values so when RH is high VPD is low.

    If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).

    Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.

    It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.

    By measuring the temperature and relative humidity within the crop canopy the calculated VPD is still a useful measure as it combines both temperature and humidity into a single measure in a way that approximates the well-being of the crop. As an example, for many crops it is suggested that RH should be kept between the following limits at the stated temperatures:-

    Temperature oC Min RH (apply fogging) Ideal RH Max RH (for disease prevention)
    15 - 50 73
    20 46 64 80
    25 60 73 86
    30 70 80 89

    You can see from the table that the higher the temperature is the more humidity is required by the plants. The above makes it difficult to specify control parameters as different RH settings are required at different temperatures.

    Now look how much simpler this is made by using VPD as the whole of the above table is contained in just three VPD values as follows
    Maximum VPD (too hot and dry - apply fogging) VPD ideal

    VPD too low (too cool and humid -warm/ dehumidify)
    1.25 0.85 0.45

    AutoVent 2 and 3 environment controllers estimate the VPD based on the air temperature and humidity in the crop canopy. It will only be close to the true figure for a healthy transpiring crop. The VPD calculator below allows the VPD to be estimated based on both air and leaf temperatures. This clearly shows the possible error in VPD due to just a 1 deg C difference between air and leaf temperature.

    As a general rule, most plants grow well at VPDs of between 0.8 to 0.95 KPa

    Fogging or other humdification is usually applied at VPDs above 1.25KPa and heating and dehumidification at VPDs below 0.45KPa
    VPD Calculator
    Calculate VPD on your own using this

    Here is a list of temps in Fahrenheit with correlating relative humidities to keep the VPD between 1.25 and .45 KPa

    69F - 70F: 50% - 82%
    71F - 72F: 53% - 83%
    73F - 74F: 55% - 84%
    75F - 76F: 58% - 85%
    77F: 60% - 86%
    78F - 79F: 63% - 87%
    80F - 81F: 65% - 87%
    82F - 83F: 67% - 88%
    84F - 85F: 69% - 89%
    86F: 71% - 89%
    87F - 88F: 73% - 90%
    89F - 90F: 74% - 90%
    91F - 92F: 75% - 91%
    93F - 94F: 76% - 91%
    95F: 77% - 92%
    96F - 97F: 79% - 92%
    98F - 99F: 80% - 92%
    100F - 101F: 80% - 92%

    And here's a list of temps in Fahrenheit with correlating relative humidities to keep the vpd between .8 and .95 KPa. This is the sweet spot

    69F - 70F: 62% - 68%
    71F - 72F: 64% - 70%
    73F - 74F: 66% - 73%
    75F - 76F: 68% - 74%
    77F: 70% - 75%
    78F - 79F: 72% - 76%
    80F - 81F: 73% - 77%
    82F - 83F: 75% - 79%
    84F - 85F: 76% - 80%
    86F: 78% - 81%
    87F - 88F: 79% - 82%
    89F - 90F: 80% - 83%
    91F - 92F: 81% - 84%
    93F - 94F: 82% - 85%
    95F: 83% - 86%
    96F - 97F: 84% - 87%
    98F - 99F: 85% - 87%
    100F - 101F: 86% - 88%

    A couple of threads on VPD:

    Get those foggers pumping

    its related to veg, right?
    cant imagine 80%rh in week 7-finish
    just lost 4 main coals 5days ago and it was 78%RH for only 3days at 80-90°F...


      Well a big factor is where you live. If you live in the Midwest, you probably will have mold problems if precautions are not taken. If your out west where its real dry then running 80% humidity is pretty safe. Seal up and get some heap filters.


        Originally posted by cannaisok View Post
        its related to veg, right?
        cant imagine 80%rh in week 7-finish
        just lost 4 main coals 5days ago and it was 78%RH for only 3days at 80-90°F...
        Pulling the RH down for the last 2 weeks of flower helps to frost everything up.

        Was that your RH and temp during lights on and off? In my flower rooms the RH goes way up during lights off and the temp goes down ... without a dehumidifier during lights out I'd have major problems.


          Great post. Those seem spot on.


            good thread, new growers, especially those using small spaces or tents..... this should be the first step to grasp and understand. Media and nutrition are easy, dialing in environment so the plants maximize the light and food is where most new growers overlook the details and question their results.

            even experienced growers going from bigger rooms to grow tents, need to totally rethink how they set up environment.... I know because I'm one of them. It took me some convincing to not use an intake fan, just didn't make sense to me. Added the small humidifier and rode out the temps and what do you know. My grow log shows similar numbers as the chart posted above and I was extremely happy with the harvests.
            HERE take the doob..... i got another fuggen fish on <(---<


              Used a dehumidifier once ,did not like it.Temps go up,and your ac kicks on.Intake/exhaust fans with climate controllers is where it's at.

              I live in a humid environment in the summer.Fans kick on when lights are off ,some nights humidity climbs to upper 70% with the fans running,but as long as you have the right air circulation you're good.

              I have never ever had mold problems.

              Besides it's cheaper to run inline fans than ac/dehuy....


                for me I try and only run in the cooler months of the year so for me that is October to end of june. No AC , no dehumidifier , just a 450 cfm exhaust, passive intakes and a blower right under the 940watt bulb. sitting in a 6x6x7 tent in a 12 x 13 x 8 room with the window open when the lights on and closed when it's off. My exhaust only runs during lights on and blows out thru filters and into the house but the blower under the bulb and a small 6" desk top fan stay on 24 7
                I live at 3000 ft elevation in western Canada and we do have a naturally drier air here tho this year has been an extreme exception to that rule with more rain than sun it seems.
                I realize what works for me doesn't work for everyone but I was having problems dialing my tent in and it took deleting the intake fan and adding a small room humidifier (lights on only) and everything balanced out for my climate..... for the cooler months of the year.
                for me it makes more sense to shut down in july and august and into September so I can do outdoor and go play and have some downtime. Something I didn't do this summer and I'm kicking myself LOL
                HERE take the doob..... i got another fuggen fish on <(---<


                  what do you guys keep you night time temps and humidity?


                    Originally posted by RedBeardy5 View Post
                    what do you guys keep you night time temps and humidity?
                    Around 70 degrees and 65% - 70% RH.


                      in the dark temps are at 25-26°C(78°f) and light on it was 29-31°C(87°f). 4x4tent with airfilter exhausting in a room with tilted window.RH in this room is 58-64%.
                      When the mold appeared, I had no bigger fan running inside the tent. only 2 pc blowers blowing hot air away under the reflectors. reflector was just 4-5inches above tha plants.
                      so maybe there was a little lack of airmovement inside the tent that caused the mold?

                      now i have a oszilating fan inside and finally managed to bring rh down below 60% by opening all windows and doors in my flat all of my girls seem to like it. weet 10 and week 3 are doing great.
                      right now its 87°f and 55%rh.
                      i was thinking it is optimum but vdp theory is saying different.?
                      would you suggest me to go up with my Rh up to 80%?

                      will more airmovement prevent molds in higher rh situations? is there scientific evidence? some people statet that.

                      thanks for help


                        in the winter i grow some shroom for myself maybe my enviroment is not 100% sterile but nature isnt sterile too, mold is not a problem of bacteria/spores its a problem of conditions and enviroment and less than ideal conditions for the plant that it cant withstand them.
                        i def wony buy a hepafilter or sth like that cause i grow shrooms without contaminations by simply dailing in the ideal enviroment. as long as enviroment is right the contam wont appear even if your coated in spores and s**t.
                        my goal is to get my plants dailed in like that.
                        only heared oldshool wisdom like as long as youre below 50%rh in flower everything will be fine very scientific


                          i def wony buy a hepafilter or sth like that
                          Some people never struggle with mold. They run mold resistant strains and/or live areas that simply aren't conducive to mold. The rest of us buy hepa filters and dehumidifiers. At this point in time, growing pot is way too risky/lucrative to skip out on regular sterilization and proper air filters. "Anything worth doing, is worth doing right." -H.S.T.


                            Easiest way to overcome humidity problems, intake cooler dry air, filter it if you need to, then have a lot of air movement in your outtake, this lowers the layer of humidity around your plants allowing for more transpiration which does lead to more humidity sure but it also leads to more drying. If you take have more air flow out with drier air in take you reduce your humidity, if you keep it stable within your room but using air flow fans it stays more even in your room/tent whatever and a higher humidity is less of an issue 78 will still be too high. Next thing you need to consider if your dew point, dew point will create moisture on your plants which will lead to mold and fungus. Try to make sure you airflow with in/outtake and room circulation fans doesn't move your plants as it will cause excessive transpiration by making the microclimate too dry. If your fans move your plants they aren't doing their job, they need to move the air and not your plants, when the air hits your plants to make them move, it removes too much of the humidity layer of the leaves are causes them to excessively dry.
                            Your best you try, to harm I and I, Aiming to kill.
                            But I love you still Cause you are here to make prophecy fulfill.


                              Thanx friends!
                              Treehouse youre right, i see your point, if I would grow on a larger scale or even as commercial/medicinal grower whos responsible for his product, then of cause its a no go to risk moldy endproduct. but i am growing on 2x2 feet in a 4x4 tent

                              thank you for your detailed post man!
                              right now my 1foot diameter fan is set to 12,5W only and is constantly rotating blowing on the upper 1/3 of the plants, you would suggest me to blow over thein heads is this right? so they wont move too much from direct airflow mostly moving from overall movement in the room.