Hey guys and gals, I am trying to do as much right things in these grows as I can to get an awesome crop. That being said I am getting conflicting information on where the VPD should be in the flowering stage. Below is two different charts from reputable places. Which one is right? I have been using the first one which comes with an interactive graph to put in your temps and rh.
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Trying to maintain a precise VPD can be quite challenging. Many growers keep it simple and shoot for 50 to 65% in veg and ~35 to 45% in flowering. The lower RH in flowering helps to prevent mold and mildew.
All that a precise VPD helps with is achieving ideal leaf transpiration. iirc the improvement that comes with an ideal leaf transpiration rate is marginal so long as the RH is relatively accurate. If VPD is off a bit it isn’t going to affect much. It’s therefore my opinion that time and effort put into chasing VPD is better spent elsewhere.
I’m not sure that anyone can tell you which is the most accurate, but we’ll see what others have to say.
You may find this helpful.
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I run 80 degree 60% humidity in flower, around 1.4kpa.
I don’t start to worry until I’m over the 1.5 close to 1.6 range.
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I use .8-1.2 for Veg and 1.2-1.5 for flower. My temps only fluctuate 5°F and for me it is easy to keep my humidity in the range needed based on my current tent temps. When I started back growing I used the standards Midwest suggested but wanted to give VPD a shot and I use this metric now instead of a larger temp/RH window. Here is a smaller plant in early veg that showed me a pretty big difference in 6 days once I dialed in VPD.
Day 1 getting VPD to .9
6 days later keeping the VPD at .9
I streamlined this chart to reflect my conditions. If it is similar to your default temps, Maybe it can help you. I got my info from the web as well.
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I don’t have much of a problem getting it where I need it. The problem is finding a chart that is correct. If you look at the two charts I put up they are both different. Where one says perfect for flowering the other says “stress”.
I am just looking for a correct chart to follow to grow the best plant I can. Thanks for your help. I appreciate it.
I don’t try to follow that. With the changing seasons and temps the air temps and humidity change not just daily but sometimes hourly.
We don’t have too much of a low humidity issue except during extreme cold when it dips quite low. More often I have to deal with high humidity like 75% when the plants are starting to flower. Before that I just let it be high as the plants thrive at that point.
I have a dehumidifier but it raises the temp a lot. This last grow I just started removing lower growth and some bigger fan leaves and that helped lower the humidity and keep the temps down too. Didn’t seem to hurt the bud development. Of course you can never really tell because the final outcome may have been better, but the risk of mold is not worth it.
Outside in nature this happens uncontrolled. The plants do fine.
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A suggestion that others here in the community regularly use if you do want to go that route. Some growers in the forum use the following chart and consider it to be accurate. It has been posted many times over the past few years.
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5
10
15
20
The difference between the temperature in F and the RH% by stage of growth
In general I just use the 5,10,15,20 rule in my head and that meets the basic VPD charts.
Seedling 5
So if temp is 80 I shoot for 75% rh
Veg 10
Temps 80 shoot for 70%
Early flower 15
Temps 80 shoot for 65%
Late flower 20
Temps 80 shoot for 60%
This made it easier for me not having to always refer to the VPD chart. KISS
Different strokes for different folks.
Many suggest much lower RH in relation to temps in late flower due to possible bud rot/mold. Using UVA/B bulbs during flower greatly inhibits those spores that cause those things.
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Take a sec and read this, this is the effects of being outside of the VPD range. I would not follow the chart that suggests 1.35 is too high…
Don’t get to carried away with trying to make it perfect you will pull your hair out.
Just understanding what it does to plants uptake of water and nutrients will explain it all.
Higher temps need higher humidity, lower temps need lower humidity.
Think of it as evaporation.
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I never thought of it this way! I like that!
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Just makes is easier is all.
A simple math that one can do in a couple seconds rather than looking at those charts. KISS
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Do you keep your temps at 80 through the whole grow or was that just a for instance? I can control temps and rh so I I know where it should be through the whole growing stage I will write it down and follow it.
Thanks
Ruby is doing great
I just want awesome!
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Temps vary but generally somewhere between 83 and 78 lights on and down ~10-15 lower lights off.
I also try to think how growing outside changes from spring to mid summer to fall and how Mother Nature changes those things so in later flower I look to get temps down a bit to better align with what happens outside…fall harvest and all.
It’s not necessary I just look to mother nature and how it has worked for millennia outside of our little boxes we live in and then put other little boxes into that box. Haha
There are ranges the plants are happy just keep within those and all will be fine.
Here is my environment the past 30 days with temp/rh/vpd
I have plants in both veg and flower in there due to some seen and some unforeseen events but they will be fine in the basic range.
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Thanks a bunch for your analogy on how to keep it simple. I will use this for now on. My house humidity is at 35% in the winter and not much higher in summer due to the AC unit.
So I have to use a humidifier.
Thanks again!
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So, if im struggling to keep my humidity up at ideal levels, its better to keep temp. at a lower degree then???
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An increase in temperature will give a increase in VPD. A humidity increase will give a decrease in VPD.
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Temps go up humidity goes down and vice versa
Hotter temps can hold more water so if humidifier is putting out the same amount in the air and doesn’t increase and the temps do the “relative” humidity goes down.
And if the temps go down while the humidifier is still putting out that same amount the relative humidity increases.
Assuming the RH in the house hasn’t changed and you haven’t changed the amount of air exchange say from the inline fan.
Even if I leave the door open to tent room right now that will in affect lower my tent temp a degree or two. Because my humidifiers are on an inkbird controller my set humidity point won’t change so my humidifiers won’t have to run as hard to maintain the RH that I set.
Increasing my house temp from say 63 to 69 will affect my basement temps and thus my tent room temps and thus my tent temps.
I leave doors closed or open in adjacent room/s and top of stairs etc and they all can have an effect on my tent environment within reason.
It can be a dance at time.
And then there’s the outside temps and RH which also affects things.
It’s warmed here a bit from the last week where it was 5-10F and humidity outside was maybe 60-70% but when it’s that cold that isn’t really much water in air if you took that cube of air brought it in and warmed it to 70F it would drop to maybe 10% RH
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Not necessarily
Only if the amount the humidifier is putting out either by setting or run time doesn’t change or temps etc.
VPD is simply the difference between temp and RH leaf temp aside.
5
10
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20
KISS
here is a great place to learn about VPD. It has some great references but some here don’t agree with the values given. Either way it’s great reading why it’s important. Happy Growing everyone.
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