Yeah, that’s pretty much how I feel about it. If you were trying to maximize profits, perhaps it would be worth chasing VPD. That being said, I do attempt to set my controllers to the proper VPD ranges, 1.2 for veg and 1.5 for flower.
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All VPD does is tell you how the plant is transpiring. Maintaining it in an ideal environment should allow the plant less stress. Too low VPD and the plant transpire’s more, meaning it can burn easier from using more nutes than it would otherwise. Too high and the plant can’t breathe, visible water on leaves, mold can be an issue.
Ideally I’d say you never want RH above 60% regardless of your temp and never below 30%, assuming temps 70-90s Fahrenheit. Yes the chart says you need more RH as you get into higher temps, but again, you’ll risk mold and whatnot to do so, as you saw here
In flower that is DEFINITELY a red flag. Veg that usually isn’t an issue, tho PM could be a thing. Keeping RH at 60% or under in the flower area should prevent mold/pm/etc. If your temp is really high, you may need to feed a lil lower EC but that’s about the only drawback. VPD doesn’t really matter otherwise, it’s just a guideline.
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Many thanks for your feedback. I am questioning my whole “Grow by the numbers” approach.
Ok, you are piercing another belief of mine in one of my charts. I have used this chart and pretty have lived and died by it to determine EC.
Context is rDWC:
So, if interpret what you are saying correctly, then this condition might not be telling me to raise the EC “if” the VPD is also low?
Another myth being shattered, actually I am grateful.
A big thanks to you and all of the other participants in this awesome thread. I appreciate the learning opportunity!
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IME with LEDs, leaf surface temperature can vary from being the same or slightly above ambient to 6F+ below. In conjunction with reading plant health, I find that a healthy temperature delta of minus 3-5F leaf vs ambient means that my plants are able to effectively move water and nutrients. When my LST is the same as ambient, I start to look at two things: environment and access to water.
If environment produces a VPD that’s in range but the plants’ leaves are the same as ambient, this tells me access to water is low, causing slower transpiration, slower evaporation, and less leaf cooling. Otherwise, it’s likely that the environment is preventing transpiration by reducing evaporation at the leaf. In my location this usually means high humidity causing low VPD, slow evaporation, and low to no leaf cooling, thus no temperature delta.
This will vary from grow to grow, as each cultivar will have its own preferences on temperature, VPD, light intensity etc etc. The leaf temperatures are valuable pieces of data, but by themselves they mean nothing. I know my lights, at my height and intensity but different lights, even different LEDs at various heights and intensities may produce different effects on leaf temperature. The grower needs to correlate observed leaf temp deltas to plant health to understand how healthy transpiration translates into data for their environment.
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The closer you can get to vpd around 82 to 85f and 75% to 80% in veg thru mid flower, the healthier and more vigorous your plants will be. They end up getting an oily thick coating over the leaf when they are humming. Bud rot becomes the concern after mid flower and thats when I turn down temps and rh to protect against botrytis and preserve cannabinoids and terps.
Prune overlapping leaves to reduce the chance of mold thru condensation.
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Great question. I’ve had the same questions since learning about and trying to at least be mindful of vpd. If the vpd is a result of temp. and rh (more or less), then as the chart(s) show, there are many combinations of temp. and rh that result in the same vpd. So, what’s the determining factor then; temp.? rh? I thought about it and considered that maybe the actual temperature of the plant is what is most “important”, or the “starting point” or determining factor, since the temperature of a living organism plays into it’s metabolism rate, right? When an animal or plant is warmer it’s metabolism increases, doesn’t it?
So I started to look for a number; an “ideal plant temperature for cannabis” - a target temperature for the actual plant to be at for optimal metabolism. Anyways, I came up with very little in the way of “a research paper” or “an experiment”, but I did come across a recommendation of I think 80 or 82F. And recently wolverinegrown also talked about this and IIRC he mentioned that he found an actual source for this number (I can’t remember, but I don’t think he said the source or gave a site/link or anything), and he said 82F. It was on the hlgshow podcast with wolverinegrown, on yt. Easy to find.
I usually see a lst offset of -5 to -8*F from the air temp. I checked once on a plant that was not doing well, and had slowed growing quite a bit, the lst was different (higher) than all the healthier plants around it. The temp was closer to the air temp. I notice leaves will often ‘feel’ cool to the touch (transpiring, I guess), when touch them and their “working”.
There’s a chart/chart maker by pulse, I think, that allows for an lst offset. It makes a big difference in the chart and vpd temp/rh recommendations.
It’s also to do with how the charts were developed (the equipment, methodologies, etc. that were used). There are many charts, by various groups, and also some (most, last I checked) don’t take leaf surface temperature into account.
Do you have a “controller” that allows you to set a vpd and then the environmental devices do what they need to do to maintain that vpd?
Isn’t this backwards? Or is the wording just confusing me? Higher vpd (number) causes more transpiration, lower vpd (number) causes less transpiration.
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Oh my bad, been awhile since i bothered looking at the chart/numbers but it seems all the charts have higher temp and RH = lower vpd #. Oops.
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All good. Just checking my sanity, haha.
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Yes, the Niwa grow hub has two modes that work based on VPD. One holds temp to the setpoint and varies RH, the other holds RH and varies temp. I use former in my lung room. 
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Oh, you use the Niwa grow hub? I thought you were a diy controller guy (thought you had an HA setup or something)…?
I have trolmaster hydro-x. Not super impressed.
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I was listening to hlgshow yesterday. Another mention of the 82F (plant temp), in this case associating it with the plant temp. of a plant in an HID room with air temp of around 78F. LINK (time synched). I think I remember Stephen mentioning this number before as well, years ago, so it’s a little foggy.
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