VPD vs temperatures in veg

[rama777]

Yo! So technically it’s possible to get optimal VPD at 40 degrees and 120 degrees but we don’t do that for obvious reasons.

My question is, at what leaf surface temperature both high and low would you consider it worth making modest sacrifices of ideal VPD for the sake of temperature?

[rama777]

And specific ti my context, I can get .8-.9 VPD for my 10 day old seedlings at 69-70 degree surface temps. Or I can have it more like 1.1-1.2 VPD at 74-75 degree leaf temps. Which is superior? My guess is the higher VPD and temperatures would make for the biggest growth gains. But that’s just a guess.

[SeasonsLater]

generally you want a vpd of 1.0 for veg and 1.3 for flower and even these can flux by genetic somewhat.  As with any living thing and plants they will survive at certain temperatures and will thrive at others.  Mr. G provides all the ideal temps and RH for each in his book and it is a great baseline.  When temps are out of the range the plants will work harder and get stressed more.  VPD is the way your plants transpires.  A good analogy that I use to wrap my head around this…  Try running a 5k or a mile in 20 degrees vs, 60 degrees vs, 100 degrees and you will have vastly different experiences in total time and effort exerted.  So you run the 5k in 20 degrees but now you have frostbite on your hands and toes, where you ran the 5k in 100+ and have sun burn and and heat stroke and partially dehydrated as a result.  When you grow outside of the optimal VPD parameters it is possible but not the most advantageous as other variables that were not necessarily a problem become enhanced as a result.  Some will mitigate excessive heat with CO2 or in my case my lung room in the summer during the day is hotter than the evening so I have flipped my day and night schedules on the growlight to help with leaf and tent temperatures getting out of control.  Also in the winter I benefit from keeping my tents in the same room as my furnace as I get free heat in the lung room which helps to stabilize extreme fluxations in temps i.e. 10/15+ degrees  Hope this helps.

[CamZ]

I think you are viewing the situation wrong. Warmer temperatures speed up enzymatic processes. I would rather have a higher temp and lower vpd than the converse. The plant has the ability to regulate its water consumption by opening and closing the stomata. It doesn’t have the ability to regulate temperature in the same way (yes it can cool itself down through transpiration, but it can’t generate any substantial heat). In an ideal world I’m shooting for 77f and 70rh for seedlings. Too much heat and they can get a bit stretchy.

[rama777]

Makes sense. I’d rather have higher heat and lower VPD as well. In my situation, I have optimal VPD, but it just happens to be when the leaf temps are at 69-70 degrees. I could have the leaf temps at more like 74-75 degrees, but then my VPD would be more like 1.2-1.3

I’ve been choosing VPD over a warmer temperature but question which option would be superior.

Getting closer to veg I will be on track with optimal VPD and temps. It’s the seedling phase I wasn’t sure about.

[mrgrowit]

Good question! If we are talking about overall plant growth, I don’t think we can only look at VPD here. I think we need to loop in microbe activity at the different temps into this conversation; the lower the temp, the slower the microbes operate. Sure, the .8-.9 VPD may be more ideal for the stomata opening but you slow down microbes activity breaking down organic matter when running the lower temp.

The higher the temp the faster metabolic processes in the plant can happen – assuming all other variables are the same. I’d aim for the “1.1-1.2 VPD at 74-75 degree leaf temps” you mention vs “.8-.9 VPD at 69-70 degree” since the temp is higher and the VPD is still within the tolerance range – even though .8-.9 is in the optimal range.

I personally like to run 80-82°F lights on up until 2-weeks prior to harvest where I will then drop the temp to 78°F to try to conserve terpenes. Looking the the VPD charts, the optimal RH for 80-82°F temp in Veg would be about 64-68% RH (0.9-1.1 kPa), and  about 51-60% RH (1.3-1.5 kPa) in flower. That’s factoring in a -2°F leaf surface temperature adjustment.

[rama777]

Awesome thanks!

I remember you had a guest on your show that was talking about a microscope being one of the best investments he made. You were running 7 gallons at that point I think, maybe it was 10. Anyways he said you probably didn’t actually have a lot of microbial life in pots that small.

What do you think about that assertion?

I guess I ask because I’m running 7 gallons and will continue to do so in the 3×3. Maybe I’ll try 10’s someday.

The metabolic factor is sound logic on its own, but even with me feeding organic inputs and inoculants, you think the microbes are really much of a factor? I’d love to believe they are :))

[mrgrowit]

Oh yeah I know who you’re talking about. I heard he actually got arrested for selling on the black market and is currently locked up. I honestly didn’t get a good vibe from him and got feedback privately to watch out for him. So I’m not sure how much of his info is actually trustworthy. That aside, microbes are a key component for organic matter breakdown. Even if I didn’t “have a lot of microbial life in pots that small”, the microbes there were still actively breaking down the organic matter and nutrient cycling was happening – or else the plant wouldn’t grow or be healthy. So to answer your last question, yes I think microbes are really much of a factor when growing with organic inputs.

[rama777]

Awesome, thanks for sharing. I didn’t know what to make of that myself. I mean I don’t just immediately believe anything anyone says, but his logic didn’t sound crazy so it did leave me questioning!

[rama777]

I do need to heat the veg tent somehow. Kind of challenging managing an LED veg tent and an HPS flower tent in the same room! Switching to all LED after this first harvest in about a week.

I was thinking of just hanging a 100 watt incandescent bulb near the top of the tent, connected to an ACI UIS plus so I can automate it.

Any better idea?

 

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