Jungle's 2nd Grow

And the view from my defoliating seat.

Also, FWIW, I saw GH Trio and Calmag (and pH up/down) at the local Ace Hardware today. That was a surprise.


That’s some crazy roots


RDWC, baby!

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Really nice and great Job

Evening pics.

The Mt Vernon Kush in the front is going gangbusters. The Terple in the back are … not as happy with the environment.

Oh well. I really wanted to push (with avail equip), and I guess lesson learned. Or maybe I should wait for harvest before deciding.


Anyone wanna weigh in on nighttime VPD?

Chart for attention (chart also at linked post)

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After staring at this for a bit, it seems like I need to decrease RH via my dehumidifier (which will also increase temp), yielding an increased VPD overnight… Still thinking about this.

And yikes:

Changing the VPD can be a challenge but under closed environments it is much easier to do. You can reduce the humidity using a dehumidifier to increase your VPD and you can use a humidifier to increase your VPD. Ideally you will want to use an AC unit to keep your temperature at exactly the value you want it to be and you can then use a humidifier/dehumidifier to control the exact point where you want your VPD to be by controlling the value of your relative humidity at the fixed temperature provided by the AC unit. (emphasis added)

Source: https://scienceinhydroponics.com/2017/04/vapor-pressure-deficit-vpd-in-hydroponics.html

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Tagging: @GreenJewels, @db_cooper.

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Thanks for the tag
Good article on vpd. Even if not in hydro, it applies. My flower tent VPD at night range has been not so good at 1.4 I have noticed wetness on a few leaves of bubble monkey in the morning when the lights come on due to transpiration at night. I just put in a new exhaust fan a couple days ago and re arranged stuff. Come spring/summer I usually don’t have an issue because the humidity goes up a little. I keep my vpd in good range in the veg tent but the humidifier takes care of it nicely. Come to think of it. That $5 garage sale humidifier was the cheapest grow related purchase I’ve ever made :rofl:
I’ll work on getting my night time vpd in range.

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Turns out I’m 75% idiot. The VPD calculations I made were wrong due to Order of Operations and Lack of Parenthesis.

Here’s the chart from yesterday ~6am through today ~6am:

  1. Because I screwed up the calculation, instead of being generally inside my desired VPD range (1.2-1.5), I’m too high. I was wrong yesterday.
  2. The new chart shows that yesterday daytime, VPD was ~1.6, before dawn yesterday VPD was ~1.7.
  3. I adjusted my controller to increase the tent temp overnight by 2F, and decrease the overnight humidity to 40%. This was a success in that it increased VPD, but a failure in that I actually needed to decrease VPD.

Even assuming that I was going in the right direction by increasing temp and decreasing humidity, all it really accomplished is making the dehumidifier run continuously (the temp came up to ~29C). This had the effect of increasing the VPD (when it really need to be decreased).

I’m going to go the other way 'round tonight, decrease temp target and increase humidity target for overnight. I don’t have a way to cool the tent cooler than the ambient, which is generally ~40%, 75F (24C).

I still need to decrease VPD during the day… so I’m going to adjust the daytime RH target up a bit. But there’s a limit, I don’t want to get past 70%, I don’t want to vent any more than I need to (CO2 supplement).

I’m at the end of Flower Week 1, I do need to start decreasing the RH at some point to avoid problems with the buds (mold).

If anyone wants to know exactly what I’m doing to measure, download & chart the data, I’ll write it all out. The bottom line is I have a “Wirelesstag” sensor, which sends data into the cloud, and I download the data into Excel.


Nice setup you have @Jungle I’ve thought about getting a sensor push just to be able to monitor my vpd better.

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Ok, I’m getting somewhere.

I adjusted the controller a little more than an hour ago, increasing the humidity target, and I seem to be smack-dab in the middle of my goal range (see the blue box).

Now I just gotta see if I can get the nighttime VPD into the range as well. I think I can probably manage that tonight or tomorrow am. Whew.


Ok, I’ve got the overnight VPDs in range. Daytime VPD is in range (starting ~2:30pm yesterday).


  • It’s difficult to look at singular VPD calculations. It swings a lot as T&H move around naturally. Compare the green squiggles to the black moving average. I’m not sure how you’d do this very well without a way to get average VPD values.
  • An increase in daytime humidity made my overworked dehumidifier happy, it ran about half as much. It also vented about half as much, which is really good for CO2 consumption.
  • The nighttime T&H similarly required less intervention (but lots of venting, which doesn’t bother me overnight as there isn’t any CO2 consumption).
  • Ambient basement conditions are RH=40, T=71F. I decreased the whole house humidity a few percent, this may have helped. It’s winter here, but not so cold out that <=40% RH is necessary, but the house is still comfortable.

What do you folks do? Anyone pay any attention to VPD? Anyone obsess about it like I have this week? Why do I feel like my charts are causing MEGO from you folks (“my eyes glazeth over”)?

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I’ve never looked at that. But it’s very interesting :thinking: And I’m watching. And learning. :sunglasses::v::call_me_hand:


Did you ask for more details? Well, here you go.

Environmental Equipment


  • CO2 - 1100ppm ± 50ppm
  • Temp - Daytime: 88F, Nighttime 50F, ± 6F
  • Humidity - Daytime 75%, Nighttime 50%, +0-5%

This yields VPD between 1.2 and 1.5. This is higher than what you’d run normally, but with the CO2, and higher temperatures, that’s my plan. Non-CO2-supplemented grows should be between about 1.0 and 1.2 for flower, I think.

I’ve been supplementing CO2 for about a month, and the 20# tank continues to spit out CO2. An exchange is ~$40? Hopefully I can get through the whole grow with just one exchange (it’s still running on the original 20# tank which was only partially full when I acquired it).

There’s no way I could do this in the setup I had previously (a panda film-draped off room). I don’t think my cheap tent is as well built as a nice Gorilla or something, but it seems to be airtight enough. I wish I had more height, though.

Have you thought about CO2? Why or why not?

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I continue on with the craziness.

I’ve now constructed a script that’ll send me updated graphs of the last 24-48 hours (from 00:00:00 ‘yesterday’ 'till ‘now’).

I’ve got this pretty well dialed in to within [1.2, 1.5]. Hopefully that’s not bad VPD numbers to be using :slight_smile: . (See above, CO2).

Also, I found this article interesting… Tomatoes aren’t cannabis, but I think it still applies (C3?) : Coordination between vapor pressure deficit and CO2 on the regulation of photosynthesis and productivity in greenhouse tomato production | Scientific Reports

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Thanks for sharing the articles. I haven’t used CO2 in my grows. Well, except that time my gas water heater (in the grow room) exhaust came off :grimacing: all fixed now. I love my lights but its not enough light to make using co2 worth the effort. I had put some thought into it. Maybe in the future i would use it but for now, keeping my VPD in range would help the girls. Great subject . :v::green_heart:


Well, ok. But CO2 just changes my VPD target range. It’s the same hassle to adjust things to get VPD in-range. Without CO2, in flower, I think you want 0.96-1.2. Im just higher at 1.2-1.5.

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Sorry if i misworded, I was referring to the money and effort it would take for me in my personal situation to add CO2. Especially with my vpd frequently going to high. I agree CO2 is beneficial to the indoor plants when light levels are proper and vpd in range. The science article showed an increase in biomass with low vpd and high CO2. Something we all want. CO2 is just not for me right now in my basement grow.
I was trying to answer your question. Sorry if I offended in any way friend :v::green_heart: