But yeah, you want your VPD around 0.5 or so right now, if you are aiming for optimal growth in the average cannabis plant, but you’re more than double that…I’m running about the same in bloom at the moment. That humidity seems very low for a dome. I’m sorry if I’m being negative. I’m just trying to steer you away from the diseased plants I started with in my first year.
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To Decrease the leaf temperature/vpd. I could put them back down and lower the dim to 25%… is that going to make an actual difference? First time using LED so this is all new to me.
I don’t have the sensor inside the domes. They’re just plastic domes that are over each each individual seedling.
Oh, it’s all good man. I don’t think I can get a .5 VPD without lowering my temps down into the 60s without a germination set up… possibly with the humidifier… germ dome will be here in a week 
Well it’s a two-way street, actually. VPD matters, but so does high temperature and humidity for root propagation. I personally don’t run seedlings under 65% or 79F, usually higher for both for the first few weeks.
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For example
Given: 69% RH and about 80F
and let’s assume the gaseous exchanging from transpiration has an evaporative cooling effect on the plant that lowers it by 5F from the air temperature.
69% RH and 75F is approximately 0.58 VPD.
For VPD you always have to assume a lower leaf temperature than the air, unless the leaves are dead.
And it’s the leaf temperature that matters, not the air temperature. The air temperature just adjusts the leaf temperature.
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Got it … sounds like I just need more water in the air in there … Without sacrificing too much temp.
Might as well hook that humidifier up sounds like the perfect solution right now… at least until I get my clone/germ dome
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Using that site I linked, you can find a similar VPD for 75F (air temperature, and assuming -5F leaf temperature) and 64% RH, that also gives you about the same VPD.
But propagation demands high temperature and humidity until a root system is well-formed, so you can only go so low until VPD doesn’t help you (or too high).
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Yeah, when you raise temps to optimal propagation levels, humidity has to rise a lot more. It’s a tradeoff of finding the sweet spot for your environment and how hard you want to push the plant for fast growth (or pulling it for slow growth).
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And it makes sense that humidity needs to rise more, from a plant’s perspective:
Since RH is not the real air (absolute) humidity, but relative to the air temperature, more humidity can fit in the air with higher temperatures, which means less space with water vapor at the same humidity level.
Now, without a fully-established root system, those pores on (mostly the) leaves of plants (called stomata), open and close to transpire when there is a difference in root water uptake from what is expelled into the atmosphere. Transpiration rate is affected by the environment and photosynthesis, and at night, plants respirate more than they transpire, meaning they release CO2 and water into the air.
Once roots are formed well enough, transpiration rate increases due to the increase in water uptake, and it’s easier to keep things under control for the rest of veg, in my experience.
When flower hits, I aim for a higher than normal VPD (lower RH) to mitigate bud rot.
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For sure. This is part of the fun for me, problem-solving.
Understanding VPD and even further how the leaves work as you just explained, goes without saying- Is very helpful for that!!
Before I made this post, I didn’t even know what VPD was. 
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Sure, no problem.
Things will change when they grow to have more biomass, and also at night when they respire. Don’t look through a microscope here. It’s going to take some time and seeing all the ranges you get with your setup through a grow or two.
And I think this summarizes why I spent a couple months designing my own software to automate this. Again, I’m lazy and don’t trust one source of truth. My tents all have 4 hygro-thermometers in them, lol.
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Because transpiration rate is affected by nutrient uptake (or absorbtion through leaves), as well as photosynthesis, it’s important to stay in the correct DLI range for the stage of growth TOO.
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Also don’t make the mistake of measuring DLI with your tent open. The walls and the material of the tent greatly affect the photon density reaching your plant.
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Soak the seeds in 2:1 water:hydrogen peroxide for 24 hours and prepare a 50/50 vermiculite/perlite mix soaked in 2:1 water hydrogen peroxide as well. After the 24 hours let the vermiculite/perlite mix drain, crack the seed either by hand or your preferred tool, then toss in your seed and wait.
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Thanks man I’ve tried peroxide and cracking the seed no luck so far with it…hopefully getting some new ones in the mail this week
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Give the vermiculite/perlite combo a try. I used to use soil + that method on old seeds and germ rates started to drop but the verm/perlite got me back up to the high 90s for germ rate.
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It’s causing me anxiety now and I’m just sort of over it. I feel like I’ve tried everything in some form except for maybe giberrilic acid…
Been going at this for over a month now with this batch. Have seen 4 taproots in probably 50 seeds. Maybe I’ll give them another shot next time but for now I’m just waiting on fresh stuff and clone off what I have.
Sorry man, I was in a bad mood yesterday feeling defeated. I went ahead and tried this. Fingers crossed.
Thanks again for the advice
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