Will taking my tent to a slight negative pressure lower the transpiration?
For example, on the chart below, if in late bloom I was reading 77 degrees and 40% rh. Would negative pressure help move the transpiration rate of the red zone and into the yellow zone.
Welcomed to the community @Ickey
I don’t follow that chart but if just said you in late flowering and you temp is 77 and you HR is 40% you in good rank just little up on temp but not to much
Happy growing 
Just giving an example with those numbers. I have been playing around with my tent pressure, and it occurred to me that more information would be good. My 40 years in the HVAC field taught me that changes in pressure move boiling points and evaporation rates. I am not sure if the same rule will apply here in the plant world?
I guess another way to pose the question would be this.
Does altitude effect a plant’s transpiration rate?
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Negative pressure on tent and vapor pressure deficit are two different things. In a perfect world you want Negative pressure on tent if usinh carbon filter to make sure you don’t have scent leaks, and to keep your vpd proper for stage of plant growth you are in. So short answer is that you want both. This is most easily done by balancing temp and rh of your intake air with amount of air exchange rate.
You don’t exactly have to be perfect to run a good grow either. Just use as guideline and do best you can.
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Really interesting question @Ickey! It looks like there is research being done on that, and that maybe fluctuating between high and low can be beneficial. I’m not sure how much you could really affect atmospheric pressure in the tent, you’d need to measure.
@BobbyDigital, did I read about you cycling between high and low pressure in your environment? Or at least positive and negative. Someone on here was already trying that (of course).
No. If anything it would move it further into the red. Lower air pressure may increase the rate of evaporation a bit, worsening your low humidity problem.
Using nature as a guide, I have been reducing light output and putting the tent in negative press after watering. Then the following days back to full light and ramping up pressure to positive before next watering. While maintaining correct temp/humid. I only have this much control in my blooming tent. If I water early in the morning. I will start bringing the light and pressure back up after about 6 hours. My last harvest after doing this seems to have produced the best yet smell, taste, and potency. And this comparing B. Banner to B.Banner. But I need to note my tent has been cooler than normal during this grow also. I should note, using soil.
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Thanks Dr Woo
I was afraid that I may have that backwards. But it does have an effect?
So does that mean it will be best to harvest after a positive pressure cycle?
The only thing that I haven’t seen mentioned yet is that hot air can hold more water than cool air.
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Ah…. You’re in flower. I assumed you had seedlings and couldn’t get humidity up. Negative pressure may affect transpiration a bit by affecting evaporation rate, but I’d guess (not know) the effect is minimal. You’re likely affecting temp and humidity more with the high negative pressure. This will have a greater effect on transpiration than pressure will directly.
I am not taking the tent pressure too extremes. Just changing the input fan speed a little to obtain slightly neg, equal, or slightly positive. I mainly stay on track with matching rh to the temperatures.
So maybe I am on the right track with the reduced light and pressure after watering. If negative pressure will increase transpiration. Wouldn’t this allow them to focus less on growing and more on absorbing the water or nutrients they were just given? Then the next 3 days is full on grow.
I don’t think I would use this technique if water was needed more than every three days.
It sounds more like you’re trying to imitate the impact of VPD with pressure. I’m saying you will have more control by managing VPD directly, using temp and RH.
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I am clearly out of my league here! After looking up VPD, I think we are on the same page here. I agree, temperature and humidity are the best way to stay on track with vpd,(that’s a lot easier to type than transpiration).
This all started with a simple idea from my simple mind, and this was before I knew anything about vpd or transpiration. I noticed that after watering my plants looked sluggish. In an effort to simulate nature, I thought in nature, a cold front with clouds moves in and the pressure drops and the rain soaks the root. The next days it is sunny with a high pressure.
So on the days I water, I also reduce light output and set pressure to negative. The next three day are full light, two days at equal pressure, and one at positive. Then repeat cycle. My hope was that the pressure changes would somehow trigger the plants into producing a better more potent bud.
For all I know it might. Plants key off different environmental factors to trigger growth responses. Look up the red/far red phytochrome cycle for an afternoon’s rabbit hole exploration. I don’t think you’re hurting anything, in any case. Please report back if you yield positive results.
Thanks
It really helps that you straighten my 180 degree out of phase thinking about the vpd vs pressure.
Will do on the results.
Setting the pressure thing aside. Just before harvest. Will be best to keep the humidity as high as possible while staying in the yellow area of the above chart?
When in flower you want humidity lower for two reasons:
- VPD is high, forcing the plant to take in more nutes
- High humidity will lead to bud rot and other mold growth
If you want to stop the plants from taking in the nutes near harvest you’re better off with flushing them from the soil than by slowing the take up using VPD.
I do because my intake always runs but my exhaust shuts down with my lights. So at lights out, I’m under positive pressure and lights on I’m under negative pressure.
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