LarsVegasNirvana
Well-Known Member
Yes, I run VPD. This CO2 guide says not to run VPD if you run CO2, which is some bullshit. I'm not concerned with the dry weight of radishes, I'm not trying to grow roots.
I use this chart:
Killer info for setting up a CO2 equipped grow room!
- Thread starter OldMedUser
- Start date
I use this chart:
Keesje
Well-Known Member
I can't find this in the link from OldMedUser.
Or are you referring to another guide?
LarsVegasNirvana
Well-Known Member
About halfway down in the section "CO2 and Relative Humidity"
The experiment they're quoting was done at 90% humidity, which means basically no transpiration happening at all.
Keesje
Well-Known Member
I also can find no legitimate reason for not sticking to a good VPD when using CO2.
Can't find any scientific ground for that.
The hardest thing for commercial growers in greenhouses is to maintain a good VPD (also with CO2)
In principle, the stomata should be as open as possible to let CO2 in and stay closed as much as possible to limit moisture loss.
These are of course 2 'ideal' situations that do not go well together.
The plant therefore constantly seeks a balance between moisture loss and CO2 inlet.
That balance is always different and depends on the moisture condition of the plant, humidity, light intensity, light color and CO2 concentration. All these factors plus the internal rhythm of the plant determine whether and to what extent the stomata are open.
The plant does not respond directly to the humidity in the air, but at its own internal moisture state.
That condition is the result of the interplay between humidity, evaporation and water absorption.
It is very logical that the plant closes the stomata if water loss leads to problems, however harmful that may be for the photosynthesis.
After all, there is no more CO2 coming in on that moment.
This will be also disadvantageous for the leaf temperatures that will run too high.
Because of the higher internal temperature, the evaporation will be stimulated. Not through open stomata > even with closed stomata the leaves are not completely vapor proof.
This ultimately leads to even more water stress.
I am not a biologist and this is not knowledge that I have available. I wish I was that smart.
But I do like to read scientific reports that are meant to support greenhouse cultivation.
These papers are mostly from an important agricultural university and tested in real life with real companies.
While reading all these papers, another question occurred to me.
I always see the VPD charts on all Cannabis forums.
Now it seems to me that this is a general chart for C3 plants.
Not specific to Cannabis and for sure not specific to for example Sativa, Indica and certain strains.
But, just an example, research with tomatoes has been conducted and they found out that 0.8 was the ideal VPD that led to a significant increase in yield.
Who did this research for Cannabis? Are the charts right for it?
Can't find any scientific ground for that.
The hardest thing for commercial growers in greenhouses is to maintain a good VPD (also with CO2)
In principle, the stomata should be as open as possible to let CO2 in and stay closed as much as possible to limit moisture loss.
These are of course 2 'ideal' situations that do not go well together.
The plant therefore constantly seeks a balance between moisture loss and CO2 inlet.
That balance is always different and depends on the moisture condition of the plant, humidity, light intensity, light color and CO2 concentration. All these factors plus the internal rhythm of the plant determine whether and to what extent the stomata are open.
The plant does not respond directly to the humidity in the air, but at its own internal moisture state.
That condition is the result of the interplay between humidity, evaporation and water absorption.
It is very logical that the plant closes the stomata if water loss leads to problems, however harmful that may be for the photosynthesis.
After all, there is no more CO2 coming in on that moment.
This will be also disadvantageous for the leaf temperatures that will run too high.
Because of the higher internal temperature, the evaporation will be stimulated. Not through open stomata > even with closed stomata the leaves are not completely vapor proof.
This ultimately leads to even more water stress.
I am not a biologist and this is not knowledge that I have available. I wish I was that smart.
But I do like to read scientific reports that are meant to support greenhouse cultivation.
These papers are mostly from an important agricultural university and tested in real life with real companies.
While reading all these papers, another question occurred to me.
I always see the VPD charts on all Cannabis forums.
Now it seems to me that this is a general chart for C3 plants.
Not specific to Cannabis and for sure not specific to for example Sativa, Indica and certain strains.
But, just an example, research with tomatoes has been conducted and they found out that 0.8 was the ideal VPD that led to a significant increase in yield.
Who did this research for Cannabis? Are the charts right for it?
OldMedUser
Well-Known Member
I consider them too high myself. If I get the VPD right according to any of the many charts I've seen if feels like a tropical jungle in my room. Condenses on the walls and floor that are cooler so would for sure cause mould to grow. Our RH is very low here so it's a hassle trying to keep the RH up in the grow room. Even with a 600W HPS going right now it takes ages to get warm enough to trip the exhaust fan so my RH is pretty good right now. 72F @ 55% but it's 23 days into flowering so I'm going to want lower RH pretty soon. Adding some CO2 because I'm not getting regular fresh air coming in often enough.
Perfection is a goal rarely reached.
LarsVegasNirvana
Well-Known Member
I was with you on the higher humidity train, but then I tried to find the study you just asked for and it turns out that they found drier is better. At least that's what it says in the abstract.
https://sci-hub.tw/10.1007/s12298-008-0027-x
Keesje
Well-Known Member
As I read it, they do not state that dryer is better.
They only say that they did their tests with a RH of 55% (in the range of)
But can't find any claim that dryer is better.
Besides that, they tested with different temperatures but with the same RH
To study the photosynthetic response of C. sativa under different PPFD and temperature levels, leaves of twenty vegetatively propagated, four month old plants from a single mother plant of high yielding Mexican variety were exposed to a range of PPFD (0, 500, 1000, 1500 and 2000 μmol m-2s-1) and temperature conditions (20, 25, 30, 35 and 40 oC) under controlled humidity (55 ± 5 %) and CO2 (350 ± 5 μmol mol-1) concentration to determine suitable environmental conditions for it’s optimum photosynthetic assimilation.
With a humidity of 55% at 20 degrees the VPD would be 1.05
But with a humidity of 55% and 30 degrees the VPD would be 1.91. Way to dry for most plants.
I also wonder if they would have had the same results in their tests if they did not maintain their humidity on a fixed number, but trying to maintain the same VPD, no matter what the temperature. For example 20 C & 55% RH = VPD 1.05 and 30 C & 75% RH = VPD 1.07
How the stomata react is only partly because of the amount of light.
If one doesn't measure leaf temperature and it is not used in the chart, the chart only helps a little.
Especially now with people using led this becomes more important.
They only say that they did their tests with a RH of 55% (in the range of)
But can't find any claim that dryer is better.
Besides that, they tested with different temperatures but with the same RH
To study the photosynthetic response of C. sativa under different PPFD and temperature levels, leaves of twenty vegetatively propagated, four month old plants from a single mother plant of high yielding Mexican variety were exposed to a range of PPFD (0, 500, 1000, 1500 and 2000 μmol m-2s-1) and temperature conditions (20, 25, 30, 35 and 40 oC) under controlled humidity (55 ± 5 %) and CO2 (350 ± 5 μmol mol-1) concentration to determine suitable environmental conditions for it’s optimum photosynthetic assimilation.
With a humidity of 55% at 20 degrees the VPD would be 1.05
But with a humidity of 55% and 30 degrees the VPD would be 1.91. Way to dry for most plants.
I also wonder if they would have had the same results in their tests if they did not maintain their humidity on a fixed number, but trying to maintain the same VPD, no matter what the temperature. For example 20 C & 55% RH = VPD 1.05 and 30 C & 75% RH = VPD 1.07
How the stomata react is only partly because of the amount of light.
What most of the charts lack imo (also the one that LarsVegasNirva posted above) is the difference between room temperature and leaf temperature.
If one doesn't measure leaf temperature and it is not used in the chart, the chart only helps a little.
Especially now with people using led this becomes more important.
LarsVegasNirvana
Well-Known Member
If you notice on the chart I posted they have all the values over 70%RH greyed out. Even though it is the optimal VPD and I've tried it and it works, it wreaks havoc on your equipment. I had a clip fan violently rip itself off the tent pole when the bearings rusted out and it started vibrating. I run 70% at the beginning of flower and 60% at the end. The point isn't to follow VPD exactly, it's just to try to be as close as possible to it.I consider them too high myself. If I get the VPD right according to any of the many charts I've seen if feels like a tropical jungle in my room. Condenses on the walls and floor that are cooler so would for sure cause mould to grow. Our RH is very low here so it's a hassle trying to keep the RH up in the grow room. Even with a 600W HPS going right now it takes ages to get warm enough to trip the exhaust fan so my RH is pretty good right now. 72F @ 55% but it's 23 days into flowering so I'm going to want lower RH pretty soon. Adding some CO2 because I'm not getting regular fresh air coming in often enough.
Perfection is a goal rarely reached.
LarsVegasNirvana
Well-Known Member
They do. It's at the end of the abstract:
"The study reveals that this species can be efficiently cultivated in the range of 25 to 30 oC and ~1500 μmol m-2s-1 PPFD. Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO2 concentrations in C. sativa, reflects its potential for better survival, growth and productivity in drier and CO2 rich environment."
Unfortunately, if you read down in the paper they give even less information:
"Furthermore, higher PN, WUE and nearly constant Ci/Ca ratio under elevated CO2 concentration, reflects its potential for improved growth and productivity in drier and CO2 rich environment."
PN is photosynthesis, WUE is water use efficiency, and Ci/Ca ratio is the ratio of CO2 in the leaf vs outside. They just know from that information that it would work better at low humidity, even though they only tested various VPDs by changing the temp.
Basically with higher levels of photosynthesis and better water use efficiency caused by high CO2, the plants transpire less. I guess by raising humidity you limit transpiration even more causing a shortage of nutrients.
LarsVegasNirvana
Well-Known Member
Leave it to the national institute on drug abuse to not give us a handy chart to grow our schedule 1 narcotics with.
OldMedUser
Well-Known Member
These cheap f'n fans always seize up after a while. The bushing gets gummed up and the fan will usually run fine until you shut it off then it won't get going again. To fix that I take them apart and if an oscillating fan remove the mechanism at the back of the motor to get to the bushing.
Then spray some WD-40 in there to loosen them up and get it spinning again and run for a bit to dry out the WD then add a couple drops of 3-in-1 oil or light oil for sewing machines. Good for another 6 months or so but eventually that doesn't help much either.
Recently got a wall mount 16" but it's the same crappy Chinese motor drive so in 6 months I'll be fixing it too I bet. If I could find some decent ones with ball bearings I'd pay extra for a couple of those. Wife is picking up my new 8" inline at the PO right now.
I aim for 40% in very late flower once the colas have stacked up to prevent bud rot. Easy to get that here. RH here at my desk is only 21% at 72F and we haven't even hit the deep freeze yet when it'll be down to 10% or lower in the house and barely 30% in the grow room.
LarsVegasNirvana
Well-Known Member
I could calculate the difference for leaf temp but my hygrometers aren't calibrated well enough to be within 5% humidity anyway. It
Even if we had fans with ceramic bearings, the humidity is going to destroy your humidity sensor. Which is incredibly ironic.
I only use the small honeywell fans now. They're ultra-efficient compared to everything else on the market, and they're reliable enough that I've never had one fail.
I've never had any fungal pathogens except once when I had a tiny spot of what looked like cob-webs where a bud was touching the wall. As long as you don't have low night temps and don't have humidity swings you won't get PM or botrytis.
I guess I'm going to aim for something more like 60% at week 1 and 50% at week 8? The NIDA didn't tell me what humidity to use, and I'm not going to use 35% because I have to pay mucho dinero for dehumidification electricity.
The bearings were completely gone. The blades and the motor axle and windings fell out of the front of the fan.
Even if we had fans with ceramic bearings, the humidity is going to destroy your humidity sensor. Which is incredibly ironic.
I only use the small honeywell fans now. They're ultra-efficient compared to everything else on the market, and they're reliable enough that I've never had one fail.
I've never had any fungal pathogens except once when I had a tiny spot of what looked like cob-webs where a bud was touching the wall. As long as you don't have low night temps and don't have humidity swings you won't get PM or botrytis.
I guess I'm going to aim for something more like 60% at week 1 and 50% at week 8? The NIDA didn't tell me what humidity to use, and I'm not going to use 35% because I have to pay mucho dinero for dehumidification electricity.
OldMedUser
Well-Known Member
I don't have to use a dehuey for low rh. It's dry up here in northern Alberta except for a couple summer months and even then fairly low. I try to time my growing so I'm just vegging in the summer indoors. Cool in the basement all year so never needed AC.
I might get a small dehuey to knock back the rh when the lights go out when they are near done flowering but wouldn't worry about it for veg plants. It is Relative Humidity after all.
I might get a small dehuey to knock back the rh when the lights go out when they are near done flowering but wouldn't worry about it for veg plants. It is Relative Humidity after all.
Keesje
Well-Known Member
By raising your humidity, you indeed can cause a shortage of nutrients. But it can also cause your plant becoming too hot because it can not transpire. It will also close the stomata, and thus less absorbing of CO2. So plenty of water, plenty of CO2, no nutrients, and heatstress.
By lowering your humidity, your plant will of course uptake more nutrients, but it will lose more water because of transpiration then the roots can suck up. So more heat for your plants. Not enough water. A lot of CO2, yes (because the stomata are wide open) So plenty of nutrients, plenty of CO2, no water, and heatstress.
That's why commercial growers watch VPD, also within CO2-enriched greenhouses.
They want to find the balance between enough water, enough nutrients-uptake, enough CO2 and no stress.
I read the report again and I could read also (as you showed me te part) that they claim that in a dryer situation their plants did better.
But in my opinion their test was lousy.
Every grower knows that humidity and temperature are connected.
They change temperatures in their tests, but they never change their humidity.
Then such a test is incomplete. IMHO.
I have several hygrometers in my room.
They all give different results. Just the slightest blow of air changes them.
What I do is look at all of them and take an average.
I was in a greenhouse recently and they have these great camera's that can measure the temperature of the canopy.
They control their whole greenhouse with VPD: measuring humidity, ambient temperature and leaf temperature. Without any of these 3, a result is useless.
For example if your room temperature is 30 C / 86 F and humidity is 65% and you assume that the leaf temperature is the same as ambient temperature, then the VPD is around 1.5.
But if you would measure the temperature of your leaves are it turns out to be 28 C / 82.5 F then the VPD also turns out to be around 1.
OldMedUser
Well-Known Member
I do the same thing. 2 small electronic ones, an old analogue one that I just found out how to calibrate properly and a remote one I got at Costco with it's readout screen above my computer monitor so I can see what's going on in real time down there. Temp would get higher than it was supposed to and after a bit of sleuthing realized my cooling thermostat was shot and not turning on my exhaust fan until it got close to 10F higher than it was set at. I keep spares so 15 min later all good again.
For years I was getting nute burn starting around the 4th week of flower that would get real bad sometimes by the end. Thought I was feeding too much N so went to a Lucas Formula style of feeding with my 3-part AN nutes after the stretch and still had it. Was only a few years ago that I discovered VPD and now know it's because of my low RH that I have. Causes my plants to transpire a lot more which draws up more nutes than the plant can use so builds up in the leaves leading to Toxic Salts Buildup. In pots I started feeding at about half the normal rate and splitting that up so I feed each watering to reduce spikes in nute levels. Seems to be working as my last crop of 12 plants was much better as far as signs of burning goes.
This 'hobby' can sure be a complicated science experiment can't it.
LarsVegasNirvana
Well-Known Member
Stoma don't open and close based on humidity, they do it based on the difference in humidity between the inside of the leaf and the outside. If the roots can't provide enough water then the stoma close. The plant is already solving all of these problems you think you're solving for it.
Your plants aren't going to magically get hotter than the environment they're in, so simply lowering humidity won't cause them to overheat.
The suggested humidities in this guide are not that crazy or far off from the VPD charts we're used to. Starting at 65% in veg and going down to 50% in late flower? That's not that much lower than what I run now.
OzyM8
Well-Known Member
Here’s a decent VPD chart you can get from Pulse Labs. You can put your LST off set from ambient temps, and adjustments for growth stage. Also work in C or F.
An inexpensive infrared temp gun is a handy tool. The Flir infrared cameras and software are expensive for hobby gardeners
getpulse.co
An inexpensive infrared temp gun is a handy tool. The Flir infrared cameras and software are expensive for hobby gardeners
The Ultimate Vapor Pressure Deficit (VPD) Guide
"Why devote an entire guide to VPD?" you might ask. The answer is that the vapor pressure deficit (VPD) is extremely important for growing plants. VPD helps you identify the correct range of temperature and humidity to aim for in your grow space. With VPD you can achieve the best results while...
getpulse.co
LarsVegasNirvana
Well-Known Member
Here is a good explanation of WUE:
https://www.frontiersin.org/articles/10.3389/fpls.2019.00103/full
As WUE increases, the plant takes the same amount of water up through the roots, but it transpires less through the leaves. The whole point of VPD is to maintain the proper amount of transpiration for the growth stage we're at. If you lower it with CO2, you can't lower it again with VPD. If you don't lower it with CO2, you have to lower it with VPD instead or it'll be too high.
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LarsVegasNirvana
Well-Known Member
the difference of a couple degrees in LST differential is going to translate to like one percent humidity and most people's humidity sensors are only accurate to 5% anyway, and probably haven't been calibrated properly anyway.Here’s a decent VPD chart you can get from Pulse Labs. You can put your LST off set from ambient temps, and adjustments for growth stage. Also work in C or F.
An inexpensive infrared temp gun is a handy tool. The Flir infrared cameras and software are expensive for hobby gardeners
The Ultimate Vapor Pressure Deficit (VPD) Guide
"Why devote an entire guide to VPD?" you might ask. The answer is that the vapor pressure deficit (VPD) is extremely important for growing plants. VPD helps you identify the correct range of temperature and humidity to aim for in your grow space. With VPD you can achieve the best results while...
They should have a calculator that includes CO2 concentration too. That would actually be useful.
OzyM8
Well-Known Member
I like to think of it being within a target zone. Environmental controls are going to have working offsets, min / max settings etc, and ramp up / down time to reach targets anyway. LST is going to be an average of the canopy too. Being within a few .points of the wanted VPD is a desirable goal.
I.e if in veg and the average LST is -1 on ambient of 27C then you may want to shoot for 64-68% RH. If same and LST was -2 then something like 58-62% may be better.