What's new
  • Happy Birthday ICMag! Been 20 years since Gypsy Nirvana created the forum! We are celebrating with a 4/20 Giveaway and by launching a new Patreon tier called "420club". You can read more here.
  • Important notice: ICMag's T.O.U. has been updated. Please review it here. For your convenience, it is also available in the main forum menu, under 'Quick Links"!

Vapor Pressure Deficit - VPD and Nutrient Deficiency

K

Kiva

New member
For all you VPD aficionados: The new update on my SmartBee Controllers system now incorporates VPD. You should check them out.
 
L

linky

Member
anyone growing using the vpd chart in a perpetual room? I would like to set my room up (have everything to keep my room at whatever humidity I choose) but my room is perpetual. Are there issues with high humidity in late stages of flower (I always have plants in the last couple weeks of flower).
 
Three Berries

Three Berries

Active member
I have an AC Infinity setup that I can monitor the VPD and react to temp and RH. Question is where do you put the sensor?

In the past I put it up high usually where the air was exhausting and hottest. But if VPD is the real concern wouldn't it be better to have the sensor near the leaves?

I have a couple small 4" fans. One on the floor blowing up and another up high blowing down.

Been trying to maintain 1.00-1.10 for the last year or so.

I tried putting in a +2F offset for leaf temp and that really changes the VPD value. But when I measure the leaf surface temp it's almost always the same as air temp. Probably due to the air blowing around.

Still I've moved the sensor down near the plant.

Interesting to see the plants still sucking up CO2 with lights off. I have noticed a spike when lights off.
 
S

St. Phatty

Active member
Vapor pressure is super important.

One of my back-burner technical/research interests is how Solid particles obey the rules of Chemical Diffusion - that are supposed to only apply to gases.

Smoke particles are one example.

Dust particles are another.

Covid19 virus particles are another.

All 3 spread like a gas. A killer virus that diffuses like a gas is a BIG PROBLEM.

I think Physical Chemistry is one of the sciences that describes Vapor Pressure etc.

HIGHLY RECOMMENDED - "Concise Physical Chemistry" book by Donald W. Rogers. Someone gave me the *.pdf.
 
P

Piff_cat

Well-known member
Three Berries said:
I have an AC Infinity setup that I can monitor the VPD and react to temp and RH. Question is where do you put the sensor?

In the past I put it up high usually where the air was exhausting and hottest. But if VPD is the real concern wouldn't it be better to have the sensor near the leaves?

I have a couple small 4" fans. One on the floor blowing up and another up high blowing down.

Been trying to maintain 1.00-1.10 for the last year or so.

I tried putting in a +2F offset for leaf temp and that really changes the VPD value. But when I measure the leaf surface temp it's almost always the same as air temp. Probably due to the air blowing around.

Still I've moved the sensor down near the plant.

Interesting to see the plants still sucking up CO2 with lights off. I have noticed a spike when lights off.
Click to expand...
since vpds function is opening/closing stomata you want the reading to be on the leaf surface. ultimatly its leaf surface temp and humidity that matters most. also a good idea to get a baseline for the temp difference between leaf and air. this offset can vary depending on your light type, hang height, distance from canopy fans etc. a good way to think about vpd is the humidity as a shield which lets you run higher temps and increase growth rate. but there is also a balance between health/yield/ and resin.

since we grow for secondary metabolites an "ideal" enviorment will differ from a food crop. dont want to encourage vegetative growth after the first month or so of flower. bigger day/night temp swings and drier air at second half of flower will increase resin quantity/quality. yield and potency come to an impass . the type of lighting system is very important to come up with vpd program. led lights put out significantly less infrared heat while hps has high infrared heat which encourages transpiration. the humidity will rise in hps room much faster then an led one. its mostly about tinkering with your own room to see how plants best react. using an infrared thermometer to get a baseline difference between leaf temp and air temp is the most important part or else the whole equation is thrown off. in my room i like to run80-82 F/ 70-75 rh in first half then progressivly drop the humidity and the night time temperature in second half. drought is the number 1 condition which increases resin.
 
Three Berries

Three Berries

Active member
Well I moved my sensor to canopy height next to the plants. I have a probe up high in the tent too and there usually is not much difference due to the fans circulating. Same goes for anytime I've tried a leaf surface temp.

If I put in a 2F leaf temp offset it really changes the VPD to smaller values, ~.8xs vs 1.xxs.

I usually water when my meter tells me too and try to maintain a relatively consistent moisture level in the medium. As it is now when the VPD gets too high I know it needs watered.

Right now with high CO2 and plenty of light they are regulating their VPD RH pretty good. The fan comes on a 76F or 65RH and is usually running at the 1 or 2 speed out of ten. No misters or wet towels needed.

Lights off the room today is 58% so I might be able to get the RH down to 68% running the fan on auto..
 
P

Piff_cat

Well-known member
St. Phatty said:
Does this mean that Resin is part of the plant's mechanism for keeping moisture inside the plant ?
Click to expand...
hmm thats an interesting question. stress is what causes the plant to create secondary metabolites. as the leaves/stem dry out they become more vulnerable to UV or other stresses. as a result signal hormones like methyl jasmonate are dispatched to instruct the affected areas organs to increase transcription(production) of the synthase enzyme which transform cbga into thc inside the trichomes.
so yes you could say the plant is trying to keep moisture inside but moreso its recognizing that low moisture puts it in danger and as a result inceases resin production to offset and protect the vulnerable organs. almost like the hot dry air pulls the trichomes into high gear. this is why good hasplants come from mountanious areas they have low night time temps dry air and intense uv.
 
P

Piff_cat

Well-known member
Crooked8 said:
If this were true, why does dwc and deep flow hydroponics grow insane quality?
Click to expand...
hydro and dwc have prolific growth rates and certainly put out great bud im not saying water is bad. its about the humidity or lack there of in the air. its about manipulating the size of stomata to get maximum transpiration. move nutrients from roots to top of plant especially important for non mobile elements like calcium. there is a goldilocks zone where u can get good yield and potency but after that they will diverge. its a zero sum situation theres only so much pre cursor to be spread around. certain point dilution will happen. ddthats why i think growing large number of small plants could be good compromise
 
P

Piff_cat

Well-known member
1675636667928.png

heres a good article if interested
www.benzinga.com

Study: Underwatering And Drought Conditions Could Induce Cannabis Plants To Produce More Cannabinoids

www.benzinga.com www.benzinga.com
 
Crooked8

Crooked8

Well-known member
Mentor
ICMag Donor
Veteran
Piff_cat said:
hydro and dwc have prolific growth rates and certainly put out great bud im not saying water is bad. its about the humidity or lack there of in the air. its about manipulating the size of stomata to get maximum transpiration. move nutrients from roots to top of plant especially important for non mobile elements like calcium. there is a goldilocks zone where u can get good yield and potency but after that they will diverge. its a zero sum situation theres only so much pre cursor to be spread around. certain point dilution will happen. ddthats why i think growing large number of small plants could be good compromise
Click to expand...
I totally understand this concept. This is why I try to drive rh down as they develop further into flower. Although VPD may be imperfect, resin production certainly increases and ive tested this in many formats. It doesnt surprise me at all that stomata manipulation would cause this reaction. I was more surprised by the idea that drybacks/drought in late flower would increase resin production. This has not been my experience. The highest quality product ive experienced comes from dwc with full RH control. Weeks 1-3 dialed to 55-60% RH, 3-6 50% RH, 7-9 down to 45% or even 40% for the last week. In all media, rockwool, living soil, cocoa, and a peat mix, the quality improved in all areas. Keeping within those ranges also reduces a host of fungal issues. Rockwool in dwc clocking the highest cannabinoid and terpene numbers. Drybacks/drought not much of a factor beyond week 3-4 since most of the root zone is now submerged fully. Although i truly dont trust testing facilites but thats another story hah.
 
P

Piff_cat

Well-known member
Crooked8 said:
I totally understand this concept. This is why I try to drive rh down as they develop further into flower. Although VPD may be imperfect, resin production certainly increases and ive tested this in many formats. It doesnt surprise me at all that stomata manipulation would cause this reaction. I was more surprised by the idea that drybacks/drought in late flower would increase resin production. This has not been my experience. The highest quality product ive experienced comes from dwc with full RH control. Weeks 1-3 dialed to 55-60% RH, 3-6 50% RH, 7-9 down to 45% or even 40% for the last week. In all media, rockwool, living soil, cocoa, and a peat mix, the quality improved in all areas. Keeping within those ranges also reduces a host of fungal issues. Rockwool in dwc clocking the highest cannabinoid and terpene numbers. Drybacks/drought not much of a factor beyond week 3-4 since most of the root zone is now submerged fully. Although i truly dont trust testing facilites but thats another story hah.
Click to expand...
Controlled stress is a cool topic which doesn't get much discussion. I also think it's dependent on strain type concerning where the plant originated. And also the content inside the resin. I grow mostly haze which seems to prefer a constantly moist warm rootzone with good drainage. This must be about the ratio of available oxygen so this could be a big benefit of the dwc since it greatly increases oxygen access. Whereas hybrid types seem to prefer big swings of saturated root zone all the way to dry which spikes the ec. The terpene content and make up is also subject to manipulation especially at the end. Cold dry drying room substantially cuts down the volatile monoterpenes even when they stunk in flower. These type of programs applied to specific strains and profile goals would be great to develop.
 
Three Berries

Three Berries

Active member
I'm currently smoking a grow from Dec of 2021 that has been froze. It still is quite fragrant and sticky. But before cutting I gave it three days of dark in about 45-50F. It was the stickiest plants I ever grew. Completely covered the main trunk to the dirt. And I had grow others of this strain before. OGKush photo from ILGM

I've tried the three days of darkness three other times and never got the stickiness like that one, but IIRC it's the only one done during winter when the room was cold vs 75F or so..

And they didn't get watered either for the three days. Actually loosely covered with black plastic..
 
P

Piff_cat

Well-known member
Three Berries said:
I'm currently smoking a grow from Dec of 2021 that has been froze. It still is quite fragrant and sticky. But before cutting I gave it three days of dark in about 45-50F. It was the stickiest plants I ever grew. Completely covered the main trunk to the dirt. And I had grow others of this strain before. OGKush photo from ILGM

I've tried the three days of darkness three other times and never got the stickiness like that one, but IIRC it's the only one done during winter when the room was cold vs 75F or so..

And they didn't get watered either for the three days. Actually loosely covered with black plastic..
Click to expand...
i hadnt thought about cold stress that is cool. its primarily myrcene which is lost in low temps in drying but og doesnt have much. so maybe classifying drying regiments specific to terpene chemotype would be something to look into. drying and curing took a big hit in the change from landraces grown in country to more home growers in ithe early med movement pushing barely dry flower out. sun drying turns some thc into cbd, but it also decarbs the thca and stresses the plant at the end. alot of landrace sativa plants are intentionally abused at the end to increase secondary metabolites. similar to oolong tea procedure. not to mention stuff like underground clay pot curing or a malawi cob type situation. rupturing the glands in a 0 oxygen enviorment coats the vegetative material. hash curing has been known to do crazy stuff as well. hashishene is a terpene produced in the hash curing process when uv turns myrcene into hashinene. all the old thai sat on big ships for months with time to cure and change. this area will continue to evolve and improve. marijuana chemistry by michael starks is a great book from the 80s which documents alot of the old landrace production methods
 
You must log in or register to reply here.

Latest posts

Latest posts

Top