knna

AS time ago we had a interesting disscusion about this topic at the Cannabis World (before it was converted on a seed sale site), im going to quote myself, includiong some very useful links (hope they still work):

"As i said before, RH measure alone is insufficient to inform about transpiration rate.

If we want to compare differents setups, we will need more accurate measurements. VPD (vapor pressure defficit) is the right parameter. It includes RH and temp, combined with leaf temp. Supposing enough water avalaible and not salinity excess wich dont must succeed in a hydro system (meaning near constant and anycase, good water potential), VPD gives enough info to check transpiration rate and study TDS levels according to.

Usually, a range between 0,8 and 0,95 KPa is accepted as the best for optimizing transpiration rate. For cannabis cultivation, its better the high end of the range in flower stage, because is prefered a little low humidity envirovement to produce more resin and avoid mold problems. So, trying to get 0,90-1 KPa of VPD would be the target.

But, anyway, when comparing transpiration rates and TDS levels, the most informative data is give the VPD at it was produced.

Explain fully what VPD is, how to compute it would require a very large post, and all the info is already in the net, so ive selected some links in order to understand it:

This is a great ppt presentation (dont work now)

This wikipedia entry is very usefull. Short, and providing links to more info, about units used (and conversion to others) and to a VPD calculator. The usual range of leaf temp is about 3ºC less than ambient (at ambient CO2 concentrations, about 300ppm) to only 1ºC less (at high enriched CO2 concentrations, over 1000ppm). Canopy exposed to high IR content light can be perfectly over ambient temp.
Direct link to VPD calculator.
This provides a VPD calculation method. It use psi units, to convert to KPa: 1psi=6,895 KPa.
http://www.hortnet.co.nz/publication...umid01.htm#topThis is very good explaining concepts, notice there are 7 diff pages.

Finally, a excelent study about all this parameters: transpiration rate, VPD, light quality and irradiation:

Spatial distribution of leaf nitrogen and photosynthetic capacity. Only CO2 supplementation remain unstudied, but has several interesting links.

http://www.ars.usda.gov/SP2UserFiles...nspiration.pdfI put this because it study CO2 in relation to VPD. (dont work now)

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knna

I continue quoting (my posts and Lucas related post):

"Thanks Knna

very good info links on the relationship between temperature and humidity..

I used a farenheit to celcius converter here Celcius and Fahrenheit Converter to calculate the celcius equivalent of 80 degrees farenheit, then used the calculator you linked to.. VPD Calculator thanks!

I reached a couple of preliminary baseline conclusions.. assuming the target Kpa is .9 and that leaf temp is 3Celcius degrees below air temp..

if the temperature is 85F, humidity should not be higher than 62%

if the temperature is 80F, humidity should not be higher than 58%

If the temperature is 75F, humidity should not be higher than 53%

If the temperature is 70F, humidity should not be higher than 47%

the rule of thumb seems to be a target temp of 80F should have just under 60% humidity.

An interesting observation is that as temperature drops by 5 degrees F, required humidity also drops by 5 %

so, 80F @ ~60%, or 70F @~50%, are good ballparks..

thanks!
Lucas

ps. leaf temp will be a function of how close the light is.. so if there is less than a 3Celcius degree difference, the light could be closer..

"Yes, the concept is at

higher ambient temp-higher RH
higher CO2 concentration-higher RH

And in general, at higher metabolism rate, higher RH. Always between limits. Both, excesive RH and low RH lead to transpiration become difficult.

Time ago, i did some calculations:

T(ºC)-T(F)-RH-VPD(3ºC)-VPD(2ºC)
18-64-35-0.98 1,1
20-68-35-1,12 1,25
23-73-35-1,35 1,5
26-79-35-1,63 1,81
30-86-35-2,08 2,29

18-64-40-0,88 0,99
20-68-40-1 1,13
23-73-40-1,21 1,36
26-79-40-1,46 1,64
30-86-40-1,87 2,08

18-64-45-0,78 0,89
20-68-45-0,89 1,01
23-73-45-1,07 1,22
26-79-45-1,3 1,47
30-86-45-1,66 1,87

18-64-50-0,67 0,79
20-68-50-0,77 0,89
23-73-50-0,93 1,08
26-79-50-1,13 1,3
30-86-50-1,44 1,66

18-64-55-0,57 0,68
20-68-55-0,65 0,78
23-73-55-0,79 0,94
26-79-55-0,96 1,14
30-86-55-1,23 1,45

18-64-60-0,47 0,58
20-68-60-0,53 0,66
23-73-60-0,65 0,8
26-79-60-0,79 0,97
30-86-60-1,02 1,23

18-64-65-0,36 0,48
20-68-65-0,42 0,54
23-73-65-0,51 0,66
26-79-65-0,62 0,8
30-86-65-0,81 1,02

18-64-70-0,26 0,37
20-68-70-0,3 0,43
23-73-70-0,37 0,52
26-79-70-0,46 0,63
30-86-70-0,6 0,81

I work within these ranges:

18ºC-64,4F - 35%RH. Max 55%
20ºC-68F- 35-45% RH. <60%
23ºC-73,4F- 45-50% RH. 34-64%
26ºC-78,8F- 50-55% RH. 42-67%
30ºC-86F- 60% RH. 50-70%

Range are computed between 0,55 (min) and 1,4 KPa.

Quote:

leaf temp will be a function of how close the light is.. so if there is less than a 3Celcius degree difference, the light could be closer..

Yes, but if you have leaf temp at canopy less than 3ºC below ambient, something is going wrong (bulb too far or some kind of lock, reducing metabolic activity).

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GreenDragon2k

daaayum! im gonna have to put down the pipe for a couple hours and come back and read that later Great info Thanks!

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knna

Last quotes

"> evidence of photosynthetic acclimation to elevated carbon dioxide in soybeans results from greater susceptibility to water stress in plants grown at elevated carbon dioxide

translation, if you raise CO2, plants will be unable to use it if the air or roots are too dry.

"On days with either dry soil or dry air, acclimation was apparent. On days shortly after precipitation events and on humid days there was no evidence of acclimation."

http://www.ars.usda.gov/research/pub..._NO_115=112263
---

thanks to plantbuilders acclimation info, it is clear to me that 40% humidity would be overkill.

Thanks to knna's contributions, I take note of the plants Increased need for humidity, as CO2 levels and temperature levels increase.

But look closely at the max humidity levels the Kpa calculators are producing, the max RH is close to 60%, and many grows struggle with numbers that are Way above that, especially in the dark cycle.

So I conclude, Humidity over 60%, or under 50% is going to reduce the maximum ability of the plant to use CO2, even if they get plenty of light.

I agree with DeltaNugz that 1200ppm CO2 and 85F are good max metabolism parameters. And I agree leaf temperatures are key to the game. And I agree VPD in the .9 range, +-.1 is a good target.

To implement these parameters, I would start by controlling for light distance first. Based on lamp size, I target 24" distance from 1k, 20" from 600w, 18" from 400w as rules of thumb.

With the light at that distance, I would then control for leaf temperature at the hottest point on the plant, of no more than 82F.

Then control for room temperature, starting at 75F, no higher than 85F, knowing it may be necessary to run the room lower than 85F, to achieve a limit of 82F leaf temp.

Room temp is not at all the same as the temperature of the air between the lights and the canopy, so I would most definitely not set the AC to 85F.. I like 75F, cause you wont actually get the top canopy down to that temp, since the lights add heat.. hope that makes sense..

As to humidity, given the ability to dehumidify, I would set max humidity to 60%, as knna's info has focused me on that as the max limit, for any temperature, with lower temps, in the 70's actually benefitting from being closer to 50% humidity.. 40% RH is unnecessary overkill

so now our parameters are

CO2 @ 1200ppm
Max Leaf temp for max photosynthesis, 82F
Humidity maximum 60%, minimum 50%
Room temp max 85 (careful with this one, room temp can be cooler than canopy temp, dont let leaf temp go over 82F)

fwiw, if these parameters are not met exactly, it simply means the plant is not growing as fast as it could, but it can still be awesome

Im learning a Lot, thanks again for focusing us on such practical questions DZ, and thanks for the VPD info knna..

Lucas"


"Glad the VPD info was useful for you. This is the wheel of cultivation: ive learnt a lot from you, and now i can apport something

I agree with Deltanugz, the recomended level of VPD is 0,8-0,95 KPa in veg and early flowering, and raising it (reducing RH) when flower stage go advancing. After the first month in flower, i try to get a 1Kpa level, ive notice it works fine. Probably it cost me some yield, but ive observed that resin production is a lot higher.

Although VPD continue ruling the transpiration rate at night, this rate is far lower that the light transpiration, so i think isnt so important. Better if the VPD range is within the reccomended limits, but i believe it will affect much less the grow rate.

About CO2 at 1200ppm, the RH level to get 0.9-1KPa is over 70%. That is a dangerous level to deal with mold, so a logical conclusion is CO2 enrichment apport little in late flowering stage (2 weeks), because at reasonably RH levels (55%), VPD is outrange, and excess transpiration and probable stomata closure will lead to CO2 unused, without advantages over ambient CO2, or at least over slight CO2 enrichment (500-800ppm)."

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outdoors

Thank you Knna! Great information here.

Peace-
OD

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Freakshow68

Knna, you rock. We should have a Garden Science and Research Library for you to publish these works.

Excellent Info!

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brassic

some of us dont have the means of controlling temps and humidity so we have to rely on a bit of luck and the fact that nature tends to kick in and give a helping hand. Plants are very Resilient buggers and tend to adapt to their surroundings. Mine go from 75 degrees down to 50 and 60 at night coz I dont have much heating in my home and also the humidity is up and down coz its always raining here. Im not saying there aren't ideal temps but its not an exact science and mine always grow fine infact I've grown one in the kitchen window before now and it was a lovely plant with soil from the garden, in my young and reckless days lol. Plants can take a fair bit of stick, your worst enemy than anything else are bugs and disease. These are your main natural predators.
sorry if I hijacked the thread slightly, just my 2 penith.

alteredego

Very good info there knna.

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WhAcKeD

Holy crap.
That was exactly what I was looking for knna. Not that i can claim to fully understand it all, lol.

Thanks to you too, alteredego, that was the other piece to the puzzle.

So I think I've got it, but if someone doesn't mind double checking me...

Optimum Growth Conditions:

1200ppm Co2
82 F LEAF TEMP, DAY AND NIGHT*
60% RH **

* During stretch, anyway. The obvious implication here is that in order for the plant to see a consistent temperature day to night (and therefore minimize stretch) I'd have to run a space heater at night, and probably shut off the A/C. Both are possibilities.

** RH should probably be backed off, and possibly lights raised too, toward the end of flowering, especially if additves (Bushmaster and Gravity come to mind) are used that will increase bud density and promote mold growth.

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The Trefoil Knot - a calculated experiment in soil

analog

I find daytime (lights on) temperature and RH to be easy to control in my situation.

Temperature = around 80 farenheight
RH = 50 to 53 percent sometimes 45 to 50.

Night time (lights off)

Temperature = 65 degrees farenheit approximately
RH = 60 percent (usually, if it is not 60 its low 60s)

The reason I bring this up is that controlling night time RH is difficult to do for me because, well, lights are off and there's less heat. Late at night between 10 pm and sunrise there is NO heat in the apt. at all - ambient temps in the apt. are 65 degrees F. So, I do not change my ventilation, I keep it on and make sure there's a cross breeze between nearby windows.

When the radiators come on I have to have nearby windows 1/2 open.

No sign of mold at present.

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