Lets talk about VPD...it's worth the converstion.

WaxyTaxi said:

thanks led, got some reading to do...

this or another thread with valuable vpd info should be a sticky xray, I agree.

Jackstrat- glad to see you enjoying this data too, not sure to be honest amigo.. I'm new to VPD(almost been a year now) and very very few people I know have the slightest clue about it.

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Good stuff. The explanation, in my view, is excellent; just enough geek to sound as impressive as it is and just enough regular folk to be understandable with a little thinking about it. I mean to say that is really good growing knowledge stuff. Stuff we may intuitively know or sense but couldn't readily explain.

Good stuff!

Jackstrat - I would have to say, in my opinion, while I definitely hear where you're coming from and fitting differential in there made me have to think for a minute, that deficit is the correct word. While the word differential most definitely has its place within this conversation about VPD, I'm going to venture out and say, it would refer to the difference between the temp and RH (as represented by the values on your metering) and the value that is presented to us as a result of this differential between these two factors is what would be referred to as a deficit. Labeled as such in leu of differential because it is what the ambient air LACKS (is deficient) before it becomes fully saturated with water vapor (100% RH). So finding out what the differential of temp vs RH is, is a step on the way to finding out what the deficit is. Just my opinion...anyone feel free to oppose it. I welcome healthy debates it is how true knowledge is attained and, ALAS, we learn shit....hope my ramblings helped.


peace and CBD grease

What is better with VPD??


A) canopy RH 65-72 canopy temp 28.5-29.5C
bush RH 70-75 bush temp 25.8-26.5C



B) canopy RH 45-55 canopy temp 26.5-27.5C
bush RH 55-60 bush temp 24.8-25.5C

Thanks alot for this info, it gave me a better understanding about plant growth in different environments.

Excellent thread

Excellent thread

Hi folks, just thought i would throw in my 2 cents worth. As a commercial greenhouse grower of 30 odd years, collecting, controlling and logging vpd and T.diff values is standard daily fare for me. The real value of this data is not in just setting and controlling it, but its ability to be used for what is known as plant steering. I have not seen this term used for cannabis cultivation but it is essentially what all production horticultural based environmental, rhizosphereic and plant cultural activities seek to achieve. VPD and Temp diff manipulation are simply two of many tools in the growers steering arsenal. Steering is undertaken either in a vegetative or generative direction dependent on cultivar, time of year, position in plant life cycle and many other variables. All designed to achieve maximal yield based on the genetic potential of the particular cultivar. Poor steering may significantly reduce yields and quality. Learning why and how to manipulate VPD and T.diff for yield and quality maximization is what we as growers are all seeking to achieve. I do it at work on a large scale and at home on a much much smaller scale. Hope this blurb is of value. Cheers and happy growing.

^^ nice! thanks for sharing!

here are the first two ag extention articles that popped up when i googled "plant steering."

very short and simple, but good intro to the term.

plant management for generative or vegetative steering

steering your tomatoes toward profit

Aye, feel free to share more of your insights

Steering continued

Steering continued

I notice growers deleaf their plants to achieve better light penetration. How about if i told you deleafing has more to to do with aggressive generative steering than it does direct light contact with the lower flowers! If you understand the terms assimilates and exudates, hort science and steering specifically becomes much easier to grasp. Deleafing redirects assimilates to apical meristems and floral growth and development. This is a big reason deleafing is effective. Photosynthetic mass must be adequate for vigour to continue but lower leaf removal is often just as effective as upper obstructive leaf removal in improving flower development. Generally speaking, leaf removal is generative and retention is vegetative. Narrow temp diff, lower Mean temp and lower vpd's at higher temps are also vegetative maneuvers. Lower EC, ...the list goes on. glad i could be of some value here. Hort is still my passion even after all these yrs, Cheers and happy growing.

Deleafing prior to photo-period change is another positive generative action. Provided the existing floral mass has sufficient photosynthetic surface to maintain vigour, upper leaf removal can also assist with assimilate targeting to flowers and thereby increasing bud development. Keep in mind however, that as plants approach senescence, nutrient sequestration from older foliage is also important for good floral development. The root system becomes inefficient as it ages and cant provide nutrients as effectively. Some foliage must be maintained as a store house for this translocation process. Maintaining a super healthy and vigorous root zone also assists. One of the few reasons hydro methods can better soil yield on a kg per square meter basis. Better root zone health into senescence.It all comes down to balancing plant culture to perfection. Definatly an experience learned skill. I think i have rambled on more than enough for one day...
Thankyou to those who have taken the time to read and reply. One day i hope to grow crops worth more than $2.00/Kg at the market... dream on in my country. Cheers and happy growing to all.

Not sure why some of my words are being "italicized", not my intention. im a total noob when it comes to forums. Cheers.

Growaway - they are italicized because they are words icmag knows that most people (myself included) prob have no idea what they mean...so you can click on them and it gives you a definition.

On a more important note pretty, awesome info man...keep it coming. What are your thoughts on manipulating the VPD in relation to the specific effects you would like for it (VPD) to have on your plants according to which stage they are in their life cycle? If that make sense. For example should you strive to maintain a constant VPD throughout the entire life cycle or do you want to change it depending on if the plant is a seedling or in full bloom?

Great question dude. Lower VPD's generally are vegetive in their effects but with cannabis in the vegetative stage raising the overall temp whilst maintaining a lowish VPD will encourage improved vegetative development. If your variety has a tendency to remain too short particularly, this manouver may be of help. If you have a naturally stretchy cultivar, then lowering overall temp with a lower vpd will be of more benefit. Generally speaking, we steer vegetative varieties in generative direction and generative varieties in a vegetative direction. High VPD's and lower temps in floral completion have been implicated with better trichome development with some varieties. And then there is is all the implications of lower VPD's and fungal disease development, so its not as simple as do "X" and get "Y". There are cross overs and implications for everything. Start thinking assimilate sequestering based on whats warm and abundant on the plant. In the case of fruiting plants its the large ripening fruits, in cannabis its the heavy developing flowers. on vegetative plants its luxuriant green leaves. The more assimilates the plant part attracts, the greater its priority for growth and development. So, yes you should manipulate VPD based on the particular varieties needs and all the other growth factors - light, CO2, nutrient appetite etc. Balance is god, the growers mantra. Cheers.

"Balance is god, the growers mantra."

Well said.

Do you have a grow currently?

Excellent posts growaway.

I like the concept of steering the plants Growing them definitely is a journey.

Thanks for the links heady!

-pH

Dropped Cat said:

"Balance is god, the growers mantra."

Well said.

Do you have a grow currently?

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I have my usual outdoor grow well underway. I clone from over wintered mothers and plant out early which initiates harvestable flowering (bonus!) and then reveges hard becoming what i understand you youngsters call "monster clones". Stems thicker than my wrist when finished. I finally moved away from old landrace sativas to modern cultivars a few years ago. Yields and ease of trimming mostly motivated me. This season will see a mix of OG's, a GDP, and a real Durban Poison(still like this landrace too much to not grow it.) I live on the edge of the desert in the southern hemisphere. I do small indoor grows from time to time to produce new mothers and flower old ones. For work i look after up to 100,000 plants at any one time, a mix of tomatoes,cucumbers and peppers, all in hydro, but i still love my home grows, both for the end product and the process. Dont know how many grows i have left in me, so i thought it was time to share some of this info with others. I would love to photo and share but have lived with secrecy for too many years to feel comfortable with it. Hope i live long enough to see the stupid laws change. Anything that i can be of assistance with will be my pleasure to provide. Its just turned 420 at my place, BRB. Cheers.

great posts growaway! thank you for sharing your knowledge and experience!

heady blunts said:

great posts growaway! thank you for sharing your knowledge and experience!

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You are welcome. The faster we help remove the growing mysticism and replace it with good horticultural science, the easier it will become for everyone to grow good quality meds at home. This seems like a really nice and well informed community, i look forward to picking others brains also. Thanks Mr. Blunts (or can i call you "Heady"?)

i prefer heady

not much of a mister...

Heady - Happy belated birthday bro! How I missed that the other night I do not know.

Growaway - I second heady's statement...awesome post, super informative. I've got a few little ones growing now and I've been applying the knowledge I've gained regarding VPD to how I manipulate the environment with satisfyingly effective results. They're currently residing in a 4x7 grow tent while construction of their new and bigger home is under...of which I am super excited about (shout out to Heady Blunts for helping me to finalize the design) I will post pics for all who are interested soon. There are a couple of terms that you've used in your posts that I am not as familiar with as I'd like to be and was hoping you wouldn't mind elaborating...temperature differential and mean temp. As simple a term they may be if you wouldn't mind breaking them down to me (and the rest of us) as though I (and not the rest of us) were an idiot it would be much appreciated.

Waxy - what up man...how's the room going? Figure out that whole furnace fucking shit up ordeal? The hours I spent reading/posting in your lung room thread were very informative times in the life of kowhite...

Peace and CBD grease.....

kowhite said:

Heady - Happy belated birthday bro! How I missed that the other night I do not know.

Growaway - I second heady's statement...awesome post, super informative. I've got a few little ones growing now and I've been applying the knowledge I've gained regarding VPD to how I manipulate the environment with satisfyingly effective results. They're currently residing in a 4x7 grow tent while construction of their new and bigger home is under...of which I am super excited about (shout out to Heady Blunts for helping me to finalize the design) I will post pics for all who are interested soon. There are a couple of terms that you've used in your posts that I am not as familiar with as I'd like to be and was hoping you wouldn't mind elaborating...temperature differential and mean temp. As simple a term they may be if you wouldn't mind breaking them down to me (and the rest of us) as though I (and not the rest of us) were an idiot it would be much appreciated.

Waxy - what up man...how's the room going? Figure out that whole furnace fucking shit up ordeal? The hours I spent reading/posting in your lung room thread were very informative times in the life of kowhite...

Peace and CBD grease.....

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Cheers Kowhitee, thanks for the shoutout. Mean Temp. is the just average temp of the grow space across the course of 24hrs. If you were to sample temp each hour and add these values together, then divide by 24, you would get your mean temp. Temp differential is the difference between maximum temp and minimum temp. in a 24hr period. For example, if max 85F during the day and min 55F at night = Tdiff of 30F. Wider temp diffs and overall higher means temps are generally considered a generative measure in the grow space once flowering/fruiting is underway. These measures encourage assimilates towards flowering and fruiting plant parts in preference to the foliage provided the flower/fruit load dominates plant balance. Used appropriately, a very vegetative, slow n short, lower yielding variety can be encouraged to improve flower development and plant structure. Used poorly, or with the wrong variety, and you will get stretch like a bitch and/or "exhausted" plants that don't reach their genetic flowering potential.
Creating greater temp diffs that increase the overall mean temp tends to accelerate assimilate production and stretch and potentially improve yield(think warmer day temps with nights staying same). Greater temp diffs that lower the mean temp can reduce assimilate production and slow development and reduce stretch(think colder night temps with days staying the same).
These changes are relative to the 'normal' temps that you would use for a particularly strain for example with light intensity, CO2, and feed manipulation used to assist the process.
All this is great provided you have the ability to change and completely control day and night time temps (active heating and cooling systems). For most of us at home, we see the effects of on our plants of our inabilty to control changing mean temp and Tdiff. In our effort to correct it, we may smash our VPD to shit. Commercially, hydronic heating and evaporative fogging and/or A/C cooling systems are the growers friend but are not cost viable for most us at home. One day...
Still, knowing about it and doing what we can do to control or manipulate it can be the difference between an average result or a great one particularly under difficult climate conditions.
Once again, i have been asked a simple question and turned it into a wall of text. This new OG pheno i have just sampled could possibly be to blame...makes me rave on just a tad.
Cheers and thanks.
PS. 102F max temp in my part of the world today! Misting, irrigating, shading and venting time! Yehaaa!