Ramblings, tricks, vpd, boundary layer

Nothing serious -


The sole function of trichomes is still ambiguous, looking at many species from all habitats only a few reasons stand out as sole functions. Theorised and tested some though heat or cold protection, some though increasing the boundary layer or creating turbulence, others simply protection from the sun. None really gained much acceptance except in a few species where their role was easily seen. From lab studies the boundary layer and turbulence didn’t add much or enough to warrant the resources needed to produce them over evolutionary scales. Some credence was given to heat and light protection but this had the effect of warming the tissue as heat conductance rose and heat stress.

Marijuana forms it’s densest trichomes at two stages, the hypocotyl on the seedling which layer gets shed once the stem turns woody and lenticels become more pronounced. The second mid to late flowering where buds, sugar leaves and the underside veins are coated in them.

Back to all the other species with trichomes we find one reason that fits and that is species that like to increase the temperature of the seed production areas. The reason for the hypocotyl trichomes would also be the same increasing that tissue temperature for better growth and as it wasn’t much of a solar collector in size like a leaf. As soon as it turns more woody it sheds that layer of trichomes as the woody stem is more resilient to climate and internal temperature a lot more stable.

Coincidentally some light is blocked especially in certain spectrums, so on the opposite side it negates some heat stress but conversely keeps the tissue hotter as heat conductance dominated the issue. This helps the seeds grow which prefer a higher temperature to the rest of the plant . I guess the underside of leaf veins on the sugar leaves simply kept the vein a little warmer but that’s really past the point of most research I could find so I can’t really speculate that precise function.


Trichs did not raise the boundary layer by much. Looking at leaf morphologiy it comes down to resources mainly, did the evolution of that plant or this favour heat or cold protection, did it favour water/drought protection and a few other things. Where did it put it’s resources to, note a forest with trees with many different shapes, they rarely favoured what the next species did and hence all the different tree leaf shapes all next to each other.

One thing we know is that with less reliable resources leaves grow longer and thinner i.e. sativa. Great at drought prevention, less fertile ground, very hot weather. Indica bigger wider where you find more resources and slightly less demanding climate and more water stability/availability.

So the plant grew leaf shapes that focused on these resources rather than others for better survival. They are pinnate which also gives smaller boundary layer and quicker feedback I e. It’s stomata change apperature much quicker tracking real time climate unlike others that don’t on broader palmate types This favours survival in challenging situations better than a palmate type large singular round leaf which although has greater potential for photosynthesis and transpiration but dosent offer much protection to stresses like drought and heat for instance like some in the tropics where resources are abundant all year.


My issue is that simple VPD charts don’t give credence for the hundred times VPDs power that wind does. Add in air movement and boundary layers drop enough that VPD charts are now impaired i.e. humidity dosent matter that air is sucking water faster than high humidity can stop it.

Against the grain the leaf and plant functions well in windless environments, take a hot stagnant day in a hemp field, shit grows fast no problems and canopy down there is little perceptible air movement some grow tent growers would doubt the vpd readings of the whole situation.

The leaf morphologiy is great at this kind of situation, I’ve done grows with no perceptible air movement just heat exchange for good air temps and always some of my best. Just some wind can screw up seedlings and most grows past a very low level and direct wind just causes burn as the leaf can’t keep up it’s water potential against the wind sucking it out of the stomata.

Just some wind is the difference between green brown stemmed seedlings and purple ones from anthocyanins. You can try that easily repeatable and the actual correct colour for seedling hypocotyls or stems is green brown and certainly not purple which indicates stress.

The pinnate leaf serrated at edge is further better equipped to shed heat from it’s edges and induce updrafts but technically stagnant air is hard to produce, even on a no wind day in the middle of a tightly packed hemp field air is still moving everywhere. Heat was the only issue in my grows wind wasn’t ever needed in fact just dialling down my exhaust in my early grows was a big step up to growing stress free plants and think most tents indoors have gar too much air flowing through almost akin to wind tunnels not a field on a hot day growing at its max. At that point water availability was the limiting factor not wind in the field. Wind also cools leaves fat too much, few plants in the world even like wind and plant recommend indoor lab levels barely get above 0.3m/s to 0.5m/s with diminished gains thereafter.

To support that idea of a no wind grow (move some air for heat but no perceptible movement to us) Google kratky growers with lettuce tomato and so on. They just string some lights no fans and produce some wonderful indoor grows. Yer low tech whatever, them plants grow well illustrating how out of whack out views on ventilation can get.

So there’s definite limits which we easily surpass and then probably blame oh or calcium or even the breeder why it all went to pot.



Excellent write up. Enjoyed the educational insight.


Thanks but don’t take it too seriously :grimacing:

Interesting, I’ve wondered if or how there is a ‘wind factor’ attached to VPD metrics. I figure that can reflect it through the leaf temperature differential portion of the equation, & the charts would reflect any changes there by moving you to a different chart, but I would still want to see if the plant reacts quickly enough to get where the equation says it is. You’d figure that the transpiring leaf would start to cool down & maybe the stomata would start to close up during the air movement to try & retain heat &/or moisture, but how quickly does the stomata reaction happen, etc., under the conditions?

Mj is a quick responder unlike birch or other species, resource limitation over evolutionary adaptations meant that if the climate changed it’s stomata did within half an hour as oppose to hours in slow responding species where resources were greater in evolutionary time scales or even what the plant preferred in its evolutionary path.

A real vpd chart plots leaf temperature, normally around air temperature in a healthy plant, the inside of the stomata temperature as that is the equilibrium towards heat loss and transpiration i.e. lower equals less higher more exchange and certain other factors like water availability and light strength.

No matter what the vpd if water isn’t enough at root depth or light too strong theirs no way to stop stress by adjusting vpd charts.

If stomata fully close in light the leaf will severely suffer, shouldn’t ever happen because transpiration is number one for cooling the leaf and light will heat it too much if these aren’t open. Obviously they can close to a smaller aperture and naturally do outside in morning evening climates.

VPD is actually a lot more complicated than we make out, if you rocked up to a university with our vpd chart you would be laughed at as they showed the true complexity.

My take and tested in grows is that plants do well from 30-70% humidity at many temps providing water for roots is enough and light dosent exceed thresholds. How can I prove this, ok Afghanistan produces plenty of fields of marijuana, summer temps 30c, humidity 20-30% but those fields don’t suffer stress and grow great smoke. The cod charts assume the plants will pretty much keel over dead.

History of grow sites is not great, some are spitting vod, ph, wind and then you see lots of others that buck that and grow just as well if not better outside these ranges. Even worse myths like calcium and magnesium did the rounds, your water ferts and soil all have abundant cal/mag yet you still get told to add more to solve some grow issue.

I went through all, only by doing the opposite did I find that I got the growth and control I wanted. Back to those online kratky videos, they string some LEDs same brand as we use, no fans or vents and do wonders, if they came here a lot would say their climate was wrong and add fans etc.

The truth is often hard to see and a leap of faith away from mainstream advice, hopefully food for though I’m not here to argue as many would dispute you need wind or some high humidity to protect seedlings.

I’m UK, indoors air temps are like 20 it so all year a little higher in summer but winter we heat the house. Humidity can fluctuate I’ve had summers stay below 30% for weeks on end and the plant never showed it was bothered.

Seems a lot of conflicting advice, use wind don’t use wind, make seedlings high humidity or ignore humidity. You can go online and see those who did well outside the ranges we seem to push which suggests some advice is born out of bad growers takes. You know the type, they start growing never do well then go on a ten year journey pushing ph, cal mag wind humidity until one day the green finger arrives and they see the errors if their ways. Same happened to me now I do much better no fails or stress and using very little of the advice.

One day I didn’t have a fan but had nine plants ready to flower, I put them in a tent with 600w kept flaps open and worked well, next grow had a fan good negative pressure tent walls sucking in could feel the air flowing from intakes to extractor and you know what… this plants sucked and stressed.

But we’re all on a journey right, we need to try and fail to see what’s right, I use to ph my soul and feeds, how fucking dumb was I as now I do nothing and it works ten times better. Somewhere along the line you learn soil is a strong buffer if ph and you don’t need to or can change that. Hydro yes oh is a factor but good frets and water always land within the right ranges it should be quite simple.


I’ve noticed that my oscillating fans don’t affect my leaf surface temperatures very much. The main exhaust fan will exchange the air quickly & drop the air temps, & that will knock off leaf temps quickly. But they recover fairly quickly. Also if I open the door to check, I have to check within a few seconds or the outside air will start flooding in & knocking off the leaf temps quickly. For VPD I have a pretty good idea of my leaf temps & surrounding air temps & RH, so I basically just run the numbers through a calculator to know where it is. I usually have a fairly wide negative offset most of the time, so I usually end up on the lower end of the range, & for me to adjust anything to hit targets is usually not possible. I think it has a lot to do with me being in hydro & the leaves having access to water all of the time. I usually just try to stay within some sort of ideal leaf temperature range, & then I just check the leaf VPD to know where it really is. If something happens & the leaves stop transpiring, their surface temps should end up somewhere near surrounding air temps. I’ve checked lights-off temps & usually they are a degree under the air temps. But I see two common mistakes with VPD where people target/ adjust their room VPD numbers to Leaf VPD targets (all VPD targets are leaf vpd numbers), &/or they use a VPD chart not matching their actual leaf temperature offset (chart constant is the leaf temperature offset). The first one seems like it could get you into trouble more easily. The second one it depends on how close the chart is to what their actual conditions are. On top of those possible issues, there is sometimes more confusion when people are running LEDs but shooting for air temps more akin to HID bulbs with a lot of heat & IR heat into the leaves. So I think I’m dialed but have taken three wrong turns. I did all of that first starting out trying to figure it out. In any case, what do you think good leaf temperature ranges are to go for (lights-on), in general?

Why do HPS lights raise the air temps by much more than an led, if the light output is the same the spectral power of each spectrum is so small in difference the actual science starts disagreeing with many here.

How much air can a hps heat, certainly nowhere near the amount we draw with out extraction or in winter I’d be running a 600w hps to heat house not various 3kw heaters. Some overestimate heat from a light and the volume of outside air we run through out tents.

Where is your thermometer to read air temperature? It needs to be shaded from all light, air temperature is not light radiation, we do not measure air temps in the light, even the met office goes to lengths to shade all light with a Stevenson screen because we’re not trying to read the power of light just air. Also on a hot day in the desert a thermometer placed in the sun on a table will register close to a hundred degrees and that’s not the air temp really is it, the thermometer might even start to melt in the sun. You need to understand science of light and heat before proceeding here or saying hps heat air more than an led especially when we pull air faster than either can heat.

VPD is a high level calculation, plants grow just fine outside those charts we produce which are just based on temp and humidity, imagine the wrong all summer in a field of crops, still grows right, dosent keel over and die. As explained way more complicated than we make out.

It’s hard to answer your question until you learn what I’ve said and got better measurements because if I use an led or hps in my tent the air temp is always the same and barely a couple degrees higher than the outside tent temperature.

Coincidentally light almost never intercepts air molecules to the point we say it never happens, light passes uninterrupted through air. So light cannot heat air, it needs to hit something first that will absorb it’s energy and then that object can directly heat air through convection.

Armed with this knowledge hopefully you will look at your grow in a different light, re check air temps and ignore vpd whilst you fine tune the other stuff and stop worrying about heat from hps lights.

How much more powerful is ir light than blue, not very much indeed. How much ir light does a hps emit, not very much. In the selling of LEDs lots of truths were twisted, hopefully those have died down, yes more efficient than a hps but heat and light wise that difference so small it’s a mole hill not mountain. Led producers also faked their tests somewhat to give higher figures and more sales. These days were a lot more knowledgeable but initially the claims were straightforward lies which we still have the tail end demonizing other lights for

Good luck

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Run an equally wattage HPS and an LED side by side for 30 minutes.

Touch the LED bulb first, then touch the HPS.

Your question will immediately be answered.


Assume the sum total heat given by the led not far of the sum total of the hps.

The real idea behind less heat was an led efficiency scale, less wattage.

But again how much heat can a poor 600w heater give off, not much and the idea that the air travels faster through our tents than either can heat indirectly counters the whole argument.

In a sealed room led does have an edge as proved but not by much and who said a hps was hot, not plants surely they love them many decades of good growth no stress with them.

It’s a small argument not really important, what is important is to understand the science behind heat and light why thermometers don’t work when in the light.

Hopefully some agreement I’m not here for arguments all info can be substantiated on wiki if anyone needs more of a low down.

Some ideal ‘HID air temp range’ had more to do with them outputting some IR which would put some more direct heat into the leaves as compared to LED which usually never have IR - & that would be even if the air temps & all else happened to be the same.
It doesn’t really matter what the light is, if you start out looking at the leaf temperature. In any case, I get trying to track anything isn’t going to be exact. But I think these days if people are into their second or third grow & planning to keep going, then getting a temp gun & trying to get the leaves in some sort of ideal leaf temperature range seems like a good way to start. Then they can look at the leaf temperature offset & the leaf VPD, & then they can adjust air temp, leaf temp, &/or RH towards one leaf VPD target or another, if they want to. Maybe more importantly they can avoid adjustments & procedures that might move them or keep them away from their targets.
I want to at least avoid moving the wrong way or too much/not enough, so I just follow how to do it whenever I check it. I never bother to look at room vpd, just leaf vpd. I run some shades over my sensors, I definitely noticed the difference without the heat-soak. After a while I usually know what the gun is going to say before I open to check the leaves, by looking at my bluetooth air temps first. What do you think a good leaf surface temperature range would be? What do you think a ‘danger-zone to maximum’ surface temp range for buds is, in order to avoid problems there as well?

Hps puts out very little ir, it’s effects on leaf temperature infinitely small, if anything it would be beneficial and outdoors the sun emits way more so naturally indoors we really don’t have an issue. Some led marketing blew that out of proportion, pretty sure if led info on hps to be believed the sun’s ir dose would have fried most life on earth by now.

If air temps are correct why read leaf temps, , if there is an issue reading leaf temperature won’t tell you what it is.

Lots of led sites claim to know about the ideal leaf temperature, just follow them. What the ideal leaf temperature is was part of a larger equation of readings and discussion based on other factors, probably should take night time temperatures too and adjust your air temps idk complicated stuff easier to take other approaches to growing.

I don’t know many who solved issues with vpd or leaf temps either, mainly you just go down a rabbit hole whilst the real issue isn’t uncovered. In a lab if it was off you would now check one of ten other sensors like water availability or wind or stomata aperture or whatever. Hence why I don’t see the validity in reading it but yes find that good range see what you discover and good luck :crossed_fingers:

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I can see the air temp as just part of the means towards the leaf temp, & not every one has the same numbers, so I check mine to see. I’ve also unwittingly made the same mistakes I see all the time of using vpd charts without matching the offset, adjusting room vpd numbers towards leaf vpd targets, both of those mistakes at the same time, etc, until one day I saw I would have to do things that were not going to make sense, & then I went back & saw how it was supposed to work, & things made a lot more sense after that.
Going back to something that you were talking about earlier,
personally for me I like to have some oscillating fans or at least some circulation fans running inside my grow space. I have a small space & you mentioned that you were only talking about larger spaces, so maybe that is why. They are mild fans & I time them on & off.

Air speed of similar species saw -

Growth increase from 0.0 to 0.3 meters per second. Diminished returns seen after 0.5 and nothing after 1.0. The ideal air speed most labs Aim for is 0.3-0.5 but seen some go over 1.0 although I didn’t note the species just what the average and most popular was.

Hard to read these levels with equipment, it was best termed barely perceptible. But you can find this stuff on line easily and make your own assumptions. Small grows are harder to get right, a big room easier here. Labs use laminar flows and stuff, more high tech equipment we are limited and would be a lot of effort to make a lab type situation.

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Another issue with VPD, most studies used set temperature and adjusted the relative humidity. It was found that in fact Rh had less effect on the vpd and temperature was the main driver.

The above suggests if you pick a set humidity and adjust the temperature you’ll see a bigger difference than if you pick a temperature and adjust the humidity.

I don’t set humidity in my grow, swings from 30-70 rarely in-between much. I don’t notice a difference at any stage.

Studies suggest I hit a VPD of 1kpa for best results but define stressful conditions where the plant can’t keep up with water demands 3kpa perhaps or more. Vpd first off increases water demand and relaxed stomata in field grown hemp before stress conditions, it simply needs to keep up with that to survive well. Obviously a loss in growth can be seen but in some species that loss was offset by more and bigger leaves although fruit trolls down by a small amount the quality was similar. In one study set lower humidity led to pepper and tomato having extended storage life and more pulp to water ratio.

But that’s how I see vpd, not something to fuss your humidity over, temperature more important and those that want to try for that extra level need to be exceptional growers before any real appreciable extra is seen. Certainly If humidity is outside of range that’s not going to make or break a grow.

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Just going off of memory with things I have noticed in my box, if I move air temperature around & I still have similar RH, I might still end up with around the similar leaf VPD either way, due to how my leaf temperature will move with the air temp, but for example if my ambient humidity spikes (starts raining), this will push my RH in the box up, the leaf VPD will end up dropping enough & the plant will transpire less. I don’t run a dehumidifier, so it is what it is. I know if it rains for a day, the plant is going to transpire about half as much. But I try to work with the real numbers to at least see where the leaf vpd is, & more often than not it’s going to be where it’s going to be & there’s not much I can do about it, & that’s probably better than doing things like adjusting my room vpd towards leaf vpd targets, or running off of charts or controllers that don’t match my actual leaf temperature offset. Seems like I could get into worse conditions doing things like that. What leaf temperature offset range do you usually see lights-on with your plants?

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If we look at the vpd of a field in mid summer India or Afghan not in a humid location, wrong vpd numbers. Stuff still grows well assume the upper limits for vpd kpa a lot higher than the charts make out for good growth.

Heat causes extra co2 internal pathway problems that humidity dosent, heat is a limiting metabolic factor humidity isn’t like that.

But all studies looked at an ideal range, stomata can relax or contract outside that range to compensate before you reach stress levels.

My grow runs 20-25 lights on no lower than 17 off Celsius. Pretty much room temps here on the UK a little hotter in summer.

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Leaf VPD? Chart matched to actual leaf temperature offset? You’re saying the VPD of a field, so I’m guessing just Air VPD. I don’t think much about room/ air vpd, as long as the air temp & RH aren’t obviously in a danger zone. It’s just a means to get to the leaf vpd, & we need to know the leaf temp & offset to get there.
-VPD target numbers are Leaf VPD target numbers.

I wonder if those plants would be closing stomata due to lack of water & also maybe because of the UV rays. Leaf temps would probably be closer to air temps then.
-The constant of a VPD chart is its leaf temperature offset.


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Optimal leaf temperature for cannabis 25c-27c, optimal air temperature for photosynthesis 30c-32c

This best educated guess, I’d have to try read twenty studies on hemp leaf again but the above would correlate to this class of plant.

At lower air temps I’d probably assume the leaf even closer to air temps less offset so only a couple degrees off.

Leaves want to be as warm as possible without overheating, that’s their goal in life, photosynthesis and transpiration have optimal rates, give a healthy plant more co2. to see were not at optimal just within a range that works.

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That’s cool that you looked those up, I don’t really have much to go on besides what I look up too. In my head I try to shoot for somewhere around between 70f to 80f on the leaf temps, & then not to let my buds get above 82f surface temps, & 84f surface temps would be danger zone for the buds. Not sure where I came up with all of that. Right now I’m having trouble putting heat into the box without turning the light up more than it needs to be, or turning the box heaters up during lights-on, or running the central heat more than I want to, you get the idea.
Going back to what you were saying about the circulation fans, I don’t think I’m seeing those types of airspeeds out of mine, so I’m not going to sweat that.

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Lights an vox heaters don’t heat a tent, central heating now that will work but here we run 20-25 indoors any higher the central heating needs to be on overdrive and too costly.