Grow Mediums Easy VPD Calculator

[Jimmy Joe James]

Hey guys and girls! Well I've been doing a lot of research and I've been building my own little database of how to grow in tropics seeing as there's so little information on the web regarding tropical climate growing. What little is out there I'm finding it's kinda hit and miss. Anyway since VPD has been part of my research I thought I would share this excel sheet I found doing some random googling. I've found it helps calculating my VPD along with a cheap laser thermometer. If you have one just point it at the plant (it's usually 1-2 C cooler than the ambient temp. At the upper right of the excel sheet just replace the red Air temp, Leaf Temp and RH with your readings and it'll tell you your VPD easy! This is especially handy if you don't grow in northern climates (like me). Here's the excel file link.

 

[Tony21]

Hey man, very cool and easy to use. Great find

Is it correct that the leaf temperature should be measured on the fan leaf closest to the light, or maybe some way down for an average. Also does the light source, hps, hid, cmh, led, whatever, make any difference

I grow in those "tropics" too, my ambient is 32 - 40 C and humidity from 40 - 75 %. My tropics is an attic with and open window. It's going OK, but maybe it could go better, I get a red card by this spreadsheet every singly time I put my parameters in, except when input valueis fir humidity is high. That is a red hot flashing haha that Interesting
I have a quantum meter, it's not really useful and helping and this also looks like something to use with caution, depends where the measurements are taken I think. Something to tinker with too @pop22 any thoughts

Some recent shots, daytime close to 40 C peak, nighttime it can go down to some 25-26 C on a cool night haha

 

[Jimmy Joe James]

Hey man, very cool and easy to use. Great find

Is it correct that the leaf temperature should be measured on the fan leaf closest to the light, or maybe some way down for an average. Also does the light source, hps, hid, cmh, led, whatever, make any difference

I grow in those "tropics" too, my ambient is 32 - 40 C and humidity from 40 - 75 %. My tropics is an attic with and open window. It's going OK, but maybe it could go better, I get a red card by this spreadsheet every singly time I put my parameters in, except when input valueis fir humidity is high. That is a red hot flashing haha that Interesting
I have a quantum meter, it's not really useful and helping and this also looks like something to use with caution, depends where the measurements are taken I think. Something to tinker with too @pop22 any thoughts

Some recent shots, daytime close to 40 C peak, nighttime it can go down to some 25-26 C on a cool night haha View attachment 916912 View attachment 916913 View attachment 916915 View attachment 916916
View attachment 916917

I find that the light source does make a difference especially in leaf temperature and yes it should be the leaves closest to the light. The upside at least is that if the leaf temp is the same as the outside air temp it definitely a sign that your lights are too close. Also in my case since I don't worry about smell I have a big room right now with windows open getting huge amounts of ventilation cool rainforest air plus a couple of fans. When we're dealing with these tropical temps air circulation is the key if you don't have an AC. The very least you should aim is for yellow. What it'll do it at least it'll be stressed and it'll drink more water but it will grow. I've also found that in tropics that larger the rez size in hydro the better. More water = overall lower water temp I've seen in my case. I also feel in these high temps it's important to not let the rez get to the halfway point I see more stress on the girls when there's less water and the rez is at a higher temps because of it. But that's on DWC. All of this plus keeping the lights off on the hottest time of the day help me keep my temps down to between 28-32 (34C on a the worst days of july/august).

I've also found in my case silica helps ALOT my last hydro test grow with bag seed they suffered greatly due to heat stress. Also my experience attics are the worse in the tropics cuz they retain sooooo much more heat. Especially since around here most of the attics are either wood, reinforced concrete or wood with zinc panel cielings. All of those combinations in the past have retained obscene amounts of heat that have made it a pain and back then I wasn't even doing hydro. Now all that rambling aside YOUR PLANTS LOOK AMAZING!!!! At the end of the day good genetics can overcome our environment. At the end of the Day your plants look pretty good (though I see a little stress) and VPD is only part of the equation to good growing.

EDIT: Another thing I forgot at least here in the tropics I have seen that as long that there's good air circulation high humidity is just fine. In both my past indoor and outdoor grows since it's so hot and were so close to sea level compared to other areas I have found that mold isn't an issue only if the air becomes stagnant. So high humidity is the easy way out in high temps in the tropics. I still don't fully understand why but humidity in the tropics doesn't create as much mold or stunts plants or overall gives as much problems as it does growing up north.

 

[Bodacious Budmonster]

Interesting topic and one I will be considering in my next grow.
Thanks for the excel sheet, I will have a look

 

[pop22]

Interesting discussion! After having read up on VPD, it has left me wondering just how useful a tool it is. My humidity has never been exceedingly high, 65-70% but in winter it can go as low as 19%. As I have nothing to compare to, I can't say if or how much it affects my grows. I think that keeping the plants well watered is very important in low humidity. I also keep my temperature in the 70-72F range especially in winter when the RH is low to keep transpiration under control. At these temps I see little sign of stress. And then I see guys growing in tropical conditions like you and Tony, and also having good success. This certainly seems to be contrary to what the whole VPD theory claims so I'm not sure the concept has value for small indoor gardens. Maybe in huge greenhouses where environmental control is a much more difficult issue, using VPD metrics to track and control areas or zones would have value. For myself, after months of tracking VPD etc., I'm not seeing it as useful.
And your right, air movement within the garden is key to controlling mold. I've only once had any PM. Recently we had a power outage that lasted almost 24 hours. with no air movement I found a few spots of mold on one plant on the lower buds. It is the only time I've ever had that issue.RH was around 80% when power came on again in the growroom.

 

[Simplicio]

There are many versions of this available on the internet.
Humidifiers are needed in the winter.
Stay in the green:

 

[Jimmy Joe James]

Interesting discussion! After having read up on VPD, it has left me wondering just how useful a tool it is. My humidity has never been exceedingly high, 65-70% but in winter it can go as low as 19%. As I have nothing to compare to, I can't say if or how much it affects my grows. I think that keeping the plants well watered is very important in low humidity. I also keep my temperature in the 70-72F range especially in winter when the RH is low to keep transpiration under control. At these temps I see little sign of stress. And then I see guys growing in tropical conditions like you and Tony, and also having good success. This certainly seems to be contrary to what the whole VPD theory claims so I'm not sure the concept has value for small indoor gardens. Maybe in huge greenhouses where environmental control is a much more difficult issue, using VPD metrics to track and control areas or zones would have value. For myself, after months of tracking VPD etc., I'm not seeing it as useful.
And your right, air movement within the garden is key to controlling mold. I've only once had any PM. Recently we had a power outage that lasted almost 24 hours. with no air movement I found a few spots of mold on one plant on the lower buds. It is the only time I've ever had that issue.RH was around 80% when power came on again in the growroom.

From my personal experience having lived in the states in the past and now in the tropics, I can tell you that traditional humidity guidelines work best for northern seasonal climates and high altitude. Again idk the science behind it, it's just personal experience but the closer you are to sea level the more relevant VPD becomes. Here in the tropics humidity, temps and pressure can change dramatically at the drop of a hat. Our weather is highly influenced from all over the place across the atlantic even by dust storms and winds all the way from the sahara (when we get a big one of those the sand in the sky turns everything into an orange hue). So I see VPD as an alternate way to guide yourself with cannabis in an cheat a little in an environment where it's not meant to grow.

People say the tropics are ideal for cannabis but even Sativas which are known to thrive in this climate are actually native to high altitude so we still have a set of problems when it comes to growing at sea level. From radically changing air pressure, higher salt content in the air, many things that are just outright problematic for cannabis which are not prone or you don't find in seasonal climates and high altitude. Once you have an AC and full climate control I believe VPD becomes irrelevant unless you want to absolute positively micromanage every little number and statistic to optimize your grow. VPD is like silica supplements lol, if you have everything else in running perfect you dont really need it. I can't afford to run AC so VPD in the tropics is my work around to avoid problems. If I could have full climate control I'd do it and just grow with the RH guidelines, but I'm making lemonade out of this situation. But it's also interesting at least with VPD now I can understand why in some outdoor grows a plant can survive 100F temps like if it was nothing, here in the tropics it's easy to get to 100F in some areas but the humidity still stays in the 70-80's (VPD to me now explains how this can happen). In the end it's all about plant perspiration rate.

 

[pop22]

I found running both a humidifier and an air conditioner counter productive and expensive...

 

[Tony21]

I found running both a humidifier and an air conditioner counter productive and expensive...

Haha, yes and yes, almost like fighting fire with gasoline

From my personal experience having lived in the states in the past and now in the tropics, I can tell you that traditional humidity guidelines work best for northern seasonal climates and high altitude. Again idk the science behind it, it's just personal experience but the closer you are to sea level the more relevant VPD becomes. Here in the tropics humidity, temps and pressure can change dramatically at the drop of a hat. Our weather is highly influenced from all over the place across the atlantic even by dust storms and winds all the way from the sahara (when we get a big one of those the sand in the sky turns everything into an orange hue). So I see VPD as an alternate way to guide yourself with cannabis in an cheat a little in an environment where it's not meant to grow.

People say the tropics are ideal for cannabis but even Sativas which are known to thrive in this climate are actually native to high altitude so we still have a set of problems when it comes to growing at sea level. From radically changing air pressure, higher salt content in the air, many things that are just outright problematic for cannabis which are not prone or you don't find in seasonal climates and high altitude. Once you have an AC and full climate control I believe VPD becomes irrelevant unless you want to absolute positively micromanage every little number and statistic to optimize your grow. VPD is like silica supplements lol, if you have everything else in running perfect you dont really need it. I can't afford to run AC so VPD in the tropics is my work around to avoid problems. If I could have full climate control I'd do it and just grow with the RH guidelines, but I'm making lemonade out of this situation. But it's also interesting at least with VPD now I can understand why in some outdoor grows a plant can survive 100F temps like if it was nothing, here in the tropics it's easy to get to 100F in some areas but the humidity still stays in the 70-80's (VPD to me now explains how this can happen). In the end it's all about plant perspiration rate.

Yes, thats my thinking too. AC is not an
option for me too, noise, venting.
The only thing with mold is fat colas, like these Think Big and Eggnog ones, no matter how much air movements

 

[Jimmy Joe James]

Haha, yes and yes, almost like fighting fire with gasoline



Yes, thats my thinking too. AC is not an
option for me too, noise, venting.
The only thing with mold is fat colas, like these Think Big and Eggnog ones, no matter how much air movements

View attachment 917010 View attachment 917011 View attachment 917012 View attachment 917013

Dude... DUDE! With those BEASTS! I can see why you would have mold in those fat colas. BTW have you ever tried Hydroguard or Heisenberg tea as a fungicidal? A couple of drops in your foliar spray take care of it easy. Bennies love eating the mold and mildew off the leaves and buds. I don't think its the best option during flowering but I have personally used to foliar spray bennies during veg and it helped.