VPD Question

[hamstring]

Everyone seems to talking about VPD so I have a question.

I am a old school grower and have always been taught your going to get you best Veg growth around 80 deg F. In my small grow room in the basement I have a 600W HID fixture and some supplemental heating to keep the temps at 80 deg during the winter months. Even with a humidifier I am can only keep the room a to a RH of 50% . With a leaf temp around 82 degF, I am at a kPa of 2.05. According to the VPD chart (https://vpdchart.com/) for cannabis this is, "Danger Will Robinson" territory.

Now if I turn off the supplemental heating I am at 71-72 deg F in the room with a RH at 55%. With a leaf temp any where from 73-75 deg I am at a kPa of 1.35 - 1.53 which the VPD chart still calls "High Transpiration" but way better than the the 80 deg temp.

My question is, am I really better off with these low grow room temps (71 - 72 deg F) for the veg period? I of course want to follow the science but its hard for an old school grower to break habits.

 

[Dr.Young]

Its good to water in the morning early as possible to get the evaporation throughout the day and higher humidity while its needed.... Also you could start spraying plants with water to keep humidity up...VPD is definitely the key..... THe main nutrients are carbon,hydrogen, oxygen... all of that is n the air/medium... Watering practices and humidity control is everything. If you dont have co2 i wouldnt push for 80*F

 

[flylowgethigh]

Take what I say as newbie ramblings as I also search for this key knowledge.

VPD is huge in veg, as I am learning. One sure fire way to have a good VPD seems to be hanging a wet cotton cloth on the side wall, and let it make the humidity. Keeping it wet is a hassle. I use vaporizers now, and like anything with a control, they can go wrong. 75*-80*F lights on, 55-65% humidity, seems to be a happy point that isn't too hard to maintain IF the lung room is kept at 70* and humidity above 50%.

When my humidity is high, the plants drink from the leaves. When the VPD is lower, the roots feed. That is just an observation, not science.

 

[hamstring]

Its good to water in the morning early as possible to get the evaporation throughout the day and higher humidity while its needed.... Also you could start spraying plants with water to keep humidity up...VPD is definitely the key..... THe main nutrients are carbon,hydrogen, oxygen... all of that is n the air/medium... Watering practices and humidity control is everything. If you dont have co2 i wouldnt push for 80*F

Hey Doc thanks for the feed back. I guess I will stick to the lower room temps then. With the high kPa I did have to water much more frequently.

 

[hamstring]

Take what I say as newbie ramblings as I also search for this key knowledge.

VPD is huge in veg, as I am learning. One sure fire way to have a good VPD seems to be hanging a wet cotton cloth on the side wall, and let it make the humidity. Keeping it wet is a hassle. I use vaporizers now, and like anything with a control, they can go wrong. 75*-80*F lights on, 55-65% humidity, seems to be a happy point that isn't too hard to maintain IF the lung room is kept at 70* and humidity above 50%.

When my humidity is high, the plants drink from the leaves. When the VPD is lower, the roots feed. That is just an observation, not science.

FLGH
Thanks for the input. Nice to hear from other growers on what they are seeing in their own rooms.

 

[hamstring]

You know whats crazy if you look at the VPD chart (https://vpdchart.com/) I would have to be a 79% RH at a room temp of 72 deg F to get to a kPa of 0.99-1.00. Which is what the chart calls out as optimal for the veg period.

Man that seems like high RH?

 

[moose eater]

You know whats crazy if you look at the VPD chart (https://vpdchart.com/) I would have to be a 79% RH at a room temp of 72 deg F to get to a kPa of 0.99-1.00. Which is what the chart calls out as optimal for the veg period.

Man that seems like high RH?

Yes.

If I kept RH that high in my shop, I'd have greater mold & fungus issues, and water (in the winter) at the base of my triple-pane windows in the rest of the building.

 

[flylowgethigh]

I have 3 instruments (the light controller and fans have sensors too) in the tent reading humidity and temp. They all read different. Temp will vary with how close to the lights the sensor is. The little recorder (CO2 monitor) hanging on the tent side at plant top height, that also reads %RH and temp, is what I go by. If in early veg leaves are krinkled up, the humidity may be too high. They haven't got the roots to support the light, so the leaves are working hard, I think.

Take me with a lot of salt. These tents are in the same room I smoke dope in, and type on the internet, so things are kinda hazy in here.

I dunno. The microbes straighten things out. Just keep the soil wet.

 

[Dr.Young]

Its all just general guidelines... BUT in nature you have extremes... so just overall long as you are going from hot and humid as possible within reason..... and slowly taper into a general lowering of temperature and humidity you are good..... You are matching nature and the mechanics of the shit the plant understands...

You can have hot days high humidity days... high light intensity days.... low intensity..... When the plants are young they dont have established root systems so they have to feed more out of the air...You are just compensating for the lack of roots so you can foliar spray to help them stay vigorous enough to have a good root system for flower.... Going into flower with a root system that is behind, or you just transplanted and they arent kept ideal to thrive into the uppot or caught up in time you get weaker flower results....

 

[hamstring]

Yes.

If I kept RH that high in my shop, I'd have greater mold & fungus issues, and water (in the winter) at the base of my triple-pane windows in the rest of the building.

Agreed I would never aim for 79% RH in my grow room. To many other issues.

 

[hamstring]

Its all just general guidelines... BUT in nature you have extremes... so just overall long as you are going from hot and humid as possible within reason..... and slowly taper into a general lowering of temperature and humidity you are good..... You are matching nature and the mechanics of the shit the plant understands...

You can have hot days high humidity days... high light intensity days.... low intensity..... When the plants are young they dont have established root systems so they have to feed more out of the air...You are just compensating for the lack of roots so you can foliar spray to help them stay vigorous enough to have a good root system for flower.... Going into flower with a root system that is behind, or you just transplanted and they arent kept ideal to thrive into the uppot or caught up in time you get weaker flower results....

Some good points. I have very rudimentary temp and RH meter. Its just two analog meters the type you can get in any garden center. I have it mounted about 3' up the wall . The room is tiny so the light is not more than 2 ft away from the meters. It could be influencing the reading.

That doesn't negate what the VPD chart says about the RH % required for optimal Veg growth. 79% wow.

 

[Three Berries]

I've just been aware of VDP since this summer. And in my tent in the AC cooled house if I run it with a exhaust fan set at 85F or so it runs great. In the winter if I let the tent stay room temp plus whatever the lights add it also runs great but seldom gets above 70F. Usually mid to low 60Fs in the room.

 

[RockinRobot]

You can also supplement with CO2. You want higher transpiration rate with CO2. But to answer your original question Yes your plants will do better at 71-72 if you can't raise your humidity at the higher temp.

 

[hamstring]

You can also supplement with CO2. You want higher transpiration rate with CO2. But to answer your original question Yes your plants will do better at 71-72 if you can't raise your humidity at the higher temp.

Thanks RR, I am going to run it this way. Just means I will have to turn on the supplemental heating when the lights are off to keep the temps near 70 deg F.

 

[hamstring]

Its all just general guidelines... BUT in nature you have extremes... so just overall long as you are going from hot and humid as possible within reason..... and slowly taper into a general lowering of temperature and humidity you are good..... You are matching nature and the mechanics of the shit the plant understands...

You can have hot days high humidity days... high light intensity days.... low intensity..... When the plants are young they dont have established root systems so they have to feed more out of the air...You are just compensating for the lack of roots so you can foliar spray to help them stay vigorous enough to have a good root system for flower.... Going into flower with a root system that is behind, or you just transplanted and they arent kept ideal to thrive into the uppot or caught up in time you get weaker flower results....

I agree there are extremes in nature but you can see why someone would question "optimal RH at 79%. It makes it tough on anyone trying to follow the science when the science seems questionable. At least from a laymans point of view.

I guess, why bother with the directions if everything is "general guidelines". Not a knock on you its on the VPD chart. I will take the time to look at few other VPD charts before I go getting all butt hurt about it .

 

[Dr.Young]

General as in you dont need to have a set temperatures... Its natural for the room to heat up and humidity rises or falls from a perfect set number. Long as you are following the vpd chart obviously... If your current humidity is 55% and you want it 70% you can spray and get 70% for a portion of the day and it keeps dropping to a lower number thats ok.

 

[xylemhort]

Ultrasonic humidifiers work very well to increase humidity in grow rooms. It helps to use RO water, or harvest condensate to feed the humidifier so that you don't get calcium film on your grow equipment. These humidifers also help to cool your room.

 

[Switcher56]

This is the one I use and have been since the beginning. I do not particularly like the virtual one, as you have to continuously adjust the cross hairs. Maybe I am doing it wrong. It doesn't matter. I finish them off (last 3 weeks) @ 75 deg F and 55%. I dry 'em out @ 70 deg and 60% nice and slow.

 

[hamstring]

This is the one I use and have been since the beginning. I do not particularly like the virtual one, as you have to continuously adjust the cross hairs. Maybe I am doing it wrong. It doesn't matter. I finish them off (last 3 weeks) @ 75 deg F and 55%. I dry 'em out @ 70 deg and 60% nice and slow.

Thats very interesting . Your VPD chart shows a kPa of 1.04 with the RH at 55% and temp at 72 deg. It doesn't take into account the leaf temp but still quite a large delta from the one I was using. Just like everything else found on the web and here for that matter, hard to differentiate between BS and science.

Thanks Switcher for this it at least opens my eyes or more like closes them.

 

[Switcher56]

I "KIISed" it