Is RH Really Such a Big Deal?

[Jimster]

Recently I have seen a lot of posts about relative humidity and the importance of it being in the proper Zone. I have grown in relative humidity levels from as low as 10 or 15% in the winter, to as much as 80% in the summer, and I find that I see very little difference at all in yield or potency of the plants. They seem to react the same regardless, with the possible exception of when they first start I might see a little bit of tip Browning which quickly dissipates as the leaves fill in more. I grow mostly Sativa strains, so perhaps that has something to do with the lack of problems. In truth, my rh rarely goes above 30% and no issues seem present. Why do some seem to have problems... it seems higher Rh causes more problems, like PM and similar maladys.
What is your experience?

 

[MIMedGrower]

My experience is higher humidity even in veg brings mold and mildew in my environment here. I keep all rooms and my whole house 45-60% max at all times to prevent mold.

My results and yields are still getting better i dont think humidity is a real factor with our plants. But we do have to adjust watering and feeding for it.

Raising humidity a bit does reduce heat stress when the room is hot i have seen. Makes sense as higher temps hold more humidity.

In Ed Rosenthals book he tested all parameters. Humidity made no difference in yield or quality in his tests. But he does caution mold over 60%.

 

[Animal Chin]

I've had high humidity in a grow produce bud rot but was able to eliminate it the next grow by increasing air flow around the colas day and night.
So I think you can get away with high RH as long there's a decent rate of air exchange happening around the plants 24/7. But if the air is dead and still it will mold up fast

 

[Dirtbag]

I dont know if anyone here has heard of Brulosophy, but it's a network of people who do side by side tests on beer brewing "myths" to test variables and use triangle tests to determine the effects of whatever variable they manipulated. We need a growlosopher to do the same thing for pot. For real.

 

[cemchris]

I think it only really matters when running gas honestly. That has been the only time when I have seen it actually make a huge difference unless we are talking about rot or mold.

I did have a long discussion with some people about PM and humidity levels tho when dealing with the warehouse. Come to find out there is different strains of PM. Some like high and some like low. So in the end it can be a double edged sword trying to steer rooms one way or the other in the large scale to help combat a problem like that. Specially if you have more then 1 strain present already (the joys of buying other peoples businesses and there for their problems)

 

[Dirtbag]

High humidity has no place in a flower room, last 4-5 weeks I like to be 40-50%.
But I have noticed vegging plants seem happier with a bit higher humidity. I'm perfectly happy to concede I could be wrong, but understanding VPD was a pretty big part of the coursework when I was in school for greenhouse production horticulture. It might not be a super critical factor, but I do believe a tuned VPD does optimize growth rates. To what extent... We need someone to run side by side tests on this shit..

 

[BigCube]

I'm always around 50% unless I'm hanging. Then it's like 40%

 

[az2000]

I have grown in relative humidity levels from as low as 10 or 15% in the winter,

I'm in the desert and have very low humidity, especially in the winter. I think I've noticed the plants don't do as well in veg when the RH is below 20%. I often wet a towel an lay it on the floor of the tent, or suspend it with a Ecco paper clip tied to the roof of the tent. I'll fill a bowl with water and let one end of the towel rest in that (wick the water out of the bowl or bucket, cup, whatever.). I think I've seen that help.

In the summer it can be 30-40% humidity. That's the most I've grown. I don't have anything to compare to above that. But, below 20, I feel like I see a difference in the lives (wrinkled, tacoed. Just a little. Or, more if I have a fan blowing on them.).

 

[oldskol4evr]

im not sure,this i know from experience,i cant control rh at all were i live,only time i worry about rh is when im hanging and curing,then i make sure i have the ac working good hahahh,but from start to harvest i never pay it any mind,pm oh ya it is cause from extreme heat during day and when sun goes down barametric pressure rise and fall,me and were i live it is 102 right now and when the sun goes down ,temp drops to 20 degree difrence in just a hour,the swing is the cause of pm for me at least on my squash plants,i tried several thing to stop or hold at bay and just gave up,if them bitchs are putting fruit in jars and skillet all good when the slow down i take the whole plant and discard the plants on other end of property.
in the tent mine goes from 40 to 60% rh and ive never had any problems so to speek,that vpd stuff hell i cant make heads or tales from it,i run with forced air only,no heat or ac and what ever the rh is ,is what it is,drying i make sure my vent hose from inside the house goes to the tent for that only

 

[Aqua Man]

Here is my opinion. VPD when used properly is very effective. This means taking leaf temp into account. So many people do it wrong. You MUST have good airflow throughout the plant.

Now in terms of differing experiences I have an answer to that and it's simple. The faster you plant is growing the more crucial each variable becomes. So in my sealed hydroponic RDWC, high light room with CO2 I find it makes a big difference whether I'm in the ideal VPD range or not. In a dirt grow without CO2 this variable become less of an impact and wider ranges will work without issue.

It's also an important factor in nutrient uptake so when we find what RH with how much light and how much nutrients works we stay with it. If you change the RH you are likely to see negative effects if we are not adjusting other variables that are affected by this.

Just my opinion. The importance is dependent on how we grow and how fast our plants are growing among a few other things I will just leave out.

Added: also in terms of mold with a higher RH. When the temps drop at night the humidity will often spike if this happens quickly. With a lower humidity this is not nearly as much of an issue. But if parameters are kept in check using VPD and good airflow mold should not be an issue. Also some genetics do better than others

 

[Dirtbag]

Here is my opinion. VPD when used properly is very effective. This means taking leaf temp into account. So many people do it wrong. You MUST have good airflow throughout the plant.

Now in terms of differing experiences I have an answer to that and it's simple. The faster you plant is growing the more crucial each variable becomes. So in my sealed hydroponic RDWC, high light room with CO2 I find it makes a big difference whether I'm in the ideal VPD range or not. In a dirt grow without CO2 this variable become less of an impact and wider ranges will work without issue.

It's also an important factor in nutrient uptake so when we find what RH with how much light and how much nutrients works we stay with it. If you change the RH you are likely to see negative effects if we are not adjusting other variables that are affected by this.

Just my opinion. The importance is dependent on how we grow and how fast our plants are growing among a few other things I will just leave out.

Added: also in terms of mold with a higher RH. When the temps drop at night the humidity will often spike if this happens quickly. With a lower humidity this is not nearly as much of an issue. But if parameters are kept in check using VPD and good airflow mold should not be an issue. Also some genetics do better than others

Well put, I agree.

 

[MIMedGrower]

High humidity has no place in a flower room, last 4-5 weeks I like to be 40-50%.
But I have noticed vegging plants seem happier with a bit higher humidity. I'm perfectly happy to concede I could be wrong, but understanding VPD was a pretty big part of the coursework when I was in school for greenhouse production horticulture. It might not be a super critical factor, but I do believe a tuned VPD does optimize growth rates. To what extent... We need someone to run side by side tests on this shit..

Sure young and vegging plants like higher humidity. The vpd chart sure goes that way. I am definitely not disputing.

I average my whole house because of the perpetual with all different stage plants. If i had a monocrop going i could dial in further.

Maybe grow veg faster but i dont need to. And i dont think it would improve quality or yield much if at all.

 

[MIMedGrower]

Here is my opinion. VPD when used properly is very effective. This means taking leaf temp into account. So many people do it wrong. You MUST have good airflow throughout the plant.

Now in terms of differing experiences I have an answer to that and it's simple. The faster you plant is growing the more crucial each variable becomes. So in my sealed hydroponic RDWC, high light room with CO2 I find it makes a big difference whether I'm in the ideal VPD range or not. In a dirt grow without CO2 this variable become less of an impact and wider ranges will work without issue.

It's also an important factor in nutrient uptake so when we find what RH with how much light and how much nutrients works we stay with it. If you change the RH you are likely to see negative effects if we are not adjusting other variables that are affected by this.

Just my opinion. The importance is dependent on how we grow and how fast our plants are growing among a few other things I will just leave out.

Added: also in terms of mold with a higher RH. When the temps drop at night the humidity will often spike if this happens quickly. With a lower humidity this is not nearly as much of an issue. But if parameters are kept in check using VPD and good airflow mold should not be an issue. Also some genetics do better than others

Excellent post!

 

[Edinburgh]

Yes ph is a big deal, you want to get the most out of your plants right, you want to bring your plants to there full potential right? Do you want to be dealing with problems? And it's not like it's hard or even expensive, first you must understand ph in soil and what it means, a ph of 7 is neutral, cannabis grows best at around 6 to 6.5 max I'm not saying a cannabis plant won't grow with a ph of 7 or 5.5 but those are not optimal ph for cannabis, they are impeded from up takeing the proper nutrients needed to give you maximum flower production, is that not what you want for your plants? And for $15 I bought an electric ph meter so there is no excuse not to check just lazyness, I mean why wait for problems to start when you can avoid them altogether?

 

[Jimster]

Yes ph is a big deal, you want to get the most out of your plants right, you want to bring your plants to there full potential right? Do you want to be dealing with problems? And it's not like it's hard or even expensive, first you must understand ph in soil and what it means, a ph of 7 is neutral, cannabis grows best at around 6 to 6.5 max I'm not saying a cannabis plant won't grow with a ph of 7 or 5.5 but those are not optimal ph for cannabis, they are impeded from up takeing the proper nutrients needed to give you maximum flower production, is that not what you want for your plants? And for $15 I bought an electric ph meter so there is no excuse not to check just lazyness, I mean why wait for problems to start when you can avoid them altogether?

Sorry... I was talking about Relative Humidity, not Ph.

 

[Edinburgh]

Ok well i thought it was a misspell, yes you rel hum is important, I keep mine 40 to 50% max, high humidity makes your plants sweat under the lights stressing them and leavening them suseptable to mold, with high humidity often comes high heat witch stresses your plant more in severe cases leading to popcorn bud, friend the whole point of growing indoors is beacuse you can control your environment, even out of doors plants you need to find a strain that can handle the conditions, here we have many days above 90f a few days of 100f and dew points in the high 70s not many plants can handle this.

 

[Aqua Man]

Ok well i thought it was a misspell, yes you rel hum is important, I keep mine 40 to 50% max, high humidity makes your plants sweat under the lights stressing them and leavening them suseptable to mold, with high humidity often comes high heat witch stresses your plant more in severe cases leading to popcorn bud, friend the whole point of growing indoors is beacuse you can control your environment, even out of doors plants you need to find a strain that can handle the conditions, here we have many days above 90f a few days of 100f and dew points in the high 70s not many plants can handle this.

I often see the question posed about temps outdoor of 100+ and why indoor temps are an issue at that level. My thoughts on that are temps and humidity and other variables outdoor can reach extremes but in the scheme of things these periods are generally short enough that they don't usually affect the overall health of the plants. As is true with most questions about optimization of growing there is no one answer usually and it's the circumstances that dictate what the proper course of action is.

This is why this forum is so successful we have a ton of great growers with extensive experience in all sorts of different circumstances that collectively solve anything imo.

Great discussions that lead lead to real answers and while things may get heated from time to time we all get it done... Help new people and learn a metric shit ton from eachother.

Ok I'm going back to being a cranky old man now. That's about all the niceness I can put out in one day

 

[PlumberSoCal]

I often see the question posed about temps outdoor of 100+ and why indoor temps are an issue at that level. My thoughts on that are temps and humidity and other variables outdoor can reach extremes but in the scheme of things these periods are generally short enough that they don't usually affect the overall health of the plants. As is true with most questions about optimization of growing there is no one answer usually and it's the circumstances that dictate what the proper course of action is.

This is why this forum is so successful we have a ton of great growers with extensive experience in all sorts of different circumstances that collectively solve anything imo.

Great discussions that lead lead to real answers and while things may get heated from time to time we all get it done... Help new people and learn a metric shit ton from eachother.

Ok I'm going back to being a cranky old man now. That's about all the niceness I can put out in one day

 

[Edinburgh]

Millions of people are under the gun, Saturday's temps here are expected to teach 105f with rel humidity in the high 70s not many plants can take that for long and not have some reaction, inside my rel humidity is 40 and my temp 78f, were I live we are getting more and more 90f days and above with brutal dew points in the high 70s, last year we had 29 days 90f or above add dew points of 77 and Saturday it will feel like 120f , if I had plant's outdoors I would water thoroughly and keep them out of direct sunlight during the hottest part of the day.

 

[PipeCarver]

Recently I have seen a lot of posts about relative humidity and the importance of it being in the proper Zone. I have grown in relative humidity levels from as low as 10 or 15% in the winter, to as much as 80% in the summer, and I find that I see very little difference at all in yield or potency of the plants. They seem to react the same regardless, with the possible exception of when they first start I might see a little bit of tip Browning which quickly dissipates as the leaves fill in more. I grow mostly Sativa strains, so perhaps that has something to do with the lack of problems. In truth, my rh rarely goes above 30% and no issues seem present. Why do some seem to have problems... it seems higher Rh causes more problems, like PM and similar maladys.
What is your experience?

The bests explanation on the effect of rh on plants I could find was plants work on hydraulics. With a high rh the plants needed water is absorbed through the leaves slowing down its need to draw water and nutrients from the roots leaving a build up of nutrients behind in the soil that then can lead to nutrient lockout.