Day vs Nighttime Humidity

[mopwr]

Around the start of this year I got back into orchid growing, but this time I wanted to be able to grow a bit more of the more finicky plants that I could never get to do well as a windowsill grower in the Pennsylvania. In order to do this, I decided to build my own terrarium for my orchids. I wanted to have it look nice as well as have it dialed in foor growing mini to medium size orchids. I also wanted it fairly automated, with timers and all that. I setup and adjusted the led lights (dimmable) to get the right light levels (in approximate foot-candles / par) from top to bottom, added fans for circulation, added a heater into the gravel on the bottom, and added a framed plexiglass top to keep in the humidity.

Regarding the humidity, it's not completely sealed and there are gaps at a bunch of different places, but even with that - by proxy of having so many plants in there - the humidity levels can easily get into the 85-90% range. To offset that, I added venting fans that pull out the humid air when it hits 85% and drops it 80%, they run for a minute or so every half hour, give or take. The thing is, I've heard that for orchids, higher humidity is more of a need at night.

That got me thinking, and here's my question:
Is there any value in having the ability to have a day / night humidity variance?

The only immediate thing I can think of is that if higher humidity at night is of more value, I could keep the day humidity levels lower - and have even less issues with fungal / bacterial diseases. Also, I'm not sure, but I think the plants may grow even better if the humidity is say 75% during the day and 88% at night - or something like that... I'd assume that that might closer replicate what they encounter in nature if I'm not wrong.

[Clawhammer]

I am not aware of a need for higher humidity at night. Achieving high relative humidity in colder air is easier than warm air, could probably vent more at night without negatively affecting humidity. Its the stale air rather than the humidity that causes problems IMO

[Ray]

Higher humidity at night is a natural occurrence. Air at a lower temperature can hold less moisture-per-pound of dry air, so at a constant moisture supply, the relative humidity will be higher.

Unless your conditions lead to a great deal of condensation, which can result in rot if collected in crowns and leaf axils, I wouldn't worry about it at all.

[mopwr]

Originally Posted by Clawhammer

I am not aware of a need for higher humidity at night. Achieving high relative humidity in colder air is easier than warm air, could probably vent more at night without negatively affecting humidity. Its the stale air rather than the humidity that causes problems IMO

I think the idea of higher humidity at night being better comes from the idea that orchids leverage CAM photosynthesis. Taken from Science Direct's article on CAM photosynthesis:
"In CAM plants, stomata open at night when conditions are relatively cool and humid..."

In nature, there's always a variance between daytime and nighttime RH levels, and in some of the places these plants come from, I think they have evolved to deal with warmer / lower RH days and cooler / higher RH nights. I have heard from a few who believe matching that cycle helps growth, but everything is anecdotal.

[Clawhammer]

Originally Posted by mopwr

I think the idea of higher humidity at night being better comes from the idea that orchids leverage CAM photosynthesis. Taken from Science Direct's article on CAM photosynthesis:
"In CAM plants, stomata open at night when conditions are relatively cool and humid..."

In nature, there's always a variance between daytime and nighttime RH levels, and in some of the places these plants come from, I think they have evolved to deal with warmer / lower RH days and cooler / higher RH nights. I have heard from a few who believe matching that cycle helps growth, but everything is anecdotal.

Only some orchids are CAM, the more succulent ones like cattleya. Increasing humidity while stomata are open reduces growth (slows down transpiration) anyway, so not sure that's something you want.

RH will always go up at night if temp falls due to the fact that cold air has less potential to hold water vapor explained by Ray.