What's the ideal RH in an Wardian case?

For cooler growing PNGs and Pleuro types I grow them around 70-80% for the day and mid 90% in the evening.

 

Similar to Chad, I'm setting it to be 80%RH.

When the temp goes down, RH naturally goes up. Higher RH at night could be benficial for CAM plants which opens stomata at night.

All plants would like higher RH in general (so they can do gas exchange freely through stomata). But it may be wasteful (if you are using electricity to generate humidity). Other creatures (pathogens) may like high RH, too.

 

"Why the increase at night, though?"

As a non-scientific comment: RH stands for "relative humidity". Accept "relative" as "depending on temperature". The same amount of water per the same amount of air can give you 50% RH at 25C of day time and 90% RH at 10C of night time. Ignore given digits of the example - I am just guessing. For precise data search "psychometric chart". In another words, to increase RH you do not need to provide an additional moisture, - just drope a temperature.
The beauty of that equation - even in the dry season small amount of water vapour in the air can supply enough moisture for orchids at temperatures close to the due point.
I had mixed results at the time of growing under lights. Some of my those best are still non-acheavable in greenhouse environment. To the certain degree, growing in "orchidarium" you have better opportunity to control and harmonize essential parameters of your mini paradise - light, temperature, humidity, fresh air supply.

 

You are correct. I do not provide additional humidity at night or during the day for that matter. This is the level I acheive with those types of species under my watering conditions. The increase is because the colder air cannot hold as much moisture. My temps swing between 15 and 20F in the good months. So about 70F to 50F.

 

85% RH can be good or maybe high, depending on the orchids genera you grow. I would check humidity in the natural habitats i.e. using Bakers' data. Excessive humidity of ambient air potentially can suppress water evaporation through stomata, causing suppression of capillary action, which "pumps" water and absorbed nutrients from roots to leaves, resulting in weak plant growth.

 

Is this really true, Alexey? I believe that you are talking about tension-cohesion theory (transpiration is the driver of water movement). In theory, if they don't lose water, they don't have to suck up water, do they? But with no transpiration, I can see that distribution of nutrients via xylem could be slower because it becomes more of diffusion process from high concentration region (root) to low concentration region (leaves). I haven't thought what would happen to plant under 100%RH until you point it out. Pretty interesting, and I should look into it.

With strict CAM, injecting CO2 in the day time is wasteful. But quite a few CAM plants are facultative CAM. For example, some Phal studies show that they behave closer to C3 after irrigation, but after a couple days of drying, they are more of CAM.

I also considered CO2 injection, it seems to help in some experiments (I don't remember details, but it was with Phals I believe). But there is also an acclimation effect; plants grown in high CO2 concentration develops slower photosynthesis after a while. So you might see short term benefit, but the benefit might become weaker. This wasn't with orchids, and I don't completely understand this acclimation process.

With regard to C3 vs CAM, there is a table about the mode of orchid photosynthesis in Arditti's book, Fundamentals of Orchid Biology, and I listed a couple examples here.

 

Naoki, it was approximately four years ago when I read about plant metabolism (that is why terminology is not exact in my post!). Will try to find that source of info again. If I recall it right, required minimum of RH gradient between stomata and adjacent ambient air has been provided in that article. And that data provoked me to increase amount of fresh air supply for orchidariums and do not keep RH steadily high.

 

ISTR that a lot of tropical plants have the ability to exude water from the leaf surfaces, so as to force transpiration. Dunno if orchids do that, and it may be that I don't remember that correctly, either.

 

Carl, I have not found any information that orchids (or at least majority of orchid genera) have guttation mechanism for active water and solubles exudation. Seems like whenever non-damaged orchid loses water it is caused by evapotranspiration and is driven by water vapour pressure gradient.

 

Naoki, so far I was not able to find that source, which has triggered my post. Majority of the related publications are about plant adaptations to the low RH levels. That is understandable, - "excessive humidity" is not an issue for natural habitats and is not an issue for controllable environment at the industrial greenhouses. In the natural habitats plats are always experience seasonal and daily fluctuations of RH. Industrial growers will never allow or achieve parameters, which I can keep in a small volume of my orchdarium.

As a point of discussion. Sense, if not a "definition", of the low RH is well established. To date there is no such a criteria for an excessively high RH. However, if I want to keep my growing environment at the optimum, it should exist:
- rising RH of the ambient air we will reach a point, when water vapour pressure gradient will be low enough to compromise plant's evapotranspiration;
- active root uptake can compensate that disfunction to the certain level, but that mechanism works at the expenses of the plant sugars and is counterproductive.

 

Alexey, thank you for the additional info, and I can see your logic that the highest RH is not optimum. I have thought that plants would be happier with 100%RH (as long as pathogen issues don't exist), but I probably didn't think through this deeply.

I tried to look for some related info, too. The top paragraph of p.231 basically mention what you said:
http://myrtus.uspnet.usp.br/bioabelha/images/pdfs/pdfs_publicacoes4/gonda_cugnasca_2006.pdf
But, I didn't find a reference to this info. It may be included in the basic plant physiology book. I'll try to look more when I get a chance. Anyway, thank you for bringing this interesting point!