CAM orchids - mist or not to mist them, that is the question

[Marzenna]

I recently got into a discussion about watering orchids from above. I'm a proponent of the most sensible method for me, which is raining it down, so I pop my plants in the shower cabin and shower them from the top with a watering can filled with rainwater, letting the water cascade down the leaves, flush the soil, and rinse the roots. I've been doing this for many moons and hardly ever does any of my moth orchids fall ill. Detractors of this watering style have brought up some serious points, arguing that due to the stomata, through which the water could be taken up by the plants lurking beneath the leaves, and also the fact that moth orchids are CAM orchids, such watering is nonsensical and brings more risks than benefits, as plants watered in this way gather water in hard-to-reach spots, making them prone to bacterial or fungal diseases. They reckon it would be best to pour water through the pot, but making sure to steer clear of soaking the leaves. I couldn't buy into that, considering that if moth orchids often thrive in rainforests, they've clearly been evolutionarily adapted to conditions where their leaves meet water every day. However, this didn't sway my adversaries. What's your take on the matter? I know there are many seasoned orchid growers here, that you have heaps of knowledge. Do you also reckon avoiding leaf soaking for orchids is the way to go, and that it's ineffective and hazardous for them?

[rbarata]

Welcome Marzenna

Considering only Phalaenopsis... I hardly water them from above, altough I have some specimens that due to their size and growing habit must be watered like that (otherwise they would dry to death).

I consider relative humidity the fundamental parameter for the CAM process. All the water on the leaves would be dry in a small amount of time, therefore its effect is insignificant. The problem is water stalled in small areas of the leaves.

Youare right when you say that Phals are subjected to rain everydays but in nature they don't grow in pots. Their natural position in the wild is leaning to the sides. This allows water to run out of the leaves due to gravity, which, most of the time, doesn't happen when growing in pots.

Another thing to consider is temperature. Where I live, at least half of the year we have high temperatures...in this case, any accumulated water will dry in a short time. That's when I water from above (only some specimens, as I said before). So, if temps are low or mild, accumulated water won't dry out so fast, leading to issues.

[Ray]

A slightly technical twist - water is not taken into the plant via stomata, gases are. Liquids are absorbed through areas known as plasmodesmata. Now then, they are closely associated with stomata on the leaves, so it’s sort-of a “distinction without a difference” thing.

Then there is the case that, presumably as part of a water retention strategy, most of the stomata (and plasmodesmata) are located on the underside of leaves, meaning that overhead watering may not reach them.

The most significant factor against foliar application, in my mind, is the development of thick, waxy cuticle layers on the leaves - another water retention strategy - which slows losses through the surface and near-surface cells. If it slows liquid flow in one direction, it slows tin the other, too. Such layers are present in all orchids, but more developed in thick-leaved plants like phalaenopsis, cattleyas, and vandas, especially as they mature.

I think these are all things against foliar feeding, but watering? As long as the media gets flushed and wetted well, if you have decent air movement, wetting the foliage is of little risk - I did so for years in my greenhouse, and still do when my plants are out of the deck for the summer.

[Marzenna]

I've been doing that for years as well. During winter, the humidity in my house doesn't go above 40-50%, so it's very dry. Phalaenopsis watered in this manner really thrive, it clearly suits them. Any diseases occur extremely rarely. Surely, I fully agree with that, when choosing a watering method, you should always take into account several basic factors, such as humidity, temperature, type of substrate, and air circulation. Certainly, what works well for my plants may not be beneficial for someone living in a more humid climate. It might just be an impression, but it seems to me that after such watering - leaves, roots from above - the leaves are in really excellent condition. Is it just because they take up water through the roots? I think there is something more..

BTW thanks a lot for info about plasmodesmata, very interesting.

---------- Post added at 03:08 PM ---------- Previous post was at 03:05 PM ----------

Originally Posted by rbarata

Another thing to consider is temperature. Where I live, at least half of the year we have high temperatures...in this case, any accumulated water will dry in a short time. That's when I water from above (only some specimens, as I said before). So, if temps are low or mild, accumulated water won't dry out so fast, leading to issues.

I see, but in my case, the situation is quite different - where I live, most of the year is rather cool and relatively dry.

---------- Post added at 03:30 PM ---------- Previous post was at 03:08 PM ----------

Originally Posted by Ray

think these are all things against foliar feeding, but watering? As long as the media gets flushed and wetted well, if you have decent air movement, wetting the foliage is of little risk - I did so for years in my greenhouse, and still do when my plants are out of the deck for the summer.

I totally agree with that, plus I've read that the biggest risk everyone's talking about, which is the stem rot, can come not only from invasion of pathogens in a humid environment, but also from a calcium deficiency.

[Ray]

The necrosis of newly growing tissues due to a calcium deficiency is entirely different from most “stem rots”.

Calcium is an essential element in all plant tissues. Without it being present as a “building block”, the apical meristem cannot do its job and dies, then rots.

The thing is, unlike most other nutrient elements, if it is not being actively supplied, calcium is not easily transferred from old tissue to new.

If the rot is at the growth end, it may be a calcium issue; if it’s elsewhere, it’s probably a pathogen, and if there is healthy tissue beyond the rot, it’s definitely so.

[Marzenna]

Originally Posted by Ray

If the rot is at the growth end, it may be a calcium issue; if it’s elsewhere, it’s probably a pathogen, and if there is healthy tissue beyond the rot, it’s definitely so.

Ray, thank you very much, it explains a lot.

You see, I've read so much about watering that I should really have a handle on it by now. But the more I read, the more questions still pop into my head:

If orchid leaves are covered with a cuticle that protects them from excessive water uptake and evaporation, they can't be responsible for "excessive water accumulation in the plant," can they? If so, it means that the plant can only absorb excess water through its roots.

So it's the roots that are "responsible" for potential excess water in the plant and the formation of edema, am I right? For that to happen, however, the plant would have to be submerged in water with its roots for a long time. I don't believe that excessive water uptake could occur even during prolonged rainfall (3 hours for example), am I right? Of course I am thinking about epiphytes now.

I also insist on the position that water is not responsible for the issues with orchid cultivation.The problem, however, lies in the lack of proper air circulation, or in short, the mismatch of watering method to the environmental conditions of the plant.

Well, it seems that sola dosis facit venenum, like in many different situations. It's similar to diabetes; it's not just about the glycemic index, the glycemic load matters too. I apologize for the foolish comparison.

[rbarata]

Some reading sugestions

For calcium deficiency, read this one.

For CAM process, this one I think it's easy to follow.

For edema, this one.

[Marzenna]

Originally Posted by rbarata

Some reading sugestions

For calcium deficiency, read this one.

For CAM process, this one I think it's easy to follow.

For edema, this one.

Thank you!

[rbarata]

As a side note, the only benefit I see for watering by wetting the leaves is, in dry environments, the resulting humidity increase might stimulate stomata to open (due to the dry environment they'll be closed, even if it's night time, in my understanding), which will allow gas exchange.
But that will be short lived because soon the humidity will be lower again.
This is just a general assumption because CAM process depends on a lot of variables such as humidity, light, temperature, available CO2, and probably some more.

Basically, it's an effort that doesn't pay much.

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